Science: Forces for Fours 1 Glenda Leslie

4orces

4 4s 4 teachers

supporting the teaching of

Yr 4 Smooth Moves

Glenda Leslie Curriculum Consultant Science AISWA

Science: Forces for Fours 2 Glenda Leslie

Forces For Fours for teachers Many units dealing with forces start with the question:

“What is a force?”

Even in the world of university physics, this is a tough question. Stating that a force is a push or a pull

doesn’t quite answer the question. So, better questions are:

“How do we know forces are acting?”

“What forces are acting?”

“Why is it important that we know about forces?”

Knowing about forces helps explain how and why things happen and why they happen in the way that they

do.

Forces For Fours is about exploring different types of forces and the actions they cause. It is about

making explicit the forces that are taken for granted and exploring how they affect movement. Students

should be able to recognise when a force is acting and to represent forces using arrow diagrams.

Forces For Fours is for teachers to improve their personal understanding of forces to be able to

confidently teach forces to year 4 students. It gives some background information and looks at some of

the common misconceptions, difficulties and alternative thinking of students that form barriers to

developing a sound scientific view of forces.

Primary Connections: Smooth Moves unit provides students with the opportunity to explore forces and

motion. The activities help students identify forces that act at a distance and those that act in direct

contact, and investigate how different sized forces affect the movement of objects.

Science: Forces for Fours 3 Glenda Leslie

WA Curriculum for Year 4 Science Understanding Physical Sciences Content descriptor:

Forces can be exerted by one object on another through direct contact or from a distance

(ACSSU076)

Achievement Standard:

They use contact and non-contact forces to describe interactions between objects

Science as a Human Endeavour Content descriptors:

Science involves making predictions and describing patterns and relationships (ACSHE061)

Science knowledge helps people to understand the effect of their actions (ACSHE062)

Achievement Standard:

They identify when science is used to ask questions and make predictions.

They describe situations where science understanding can influence their own and others’ actions

Science Inquiry Skills Content descriptors:

With guidance, identify questions in familiar contexts that can be investigated scientifically and

make predictions based on prior knowledge (ACSIS053)

With guidance, plan and conduct scientific investigations to find answers to questions, considering

the safe use of appropriate materials and equipment (ACSIS054)

Consider the elements of fair tests and use formal measurements and digital technologies as

appropriate, to make and record observations accurately (ACSIS055)

Use a range of methods including tables and simple column graphs to represent data and to

identify patterns and trends (ACSIS057)

Compare results with predictions, suggesting possible reasons for findings (ACSIS215)

Reflect on investigations, including whether a test was fair or not (ACSIS058)

Represent and communicate observations, ideas and findings using formal and informal

representations (ACSIS060)

Achievement Standard

Students follow instructions to identify investigable questions about familiar contexts and predict likely

outcomes from investigations.

They discuss ways to conduct investigations and safely use equipment to make and record observations.

They use provided tables and simple column graphs to organise their data and identify patterns in data.

Students suggest explanations for observations and compare their findings with their predictions.

They suggest reasons why their methods were fair or not.

They complete simple reports to communicate their methods and findings.

Science: Forces for Fours 4 Glenda Leslie

Understanding Forces: background All bodies are subjected to a variety of forces. We have learned how to deal with the influence of forces

without having the ideas or words to explain what we are doing.

Standing up on two feet is a great achievement and one accomplished by children in their first year of life.

To do this successfully the body needs to produce forces to overcome the force gravity and to balance

forces around the body to stop falling over.

Playing with a mobile suspended across the struts of a pram or with a toy on the floor, children learn about

the effects of forces. One favourite game is “drop the object” when seated in a high chair, thus learning

about gravity. We learn about forces acting in the world around us through interactions with other objects.

The scientific view of forces is often different to students’ own intuitive models of forces because everyday

experiences involve some forces that are not immediately obvious. Evidence from observations in the

everyday context is often counter-intuitive to the scientific view because gravity, friction and resistance are

involved and are not explicitly understood when considering forces.

For example:

Students’ world view:

- force is required to keep objects moving otherwise they slow down and stop

To students, it would appear that the object “runs out of force”.

This happens with every push and pull experience they have ever had.

Scientific view:

Science wants students to believe that a moving object will continue moving at the same speed and in the

same direction with no more force required to keep it going. The object will change speed or direction only

if another force acts on it.

