1. Part of an assault course consists of a taut cable 25 metres long fixed at 35˚ to the horizontal. A light rope ring is placed round the cable at its upper end. A soldier of mass 80 kg grabs hold of the ring and slides down the cable. If the coefficient of friction between the ring and the cable is 0.4, find how fast the soldier is moving when he reaches the bottom.

3. Newton’s Third Law of Motion
To know Newton’s 3rd Law.
Understand how to apply Newton’s 2nd and 3rd Laws to connected particle problems.
To use the strategy of treating a system as a whole and considering individual the parts.

4. Action and reaction are opposite and equal
Newton’s Third Law of Motion
I’ve just had yet another thought
Action and
reaction
are opposite
and equal

5. The bullet flies this way
Action and Reaction
The bullet flies this way
The cowboy recoils backwards
Bird goes up
Air pushed down
Normal Reaction
Weight

6. Definition
If an object A exerts a force on an object B, then B exerts a force on A with the same magnitude but in the opposite direction.

7. The forces in the coupling are equal and opposite.
Action and Reaction Problems
The forces in the coupling are equal and opposite.
The towing force felt by the caravan is the same force acting backwards on the car.

8. Task: draw a force diagram for a car and trailer.

9. Tension an internal or an external force?
If you consider the situation as a whole the forces of interaction (tension) are not included in the equation (in this situation they are called internal forces).
For the separate parts, the forces on each other are external forces and are included in the calculations.

10. Action and Reaction Problem
a ms-2
The car has a driving force of 8000N.
Forces of resistance are 200 N on the car and 100N on the caravan.
What is the acceleration and tension in the coupling?
500 kg
1500 kg
The car has a driving force of 8000N. Forces of resistance are 200 N on the car and 100N on the caravan.
What is the acceleration and tension in the coupling?

11. Task In pairs/threes complete cars and trailers worksheet.
Complete the force diagrams for each.
Find the three equations for each.
Use these to fill in the missing information.
Also find an acceleration for each situation.
If you finish try out the exam question

12. Plenary/Extension
Three barges travel down a river in line. Only the rear barge has an engine, which produces a forward force of 400kN. The masses of the front, middle and rear barges are 1600 tonnes, 1400 tonnes and 2000 tonnes, and the water exerts resistances of 100kN, 20kN and 30kN respectively. Find the forces in the couplings joining the barges.