12/04/23

Junior Cycle Junior Cycle PhysicsPhysics

Forces and MotionForces and Motion

Edited and Reproduced by

Theresa Lowry-Lehnen

RGN, BSc (Hon’s) Specialist Clinical Practitioner (Nursing), Dip Counselling, Dip Adv Psychotherapy, BSc (Hon’s) Clinical Science, PGCE (QTS) , H. Dip. Ed, MEd, Emotional Intelligence (Level 9) MHS

Accredited

12/04/23MomentsMomentsA moment is a “turning force”, e.g. trying to open or close a door or using a spanner. The size of the moment is given by:

Moment (in Nm) = force (in N) x distance from pivot (in m)

Calculate the following turning moments:

100 Newtons

5 metres

200 Newtons

2 metres

12/04/23Balancing momentsBalancing moments

100 Newtons

200 Newtons

2 metres 2 metres

Total ANTI-CLOCKWISE turning moment = 200x2

= 400Nm

Total CLOCKWISE turning moment = 100x2 =

200Nm

The anti-clockwise moment is bigger so the seesaw will turn anti-clockwise

12/04/23Balanced or unbalanced?Balanced or unbalanced?

12/04/23

StabilityStability

1. Centre of mass is within the wheelbase – no problem!

2. Centre of mass is directly above the edge of the wheelbase –car is on the point of toppling

3. Car falls over

12/04/23

Centripetal forceCentripetal forceConsider a ball of Pleistocene attached to some string:

The ball is kept in its path by the tension in the string – an example of a CENTRIPETAL FORCE. This force also produces the change in velocity due to the direction constantly changing.

This force is INCREASED if you increase the mass of the object, its speed or decrease the radius of the circle.

Other examples of centripetal

forces: Orbits Electrons

12/04/23

MomentumMomentumAny object that has both mass and velocity has MOMENTUM. Momentum (symbol “p”) is simply given by the formula:

Momentum = Mass x Velocity (in kgm/s) (in kg) (in m/s)

P

VM

What is the momentum of the following?

1) A 1kg football travelling at 10m/s

2) A 1000kg Ford Capri travelling at 30m/s

3) A 20g pen being thrown across the room at 5m/s

4) A 70kg bungi-jumper falling at 40m/s

12/04/23

Conservation of MomentumConservation of MomentumIn any collision or explosion momentum is conserved (provided that there are no external forces have an effect). Example question:

Two cars are racing around Teville Gate. Car A collides with the back of car B and the cars stick together. What speed do they move at after the collision?

Mass = 1000kg

Mass = 800kg

Speed = 50m/s Speed = 20m/s

Momentum before = momentum after…

…so 1000 x 50 + 800 x 20 = 1800 x V…

…V = 36.7m/s

Mass = 1800kg

Speed = ??m/s

12/04/23

More questions…More questions…

1) A white snooker ball moving at 5m/s strikes a red ball and pots it. Both balls have the same mass. If the white ball continued in the same direction at 2m/s what was the velocity of the red ball?

2) A car of mass 1000kg heading up the M1 at 50m/s collides with a stationary truck of mass 8000kg and sticks to it. What velocity does the wreckage move forward at?

3) A defender running away from a goalkeeper at 5m/s is hit in the back of his head by the goal kick. The ball stops dead and the player’s speed increases to 5.5m/s. If the ball had a mass of 500g and the player had a mass of 70kg how fast was the ball moving?

4) A gun has a recoil speed of 2m/s when firing. If the gun has a mass of 2kg and the bullet has a mass of 10g what speed does the bullet come out at? (Hint: total momentum before is zero and the gun has a negative velocity afterwards)

12/04/23

Two lorries are travelling in the same direction along a motorway.

1) Calculate the momentum of Lorry A as it travels along the motorway.

2) Calculate the momentum of Lorry B as it travels along the motorway.

(3 marks)

3) Lorry B collides with Lorry A and they stick together. Calculate the common speed of the lorries immediately after the collision.

