Chapter Five: Forces

Ø 5.1 Forces

Ø 5.2 Friction

Ø 5.3 Forces and Equilibrium

5.1 The cause of forces Ø A force is a push or pull, or an action that has the ability to change motion.

Ø Forces can increase or decrease the speed of a moving object.

Ø Forces can also change the direction in which an object is moving.

5.1 How are forces created? Ø Forces are created in many ways.

Ø For example, your muscles create force when you swing a baseball bat.

Four Elemental Forces

Ø All forces in the universe come from only four basic forces.

Ø Electromagnetic forces are important to technology.

Ø Gravity is a universal force.

5.1 Units of force

Ø The pound is a unit of force commonly used in the United States.

Ø For smaller amounts, pounds are divided into ounces (oz.).

Ø There are 16 ounces in 1 pound.

5.1 Pounds

Ø When you measure weight in pounds on a postal scale, you are measuring the force of gravity acting on an object.

5.1 Newtons

Ø Although we use pounds all the time in our everyday life, scientists prefer to measure forces in newtons.

Ø The newton (N) is a metric unit of force.

5.1 Unit conversions

Ø The newton (N) is a smaller unit of force than the pound (lb).

Ø If one pound of force equals 4.448 newtons, then a 100 lb person weighs 444.8 newtons.

5.1 The force vector

Ø The direction of a force makes a big difference in what the force does.

Ø That means force is a vector, like velocity or position.

Ø Arrows are often used to show the direction of forces in diagrams.

5.1 Drawing a force vector Ø The arrow points in the direction of the force.

Drawingvectors

Ø The x- and y-axes show the strength of the force in the x and y directions.

Ø When drawing a force vector to show its strength, you must also choose a scale.

Can you draw the x-axis vector?

5.1 How forces act Ø One way forces act is the result of direct contact.

Ø A contact force is transmitted by matter directly touching other matter such as wind acting to slow a parachute.

5.1 How forces act Ø The force of gravity between Earth and Moon appears to be what people once called “action-at-a-distance”.

Ø Today we know that the gravitational force is carried from the Earth to the Moon by a force field.

Classify these forces as contact forces or the result of force fields.

5.1 Contact forces from ropes and springs

Ø Ropes and springs are often used to make and apply forces.

Ø Ropes are used to transfer forces or change their direction.

Ø The pulling force carried by a rope is called tension.

Ø Tension always acts along the direction of the rope.

5.1 Spring forces Ø Springs are used to make or control forces.

Ø The force from a spring always acts to return the spring to its resting shape.

Which of these springs is designed to be stretched? Which is designed to be compressed?

5.1 Spring forces Ø The force created by a

spring is proportional to the ratio of the extended or compressed length divided by the original (resting) length.

Ø If you stretch a spring twice as much, it makes a force that is twice as strong.

5.1 Gravity Ø  The force of gravity on an object is called weight.

Ø At Earth’s surface, gravity exerts a force of 9.8 N on every kilogram of mass.

5.1 Weight vs. mass Ø Weight and mass are not the same.

Ø Mass is a fundamental property of matter measured in kilograms (kg).

Ø Weight is a force measured in newtons (N).

Ø Weight depends on mass and gravity.

Weight depends on mass and gravity

A 10-kilogram rock has the same mass no matter where it is in the universe. On Earth, the10 kg. rock weighs 98 N.. On the moon, the same rock only weighs 16 N.

5.1 Calculating weight

5.1 Calculating weight Ø The weight equation can be rearranged into

three forms to calculate weight, mass, or the strength of gravity.

Ø  Calculate the weight of a 60-kilogram person (in newtons) on Earth and on Mars.

1.   Looking for: Ø  …weight of person in newtons on both planets Ø  Given: Ø  …mass = 60 kg; g = 3.7 N/kg on Mars; Ø  …implied g = 9.8 N/kg on Earth

2.   Relationships: Ø  W = m x g

3.   Solution: Ø  60 kg x 9.8 N/kg = 588 N Ø  60 kg x 3.7 N/kg = 222 N

Sig. fig. = 600 N

Solving Problems

Sig. fig. = 200 N

5.2 Friction

Ø  Friction is a force that resists the motion of objects or surfaces.

