P2.1 Resultant ForcesP2 Physics

KS4 ADDITIONAL SCIENCEMr D Powell

Mr Powell 2012Index

Connection

• Connect your learning to the content of the lesson

• Share the process by which the learning will actually take place

• Explore the outcomes of the learning, emphasising why this will be beneficial for the learner

Demonstration

• Use formative feedback – Assessment for Learning

• Vary the groupings within the classroom for the purpose of learning – individual; pair; group/team; friendship; teacher selected; single sex; mixed sex

• Offer different ways for the students to demonstrate their understanding

• Allow the students to “show off” their learning

Activation

• Construct problem-solving challenges for the students

• Use a multi-sensory approach – VAK• Promote a language of learning to

enable the students to talk about their progress or obstacles to it

• Learning as an active process, so the students aren’t passive receptors

Consolidation

• Structure active reflection on the lesson content and the process of learning

• Seek transfer between “subjects”• Review the learning from this lesson and

preview the learning for the next• Promote ways in which the students will

remember• A “news broadcast” approach to learning

Mr Powell 2012Index

What is the link?

Mr Powell 2012Index

P2.1.1 Resultant forces

a) Whenever two objects interact, the forces they exert on each other are equal and opposite.

b) A number of forces acting at a point may be replaced by a single force that has the same effect on the motion as the original forces all acting together. This singleforce is called the resultant force.

c) A resultant force acting on an object may cause a change in its state of rest or motion.

d) If the resultant force acting on a stationary object is: ■ zero, the object will remain stationary ■ not zero, the object will accelerate in the direction of the resultant force.

e) If the resultant force acting on a moving object is: ■ zero, the object will continue to move at the same speed and in the same

direction ■ not zero, the object will accelerate in the direction of the resultant force.

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Activate...

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Forces between objects..

Whenever two objects push or pull on each other, they exert equal and opposite forces on one another. The unit of force is the Newton (abbreviated N).

A boxer who punches an opponent with a force of 100 N experiences a reverse force of 100 N from his opponent on his / her fist.

Two roller skaters pull on opposite ends of a rope. The skaters move towards each other. This is because they pull on each other with equal and opposite forces.

1) Draw a stick diagram of the two people

2) Add forces arrows on it to show what is happening

3) Write a sentence to explain what happens?

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Quicksand – Harder Challenge

Quicksand victims sink because they can’t get enough support from the sand. The force of gravity on the victim (acting downwards) is greater than the upwards force of the sand on the victim.

1) Draw a stick diagram of a person stuck in the sand....

2) Add forces arrows on it to show what is happening...

3) Write a sentence to explain the why the person is sinking?

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Focus on a Tractor...

To pull the car out of the mud, the force of the ground on the tractor needs to be greater than the force of the mud on the car.

These two forces aren’t necessarily equal to one another because the objects are not the same.

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Forces on a ball

Force A Force B Size and direction of the resultant force

1 12 N (L) 7 N(R)

2 7N (R) 11 N (L)3 7N (L) 7 N (R)

4 12.4 N (L) 5 N (R)

RightLeft

Copy and complete the table below by working out the forces on a football.

Also add a diagram for each one numbered 1-4

You can see an example of what is meant in the picture. You can draw the arrows to scale 1N = 0.5cm

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Match the Forces

Make a sketch copy of the pictures then label them up correctly...

1

2 3

1. B2. C3. A

1. C2. B3. A

1. D2. C3. A4. B

3

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Summary Questions – Quick test

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Focus on planes....The acceleration of an object depends on the size and direction of the resultant force. When a jet plane is taking off, the thrust force of its engines is greater than the force of air resistance on it.

The resultant force on it is the difference between the thrust force and the force of air resistance on it. The resultant force is therefore non-zero. The greater the resultant force, the quicker the take-off is.

dragthrustRF

Task

Use this example to try and explain why the RF on a plane taking off would behave in the following way. Use diagrams to help for each one and assign forces.

1. On runway: RF=0N2. Halfway: RF=1000N3. Take off RF=3000N4. Flying constant speed = RF=0N

Thrust (N) Drag (N) Resultant (N)

0 0 0

3000 2000 1000

6000 3000 3000

8000 8000 0

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What is the link?

Mr Powell 2012Index

Zero Resultant Forces

Most objects around you are acted on by more than one force. We can work out the effect of the forces on an object by replacing them with a single force, the resultant force. This is a single force that has the same effect as all the forces acting on the object.

When the resultant force on an object is zero the object remains stationary if it was at rest, or continues to move at the same speed and in the same direction if it was already moving.

Use the tugboat to describe the idea of resultant forces acting on an object moving at a constant speed

50,000N

100,000N

150,000N

Drag

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Resultant Forces

40,000N

90,000N

150,000N

Drag

60,000N

120,000N

150,000N

Drag

Now use tugboat to describe the idea of resultant forces acting on an object to describe and acceleration and also deceleration...

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Fill in the blanks.

1. The picture shows a tug-of-war between the Blues and the Reds.

2. How much force does each team exert on the ground?

Blues: Reds:

3. What is the size and direction of the resultant force?

Resultant Forces

Mr Powell 2012Index

Summary Questions

a) less thanb) greater thanc) equal to

2a) it backwardsb) Is zero

a) Whenever two objects interact, the forces they exert on each other are equal and opposite.

b) A number of forces acting at a point may be replaced by a single force that has the same effect on the motion as the original forces all acting together. This singleforce is called the resultant force.

c) A resultant force acting on an object may cause a change in its state of rest or motion.

d) If the resultant force acting on a stationary object is:

■ zero, the object will remain stationary

■ not zero, the object will accelerate in the direction of the resultant force.

e) If the resultant force acting on a moving object is:

■ zero, the object will continue to move at the same speed and in the same direction

■ not zero, the object will accelerate in the direction of the resultant force.

Consolidate – How can you explain these things...