The real nature of forces can best be seen in space where the forces of gravity, friction and resistance are

insignificant and the movement of objects does follow the scientific view.

The conundrum caused by the different perspectives is very difficult to break, but with carefully devised

activities, the students will come, someway, to understanding the scientific point of view. Work in later

years will build on students’ experiences to develop these ideas further.

If teachers get students to understand the variety of forces acting on objects at this point in their

conceptual development, then the later work will make more sense and will be easier to understand.

Science: Forces for Fours 5 Glenda Leslie

Types of forces

Forces acting on contact

Forces acting at a distance

1.

6.

5.

3.

4.

2.

3.

1.

2.

Science: Forces for Fours 6 Glenda Leslie

Understanding Forces Which of the statements below do you think are correct?

- highlight the ones you think were correct

Feel free to discuss these with a partner.

True or False Statements

1. all forces need objects to be in contact to have an effect

2. when two objects of the same size and shape but different weights are dropped

together, the heavy one will reach the ground first

3. friction only occurs between solid objects

4. the Moon has less gravity than Earth

5. all metals are attracted to a magnet

6. air and water act against movement of object in them

7. there is no/less gravity in water

8. gravity stops acting when the object hits the ground

9. forces act through an imaginary point called the centre of gravity of an object

10. mass and weight mean the same thing and they are equal at all times

11. gravity only works one way – the Earth attracts us; we don’t attract the Earth

12. opening a parachute during freefall makes the skydiver go upwards

13. objects float in water because they're 'lighter' than water or sink because they are

heavier; wood always floats and metal always sinks

14. if an object is moving there must be a force pushing on it

15. if an object is at rest on a table, there are no forces acting on it

16. a solid cannot be compressed or stretched

17. if a force acts on an object it will inevitably move

18. all magnets are made of iron

19. force is a property of an object; an object has force and, when the force 'runs out', it

stops moving

20. magnetic poles are always at the end of the magnet

Science: Forces for Fours 7 Glenda Leslie

The Problems with Forces Because of the difference in views, people have common misconceptions in this area. From: http://teachfind.com/node/93227?quicktabs_1=0#quicktabs-1

Don’t worry if you highlighted most of these as it has been shown that university physics students can also

hold fast to some of these misconceptions even after graduating! It is the way we have experienced

moving objects in our world.

How do we help Year 4 students overcome some of these misconceptions???

Science: Forces for Fours 8 Glenda Leslie

Forces: an historical account outlining how our ideas about falling objects

developed

The Aristotelian viewpoint

Galileo and the leaning tower of Pisa

https://www.youtube.com/watch?v=_Kv-U5tjNCY

https://www.youtube.com/watch?v=7eTw35ZD1Ig

https://www.youtube.com/watch?v=z_sJ15feNGw

Sir Isaac Newton

Science: Forces for Fours 9 Glenda Leslie

Identifying forces What forces are acting?

How do I know a force is acting?

During these activities, use the force arrows to identify the forces acting in each situation.

Attached to this resource is a packet of arrows (red, green and blue) of different sizes.

Notice that the arrows are all of the same width and the same size arrow head. The different in the size of

the force is indicated by the length of the arrow. This is consistent with the scientific use of arrows in force

(free body) diagrams.

Activity 1: Moving Balloons Equipment:

2 balloons blown up

piece of synthetic fabric

piece of natural fabric

1 m of cotton thread

Tie the balloons to each end of the cotton thread.

Suspend the balloons so they are alongside one another and away

from anything else eg. hang from a paper clip hook taped to the

edge of a table.

Note the positions of the balloons before being rubbed with any fabric.

Explore the effects of rubbing either one or both balloons with the different fabrics.

Be sure to allow the balloons to hang freely before noting the results.

Use the arrows to indicate the size and direction of forces acting in this situation.

Results:

What are the main forces acting on the balloons in this situation?

Science: Forces for Fours 10 Glenda Leslie

Activity 2: Moving Paper Clips Equipment:

about 20 paper clips of 2 different sizes

3 different magnets

A4 cardboard

cotton thread

sticky tape

Investigate the following:

How far from a suspended paper clip will each magnet be before the paper clip

will drop?

How many paper clips will each magnet hold suspended in a single chain?

Will 2 or more magnets placed together be stronger than one magnet by itself?