Ø  Many kinds of friction exist.

5.2 Friction

5.2 Friction and two surfaces Ø  Friction depends on both of the surfaces

in contact.

Ø  When the hockey puck slides on ice, a thin layer of water between the rubber and the ice allows the puck to slide easily.

5.2 Identifying friction forces

Ø  Friction is a force, measured in newtons just like any other force.

Ø  Static friction keeps an object at rest from moving.

Ø  Sliding friction is a force that resists the motion of an object moving across a surface.

5.2 Identifying friction forces

5.2 A model for friction Ø  Friction depends on a

material’s properties such as roughness, how clean the surfaces are, and other factors.

Ø  The greater the force squeezing two surfaces together, the greater the friction force.

5.2 Reducing the force of friction Ø  Unless a force is

constantly applied, friction will slow all motion to a stop eventually.

Ø  It is impossible to completely get rid of friction, but it can be reduced.

5.2 Reducing the force of friction Ø  The friction between a

shaft (the long pole in the picture) and an outer part of a machine produces a lot of heat.

Ø  Friction can be reduced by placing ball bearings between the shaft and the outer part.

5.2 Using friction Ø  Friction is also

important to anyone driving a car.

Ø  Grooved tire treads allow space for water to be channeled away from the road-tire contact point, allowing for more friction in wet conditions.

5.2 Using friction Ø  Shoes are designed

to increase the friction between their soles and the ground.

How do you think these shoes increase friction?

5.2 Friction and energy

Ø  Friction changes energy of motion into heat energy.

5.2 Friction and energy Ø  Friction is always

present in any machine with moving parts.

Ø  If the machine is small, or the forces are low, the amount of heat produced by friction may also be small.

5.2 Friction and energy Ø  Each time two

moving surfaces touch each other, tiny bits of material are broken off by friction.

Ø  Breaking off bits of material uses energy.

Chapter 5.3 Learning Goals

Ø Determine the net force acting on an object.

Ø Define equilibrium.

Ø Draw free-body diagrams to represent all forces acting on a body.

5.3 Forces and Equilibrium Ø  The sum of all the forces on an object is

called the net force.

Ø  The word net means total but also means the direction of the forces has been taken into account.

In what direction will this plane go?

5.3 Adding forces Ø  To figure out if or how an object

will move, we look at ALL of the forces acting on it.

Ø  Four forces act on a plane: 1.   weight 2.   drag (air friction) 3.   the thrust of the engines, and 4.   the lift force caused by the flow of air

over the wings.

5.3 Equilibrium When several forces

act on the same object:

1.   The net force is zero, or

2.   The net force is NOT zero.

5.3 Normal forces Ø  When the forces

are balanced, the net force is zero.

Ø  When the net force on an object is zero, we say the object is in equilibrium.

5.3 Equilibrium and normal forces Ø  A normal force is

created whenever an object is in contact with a surface.

Ø  The normal force has equal strength to the force pressing the object into the surface, which is often the object’s weight.

The normal force is sometimes called the support force.

5.3 The free body diagram Ø  How do you keep

track of many forces with different directions?

Ø  Draw a free-body diagram that contains the objects, like a book on a table.

5.3 Solving equilibrium problems Ø  For an object to be in equilibrium,

all the forces acting on the object must add up to zero.

Is this object in equilibrium?

Solving Problems

Two chains are used to support a small boat weighing 1,500 newtons.

One chain has a tension of 600 newtons.

What is the force exerted by the other chain?

1.   Looking for: Ø  …tension on chain 2

2.   Given Ø  …weightboat = 1,200N; tension1 = 600 N

Ø  Implied: weight and tension are forces

3.   Relationships: Ø  Net force on boat = zero

Solving Problems

4.   Solution: Ø  Draw free body

diagram

Solving Problems

Ø  Upward force of chains = weight of boat Ø  600 N + tension2 = 1,200 N Ø  tension2 = 900 N