(3 marks)

Lorry A

Mass = 20,000kg

Speed = 14m/s

Lorry B

Mass = 30,000kg

Speed = 20m/s

12/04/23

Newton’s Laws of MotionNewton’s Laws of Motion

1) If an unbalanced force acts on an object that object will either accelerate or change direction:

2) That force is given by F=ma

3) When a force acts on an object there is an equal force acting in the opposite direction (“Action and reaction are equal and opposite”)

F

AM

These are my three laws of motion (summarised):

12/04/23

Newton’s 2Newton’s 2ndnd Law LawInstead of F=ma Newton actually said that the force acting on an object is that object’s rate of change of momentum. In other words…

mv

TF

Force = Change in momentum Time(in N)

(in kgm/s)

(in s)

For example, David Beckham takes a free kick by kicking a stationary football with a force of 40N. If the ball has a mass of 0.5kg and his foot is in contact with the ball for 0.1s calculate:

1) The change in momentum of the ball,

2) The speed the ball moves away with

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Example questionsExample questions1) A golfer strikes a golf ball with a force of 80N. If the

ball has a mass of 200g and the club is in contact with it for 0.2s calculate a) the change in momentum of the golf ball, b) its speed.

2) A tennis player strikes a serve with a force of 30N. If the ball has a mass of 250g and the racket is in contact with it for 0.15s calculate the ball’s change in momentum and its speed.

3) A footballer takes a goal kick by kicking a 0.4kg football away at 10m/s. If his foot was in contact with the ball for 0.1 seconds calculate the force he applied to the ball.

4) A golfer strikes a 200g golf ball away at 50m/s. If he applied a force of 50N calculate how long his club was in contact with the ball for.

12/04/23

Energy loss in collisionsEnergy loss in collisions

We’ve also said that in a collision momentum is conserved (unless an external force acts). The same cannot usually be said for kinetic energy…For example, consider the following collision. How much kinetic energy is lost?

Mass = 1000kg

Mass = 800kg

Speed = 50m/s Speed = 20m/s

Kinetic energy = ½ x mass x velocity squared

in J in kg in m/s

In the “Forces” module we looked at how to calculate an object’s kinetic energy:

Mass = 1000kg

Mass = 800kgSpeed = 20m/s Speed = 30m/s

Before

After

12/04/23

Energy loss in collisionsEnergy loss in collisionsConsider a head-on collision where the cars stick together. How much kinetic energy is lost in this example? Where does all the energy go?

In this example more kinetic energy was lost. We say it was a “less elastic collision”. An “elastic collision” is one where the kinetic energy is conserved.

Speed = 50m/s Speed = 30m/s

Speed = 10m/s

Before

After

12/04/23

The Structure of the EarthThe Structure of the EarthA thin crust - 10-100km thick

A mantle – has the properties of a solid but it can also flow

A core – made of molten nickel and iron. Outer part is liquid and inner part is solidThe average density of the Earth is much higher

than the crust, so the inner core must be very dense

12/04/23

Tectonic theoryTectonic theoryPeople once thought that the oceans and the continents were formed by shrinkage from when the Earth cooled down after being formed.

Alfred Wegener proposed something different. Consider Africa and South America:

These continents look like they “fit”

together. They also have similar rock

patterns and fossil records. These two

pieces of evidence led me to believe that there was once a single land mass.

This is my TECTONIC THEORY.

12/04/23

The Answer:

1) Scientists discovered 50 years later that the Earth generates massive amounts of heat through radioactive decay in the core. This heat generated convection currents in the mantle causing the crust to move

2) We also now know that the sea floor is spreading outwards from plate boundaries

Tectonic theoryTectonic theoryThe Evidence:

1) Some continents look like they used to “fit” together

2) Similar rock patterns and fossil records

The Problems:

Wegener couldn't explain how continental drift happened so nobody believed him

Conclusion – scientists now believe Wegener’s Tectonic Theory

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Tectonic platesTectonic plates

12/04/23

Movements of the crustMovements of the crustWhen the lithosphere (“crust”) moves three things can happen:

1) Plates move past each other, causing earthquakes

2) Plates move away from each other – a “constructive plate margin”. The gap is filled with magma which cools to form basalt. This is called sea floor spreading.

3) Plates move towards each other – a “destructive plate margin”. The thinner one slides underneath (“subduction”) and partially melts. This causes volcanoes and earthquakes.

12/04/23Evidence for sea floor Evidence for sea floor spreadingspreading

Since the Earth was formed the north and south poles have periodically “________ ____”. When tectonic plates move apart and _____ fills the gap the iron particles in the magma orientate themselves in line with the Earth’s ________ field. This means that the rock formed on the sea floor contains a “magnetic __________” of the changing field:

These magnetic patterns can be used to prove that sea floor spreading does happen, and at a rate of about 2cm per _____.

Words: impression, magma, swapped over, magnetic, year