Use the force arrows to identify the size and direction of the forces involved in these situations.

Science: Forces for Fours 11 Glenda Leslie

Activity 3: Snapping Cotton Equipment:

1 m cotton thread

weights to about 5 kg

scissors

2 paper clips

sticky tape

Tie one paper clip to the each end of the cotton thread.

Tape one paper clips to the edge of table allowing the cotton thread to dangle freely over the side.

Add weights to the free end of the cotton thread.

How much weight is require to break the cotton?

What happen when the cotton broke? (you need to observe very carefully here)

Use the force arrows to identify the size and direction of the forces involved in these situations.

What were the types of forces involved in this situation?

What other question could be investigated using these materials?

Science: Forces for Fours 12 Glenda Leslie

Activity 4: Floating Boats Equipment:

large flat clear container

toy boats made of plasticine

weights eg. marbles or metal nuts/bolts

water

Place the ball of plasticine in the water.

Use the force arrows to identify the forces acting in this situation.

Make a boat shape from the plasticine and place it gently on the water.

Use the force arrows to identify the forces acting in this situation.

Place the weights, one by one, into the boat until it sinks.

Use the force arrows to identify the forces acting in this situation before the boat sank and after the boat

sank.

What were the types of forces involved in this situation?

Science: Forces for Fours 13 Glenda Leslie

Science: Forces for Fours 14 Glenda Leslie

Activity 5: Centre of Gravity

Equipment:

sticky tape

cardboard approximately 25 x 20 cm

1 m cotton thread or fine string

scissors

ruler

2 paper clips

small weight eg. BluTack

drawing pin

Cut out an irregular shape from the piece of

cardboard. Make a series of small holes around the

edges of the shape.

Hang the shape by one of the holes from a

straightened paper clip taped to a bench.

Hook the weighted string on to the same hole.

When the shape has stopped moving, draw a line along the weighted string.

Use a ruler to mark a straight line.

At the point of intersection of all of the lines, make a small hole with a pin.

Tie a string through the hole and hang the shape.

The shape should stay in the same position as the forces are acting equally on both sides of the

string, around the centre of gravity.

Try to find the centre of gravity of different shapes.

Find pictures of athletes in different positions. Cut out the pictures and paste on both onto

cardboard, then repeat the activity.

Explain the location of the centre of gravity with respect to the type of movement required of that

body.

Use the force arrows to identify the forces acting in this situation.

Science: Forces for Fours 15 Glenda Leslie

Activity 6: Forces and motion Equipment:

elastic band

ball

Discussion points:

What forces are involved in this situation?

What can you change in this investigation?

What would you measure in this investigation?

What would you keep the same in this investigation?

How can you change the force used to move the ball?

How will you make your trials consistent?

What will you do if the ball rolls one way or the other?

How can you make consistent measurements?

Now conduct you investigation.

Use the force arrows to identify the forces acting in this situation.

Science: Forces for Fours 16 Glenda Leslie

Visualising Forces -using arrow diagrams

Force diagrams

We can show the forces acting on an object using a force diagram.

In a force diagram, each force is shown as a force arrow. An arrow shows:

the size of the force (the longer the arrow, the bigger the force)

the direction in which the force acts.

The arrow is usually labelled with the name of the force and its size in newtons.

Text books often show a force with a thick coloured arrow (as below), but it is best if you just use a pencil

and ruler to draw an arrow with a single line of different lengths to indicate size of the force.

OR

Keep the width of the arrow constant so that it is the length of the arrow that represents the size of the

force.

Book on the table

Explain why the book doesn’t move.

Floating Boats

Draw the arrows that indicate the forces acting when the boat is moving.

Science: Forces for Fours 17 Glenda Leslie

Tug-of-War Draw the force arrows on the rope in this diagram.

Who is winning this Tug-of-war? What evidence is there in the picture to help you make this decision?

How would the forces diagram change if the other side added another person to their team?

Riding a Bicycle

What forces are acting on the bike and rider?

What difference in forces are there when considering the cyclist separate from the bicyle?

Science: Forces for Fours 18 Glenda Leslie

Forces are measured in Newtons

Measuring forces

A force meter is used to measure forces.

Forces can be measured

using a force meter. Force

meters contain a spring

connected to a metal

hook. The spring stretches

when a force is applied to

the hook. The bigger the

force applied, the longer

the spring stretches and

the bigger the reading.

