6.6 Action and reaction: Newton’s third law Hovercraft Paired forces Action-and-reaction pairs Check-point 7 1 2 ? 6.6 Action and reaction: Newton's third law

Hovercraft Moves by floating on a cushion of air. Move quickly on both land and water. Contains 3 fans: Fan A (blows downwards) Fans B and C (blow backwards or forwards) 6.6 Action and reaction: Newton's third law

Hovercraft Discussion: How would you control the fan(s) to make the hovercraft (a) go forwards? Fans B and C should blow backwards. (b) turn left? Fan B blows backwards (and fan C blows forwards). 6.6 Action and reaction: Newton's third law

Hovercraft Video Simulation 6.6 Action and reaction: Newton's third law

Experiment 6h Paired forces 1 Place a car on the card laid on a layer of polystyrene. Push and observe what happens. The car moves forwards while the card moves backwards. 6.6 Action and reaction: Newton's third law

Experiment 6h Paired forces 2 Pull the rope (a) in turns and (b) at the same time. Observe what happens. Both students move towards each other. 6.6 Action and reaction: Newton's third law

Experiment 6h Paired forces 3 Switch on the fan cart. Observe the direction of air flown and the cart moved. The fan cart moves in direction opposite to that of air flow. 6.6 Action and reaction: Newton's third law

Experiment 6h Paired forces 4 Fill a water rocket and pump air into it. Pull the trigger and observe what happens. The water rocket is lifted up when released. 6.6 Action and reaction: Newton's third law

Experiment 6h Video Video Video Video Paired forces 6.6 Action and reaction: Newton's third law

1 Paired forces Expt. 6h shows that forces always occur in pairs. If a student pulls the rope, both students move towards one another. A B A exerts force on B B exerts force on A 6.6 Action and reaction: Newton's third law

Experiment 6i 1 Set up the apparatus and place sensors on trolleys. Newton’s third law 1 Set up the apparatus and place sensors on trolleys. 2 Push the trolleys and drop down the graph. 6.6 Action and reaction: Newton's third law

Experiment 6i The graph obtained is… Newton’s third law The graph obtained is… Their magnitudes are always equal and in opposite direction. 6.6 Action and reaction: Newton's third law

Experiment 6i Video Newton’s third law 6.6 Action and reaction: Newton's third law

1 Paired forces Many other examples show that if object A exerts a force on object B, object B exerts an equal but opposite force on A. 6.6 Action and reaction: Newton's third law

to every action there is an equal & opposite reaction. 1 Paired forces This is summed up by Newton’s third law of motion which states that... to every action there is an equal & opposite reaction. It applies to two objects interacting with one another. It does not apply to one object by itself. 6.6 Action and reaction: Newton's third law

2 Action-and-reaction pairs We often get confused when identifying action-and-reaction pairs. earth exerts force on person (weight) Both act on boy  not an action-and-reaction pair chair exerts force on person 6.6 Action and reaction: Newton's third law

2 Action-and-reaction pairs Which forces acting on/acted on the boy form an action-and-reaction pair? action-and-reaction must involve 2 bodies acting on each other the Earth exerts force (pull) on boy — weight chair exerts force on boy boy exerts force (pull) on the Earth boy exerts force chair 6.6 Action and reaction: Newton's third law

2 Action-and-reaction pairs 6.6 Action and reaction: Newton's third law

Check-point 7 Q1 Draw and label the action-and-reaction pair that causes the motion in each of the following cases. (a) The car moves on the card. force on car by card force on card by car 6.6 Action and reaction: Newton's third law

(b) Student A and B move when they pull the string. Check-point 7 – Q1 (b) Student A and B move when they pull the string. force on A by string force on B by string force on string by A force on string by B 6.6 Action and reaction: Newton's third law

(c) The fan cart moves when the fan is on. Check-point 7 – Q1 (c) The fan cart moves when the fan is on. force on fan by air force on air by fan 6.6 Action and reaction: Newton's third law

(d) The launch of water rocket. Check-point 7 – Q1 (d) The launch of water rocket. force on rocket by water force on water by rocket 6.6 Action and reaction: Newton's third law

The End 6.6 Action and reaction: Newton's third law

Example 15 The motion of a fan cart is ‘stolen’ by a sail! A fan cart is switched on, blowing air in the forward direction. (a) When the cart is placed on the bench, it moves backwards. Explain why. Video 6.6 Action and reaction: Newton's third law

Example 15 By Newton's 3rd law, force on air by the fan F The motion of a fan cart is ‘stolen’ by a sail! By Newton's 3rd law, force on air by the fan F air exerts an equal and opposite force F’ on the fan This pushes the cart backwards. 6.6 Action and reaction: Newton's third law

Example 15 The motion of a fan cart is ‘stolen’ by a sail! (b) When a ‘sail’ is mounted on the cart as shown, the cart does not move. Explain why. sail 6.6 Action and reaction: Newton's third law

Example 15 sail force on the card by air F’’ The motion of a fan cart is ‘stolen’ by a sail! sail force on the card by air F’’ force on air by the card F’’’ net force on cart = 0  it does not move 6.6 Action and reaction: Newton's third law

Return 6.6 Action and reaction: Newton's third law