1. Friction 4/26/2017 Learning Outcome: Starter:
Explain how the type of surface affects friction
Explain how forces affect the motion of an object
Starter:
Can you think of a situation where……..
There is lots of friction
There is only a little friction
Where friction can be useful
Where friction is not useful
Friction

2. Wednesday, 26 April 2017Wednesday, 26 April 2017

4. Wednesday, 26 April 2017Wednesday, 26 April 2017

8. Friction and motion
Friction always tries to slow moving objects down – it opposes motion. Frictional forces occur when two touching surfaces move past each other, in this case the box moving across the ground.
pull of rope on box
friction
Friction also occurs when things move through air. This is called air resistance.

9. What are the sources of friction?
Label all sources of friction acting on this bike.
pedal
bearing
air
resistance
links in
chain
brake pad
tyre and
road
Photo credit: © 2008 Jupiterimages Corporation
wheel
bearing

10. Friction experiment

11. Cars stopping
Friction is a very important force for the movement of cars. It acts in the opposite direction to the movement of the car.
thrust from engine
friction
The time it takes for a car to brake is affected by the frictional forces between the car’s tyres and the road surface.

12. Effects of frictional forces

13. How does a plane take-off?

14. Forces in water What forces are acting on the fish? upthrust
Upthrust is a force that pushes objects upwards in a liquid or a gas. This balances the weight of the fish, and is also known as buoyancy.
thrust
friction
weight
In order to move forward, the fish uses its tail to produce thrust. This force drives it through the water and is always opposed by friction.

15. Representing Forces A force is a vector quantity.
It always has a size (magnitude) and direction.
We represent the force using an arrow
the length of the arrow represents the size of the force
the direction of the arrow represents the direction of the force.

16. Tug of War Objects can have more than one force acting on them
These forces can act in different directions
The forces can be balanced or unbalanced
Tug of War
Tug of war demo.

17. What could A car be doing?
Accelerating
Decelerating
Constant speed
Stationary
States of motion

18. Balanced and unbalanced forces –
If the forces on an object are balanced:
and the object is stationary, it will remain stationary
and the object is moving, it will continue to move at the same speed and in a straight line.
In other words, the object will continue to do what it is already doing without any change.
If the forces are unbalanced, two things can happen:
The speed can change -
This is called acceleration or decceleration.
The direction of motion can change.

19. Accelerating
Constant speed
Decelerating

20. In what other state of motion would the forces be balanced?

21. Resultant force
The sum effect of more than one force is called the resultant force.
The resultant force is calculated by working out the difference between opposing forces in each direction.
What is the resultant force on this truck?
A resultant force of 100 N is accelerating the truck.
400 N
500 N

22. Force pairs Teacher Notes
Encourage pupils to predict the outcome of each simulation.

23. Balanced and unbalanced forces
Imagine a car travelling at a constant speed of 50 km/h.
The engine provides sufficient force to balance all the frictional forces that are acting to decrease the speed.
500 N
500 N

24. Resultant forces – question 1
1. What is the resultant force on the satellite?
5 N 5 N
Resultant force = 20 N – 10 N = 10 N down
The satellite will accelerate downwards.
20 N

25. Resultant forces – question 2
2. What is the resultant force on the bird?
The forces acting in each direction horizontally are equal in size, so there is no resultant force in this direction.
Resultant force = 5 N – 5 N = 0 N
The vertical forces are not balanced, the bird will accelerate in a downwards direction.
5 N
5 N
Resultant force = 5 N – 0 N = 5 N down
5 N

26. Resultant forces – question 3
3. What is the resultant force on the yacht?
10 N
10 N
13 N
20 N
10 N
The vertical forces are equal in size and opposite in direction so there is no resultant force in the vertical direction.
The horizontal forces are not balanced, so the yacht will accelerate to the right.
Resultant force = (20 N +10 N) – 13 N
= 17 N right

27. Upthrust quiz

28. Forces – summary 28