The unit of force is called

the newton, and it has the

symbol N.

Force measurers use units to measure the forces exerted:

spring balances

bathroom scales

(Bathroom scales measure ______________________________________________________________

______________________________________________________________________________________

Because we are all on Earth and gravity does not change significantly across the surface of the Earth, then

_______________________________________________________________________________________

_______________________________________________________________________________________

1 kilogram is equal to a force of 9.8 Newtons (near enough to 10 N is you are doing quick calculations.)

Science: Forces for Fours 19 Glenda Leslie

Net Force

Many force problems require you to sum all the force acting upon the object. This total force is called the

net force (Fnet) or resultant force. The net force can be calculated using free body diagrams.

If no forces act on an object, the net force on the object is zero.

Although this happens in physics problems, it is very unlikely that an object will have no

forces at all acting on it.

If there is just one force on an object, then that force is the net force. For example in free

fall, the net force on an object is the force of gravity acting on it, ie. its weight.

If 2 forces push or pull on an object in opposite directions, and the two forces are of equal

size, they cancel each other out exactly and therefore the net force is zero.

If two forces act on an object in opposite directions and they are not of the same size, they

do not cancel each other out. What is left over is the net force (difference between the

forces). Knowing the direction and size of the forces is important to know the direction in

which the object will move

If two or more forces act on an object in the same direction, then the net for is the sum of

the forces. In this diagram the net force is 10 N to the right.

From: http://gradeelevenphysics.weebly.com/forces.html

Science: Forces for Fours 20 Glenda Leslie

Force Diagrams

(Free Body Diagrams)

Wagon Rides Thinking about all the forces acting in a situation on ONE object: the wagon.

This picture shows the forces acting on the wagon.

Label the forces shown in the diagram above.

Draw a diagram of the forces acting on the wagon in the diagram above:

How does the free body diagram of the forces acting on the wagon when two children are pushing it,

change with the addition of the extra person? Carefully compare it to the first diagram of the wagon.

Science: Forces for Fours 21 Glenda Leslie

Effects of a Force

Look carefully at diagram a and b.

What does the m represent?

What is different between these situations? Why?

Science: Forces for Fours 22 Glenda Leslie

Force of Friction

Activity 7: Cars on Carpet Does the same force produce the same motion?

Use a variety of surfaces to find the effect of the surface on the distance a car travels.

Equipment:

toy car

ramp

different surfaces eg. carpet, bricks, wood, tiles

Prediction: What do you predict will happen?

Hypothesis: (how is this different to a prediction??)

Draw a diagram to show how you will set up the equipment for this investigation.

(Think: what makes a good scientific drawing??)

Table of results

(Think: what is the scientific set up for data tables?)

Conclusion:

(Think: what makes a good conclusion?)

What was the force that influenced the results?

In a frictionless world, what would be the results of this experiment?

Science: Forces for Fours 23 Glenda Leslie

Friction: Friend or Foe Figure 1 is a crude pictorial representation of how friction occurs at the interface between two objects.

Close-up inspection of these surfaces shows them to be rough. So when you push to get an object moving

(in this case, a crate), you must raise the object until it can skip along with just the tips of the surface

hitting, break off the points, or do both. A considerable force can be resisted by friction with no apparent

motion. The harder the surfaces are pushed together (such as if another box is placed on the crate), the

more force is needed to move them. Part of the friction is due to adhesive forces between the surface

molecules of the two objects, which explain the dependence of friction on the nature of the substances.

Adhesion varies with substances in contact and is a complicated aspect of surface physics. Once an object

is moving, there are fewer points of contact (fewer molecules adhering), so less force is required to keep

the object moving.

. Figure 1: Frictional forces, such as f, always oppose motion or attempted motion between objects in

contact. Friction arises in part because of the roughness of the surfaces in contact, as seen in the

expanded view. In order for the object to move, it must rise to where the peaks can skip along the bottom

surface. Thus a force is required just to set the object in motion. Some of the peaks will be broken off, also

requiring a force to maintain motion. Much of the friction is actually due to attractive forces between

molecules making up the two objects, so that even perfectly smooth surfaces are not friction-free. Such

adhesive forces also depend on the substances the surfaces are made of, explaining, for example, why

rubber-soled shoes slip less than those with leather soles. From: http://cnx.org/content/m42139/latest/?collection=col11406/latest

Measuring Friction Measures of friction are based on the type of materials that are in

contact. Concrete on concrete has a very high coefficient of friction.

That coefficient is a measure of how easily one object moves in

relationship to another. When you have a high coefficient of friction,

you have a lot of friction between the materials. Concrete on concrete

has a very high coefficient, and Teflon on most things has a very low

coefficient. Teflon is used on surfaces where we don't want things to

stick; such as pots and pans.

Science: Forces for Fours 24 Glenda Leslie

Friction and Surfaces

Explain the different frictional forces on these two boys and how it affect the resulting movement.

Types of Friction

From: http://hyperphysics.phy-astr.gsu.edu/hbase/frict.html#rou

What will happen when the applied force is removed?

Play this game:

Friction and road surfaces

http://www.3m.co.uk/intl/uk/3mstreetwise/pupils-surface-types.htm

Science: Forces for Fours 25 Glenda Leslie

Activity 8: Types of Friction

Equipment:

book or block of wood

1 m cotton thread

spring balance

clean dry sand OR talc powder

Attach the cotton thread to the book or block with a loop in the free end of the thread.

Insert the hook of the spring balance into the thread loop.

Pull the book gently using the spring balance parallel to the table surface.

Measure the force required to start the book sliding.

Measure the force required to keep the book moving at a constant speed.

Draw up a suitable table to collect your data.

Sprinkle the table surface with the sand or powder.

Repeat the measurement with the book sliding on the sand or powder.

Add columns to your data table to collect the new data.

Compare the results with and without the sand/powder.

Why is there a difference?

What other materials could be used to produce the same results as the sand/powder?

Why is it useful to reduce the amount of friction?

Science: Forces for Fours 26 Glenda Leslie

Friction in Force Diagrams

From: http://physics.wku.edu/phys201/Information/ProblemSolving/ForceDiagrams.html

From: http://gradeelevenphysics.weebly.com/forces.html

Draw the force diagram including friction.

Science: Forces for Fours 27 Glenda Leslie

Forces on a Car

From: http://www.bbc.co.uk/schools/gcsebitesize/science/add_ocr_pre_2011/explaining_motion/forcesandmotionrev1.shtml

Name the forces represented by each of the coloured arrows.

What is not ‘good’ scientific process in this diagram?

Would you use this diagram with year 4 students?

Pushing a Trolley

This has been included to show that forces can be studied for different ‘systems’.

Forces can be looked at individual parts (the trolley and the person) or the total situation (the person

pushing the trolley). For Yr 4’s, use the KISS principle: Keep It Simple, Sweetheart. ie. use the forces on

one object only.

From: http://cnx.org/content/m42074/latest/?collection=col11406/latest

Science: Forces for Fours 28 Glenda Leslie

Forces for Snowboarding This is more difficult, but where you have unbalanced forces, movement will occur.

Free Body Diagram of a Snowboarder at Rest

In the situation I depict above, a snowboarder is on a level surface, that is, the surface is perpendicular to the direction of

the force of gravity. The black arrow represents the force of gravity and the blue arrow is the force of the snow covered earth

pushing back against the snowboarder! The last part seems a bit counter-intuitive and you need to think about it.

In this case, the snow covered surface of the earth pushes against the skier perpendicular to the surface — usually

referred to as force normal. Without that force normal our snowboarder is in free fall!

Freebody Diagram Showing Forces on a 30° Run

Notice now how the force of gravity (black arrow) and the force normal (blue arrow) do not line up anymore? They no

longer entirely cancel each other out. The gravitational force can be broken into two constituent parts, one component is

parallel to the trail and the other is perpendicular to the trail.

Force Vector Decomposition

The perpendicular component is balanced out by the force normal, but the parallel component has NO opposing force (we

ignore wind resistance and frictional forces, at least for now)

and where there is an unopposed force there is acceleration.

Therefore, ____________________________________________________________________________________

From: http://www.wi-ski.com/free-body-diagram/

Science: Forces for Fours 29 Glenda Leslie

Answers to types of forces:

Contact Forces Action-at-a-Distance Forces

Frictional Force Gravitational Force

Tension Force Electrical Force

Normal Force Magnetic Force

Air Resistance Force

Applied Force

Spring Force

Type of Force Description of Force

Applied Force

Fapp An applied force is a force that is applied to an object by a person or another

object. If a person is pushing a desk across the room, then there is an applied force acting upon the object. The applied force is the force exerted on the desk by the person.

Gravity Force

(also known as Weight)

Fgrav

The force of gravity is the force with which the earth, moon, or other massively

large object attracts another object towards itself. By definition, this is the weight of the object. All objects upon earth experience a force of gravity that is directed "downward" towards the centre of the earth. The force of gravity on earth is always equal to the weight of the object as found by the equation:

Normal Force

Fnorm The normal force is the support force exerted upon an object that is in contact

with another stable object. For example, if a book is resting upon a surface, then the surface is exerting an upward force upon the book in order to support the weight of the book. On occasions, a normal force is exerted horizontally between

two objects that are in contact with each other. For instance, if a person leans against a wall, the wall pushes horizontally on the person.

Friction Force

Ffrict The friction force is the force exerted by a surface as an object moves across it or

makes an effort to move across it. There are at least two types of friction force - sliding and static friction. Thought it is not always the case, the friction force often opposes the motion of an object. For example, if a book slides across the surface of a desk, then the desk exerts a friction force in the opposite direction of its motion. Friction results from the two surfaces being pressed together closely, causing intermolecular attractive forces between molecules of different surfaces.

As such, friction depends upon the nature of the two surfaces and upon the degree to which they are pressed together. The maximum amount of friction force that a surface can exert upon an object can be calculated using the formula below:

Air Resistance Force

Fair The air resistance is a special type of frictional force that acts upon objects as they

travel through the air. The force of air resistance is often observed to oppose the motion of an object. This force will frequently be neglected due to its negligible magnitude (and due to the fact that it is mathematically difficult to predict its value). It is most noticeable for objects that travel at high speeds (e.g., a skydiver or a downhill skier) or for objects with large surface areas.

Tension Force

Ftens The tension force is the force that is transmitted through a string, rope, cable or

wire when it is pulled tight by forces acting from opposite ends. The tension force is directed along the length of the wire and pulls equally on the objects on the opposite ends of the wire.

Spring Force

Fspring The spring force is the force exerted by a compressed or stretched spring upon any object that is attached to it. An object that compresses or stretches a spring is

always acted upon by a force that restores the object to its rest or equilibrium position. For most springs (specifically, for those that are said to obey "Hooke's

Law"), the magnitude of the force is directly proportional to the amount of stretch

or compression of the spring.

Answers to T/F Understanding Forces

Answers:

Science: Forces for Fours 30 Glenda Leslie

Only 4, 6 and 9 are correct

Answers to the wagon investigation

Figure 1: Different forces exerted on the same mass produce different accelerations. (a) Two children push a wagon with a child in it. Arrows representing all external forces are shown. The system of interest is the wagon and

its rider. The weight w of the system and the support of the ground N are also shown for completeness and are

assumed to cancel. The vector f represents the friction acting on the wagon, and it acts to the left, opposing the

motion of the wagon. (b) All of the external forces acting on the system add together to produce a net force, Fnet.

The free-body diagram shows all of the forces acting on the system of interest. The dot represents the centre of mass of the system. Each force vector extends from this dot. Because there are two forces acting to the right, we

draw the vectors collinearly. (c) A larger net external force produces a larger acceleration (a′>a) when an adult

pushes the child.

Answers to the basketball player pushing a ball and a car Figure 2: The same force exerted on systems of different masses produces different accelerations. (a) A basketball player pushes on a basketball to make a pass. (The effect of gravity on the ball is ignored.) (b) The same player exerts an identical force on a stalled SUV and produces a far smaller acceleration (even if friction is negligible). (c) The free-body diagrams are identical, permitting direct comparison of the two situations. The outcome of applying the force is different for each object. From: http://cnx.org/content/m42073/latest/Figure%2004_03_02.jpg

Answer to Friction in Force Diagrams

Science: Forces for Fours 31 Glenda Leslie

Answer to Forces on a Car

The non-scientific feature: arrows of different widths are used.

This makes it difficult to compare the size of the forces eg. the weight force is supposed to equal the

reaction force. (reaction force is the Normal force and weight is due to the force of gravity)

This is NOT a good diagram to use with yr 4’s as it adds confusion to terminology and the representation of

forces.