7.3 Newton’s third law of motion Action and reaction The wall exerts an equal but opposite force on your hand. uThe pair of forces is called the action-reaction pair. If you hit on the wall with hand, you will feel pain. Why?

7.3 Newton’s third law of motion Examples of action-reaction pairs in sports: the ball also exerts a force on his foot. an opposite force lifts him up. A football player exerts a force on the ball, A high jumper exerts a force on the pole,

7.3 Newton’s third law of motion the water exerts an opposite force on the paddle. an opposite force pushes the swimmer forward. The swimmer exerts a force on the platform, The paddle exerts a force on the water,

7.3 Newton’s third law of motion object A (football player) object B (football) Newton’s third law of motion uNewton’s third law of motion: To every action there is an equal and opposite reaction. uWhenever an object A exerts a force on another object B, B will exert an equal but opposite force on A.

7.3 Newton’s third law of motion the water exerts an opposite force. force acting on the rocket force acting on the water How water rockets work? When the air forces the water out, compressed air and water The rocket lifts up. Video: How water rockets work

7.3 Newton’s third law of motion forces exerted by the air on the wing forces exerted by the wing on the air forces exerted by the air on the rotor forces exerted by the rotor on the air How a bird lift up? How a helicopter lift up?

7.3 Newton’s third law of motion Identifying the action-reaction pair Consider a person standing on the ground… What are the forces acting on the person and on the ground? Which two forces form an action-reaction pair?

7.3 Newton’s third law of motion Action and reaction act on different objects. No gravitational force exerted on the person by the Earth mg supporting force exerted by the ground on the person R Do these two forces form an action-reaction pair?

7.3 Newton’s third law of motion Consider the force on the ground… R and R’ form an action-reaction pair, R R’ supporting force exerted by the ground on the person downward force exerted by the person on the ground but not R and the weight mg.

7.3 Newton’s third law of motion Apparent weight in a lift When you are travelling up in a lift… What do you feel when the lift just begins to move upwards? What do you feel when the lift is about to stop? heavier lighter

7.3 Newton’s third law of motion Imagine that you are standing on a weighing scale in a lift … uR gives you the feeling of your own weight. uR = the reading on the scale uNet force = R – mg Weight mg Supporting force R = ma

7.3 Newton’s third law of motion mg R = R – mg The lift moves upwards at an acceleration a a = 0 You feel heavier. R > mg The lift is at rest (take upward direction as positive) Net force = 0 R = mg mg R Net force = R – mg = ma a > 0

7.3 Newton’s third law of motion mg R = R – mg The lift moves upwards at a deceleration a a = 0 You feel lighter. R < mg The lift moves upwards at a constant velocity (take upward direction as positive) Net force = 0 R = mg mg R Net force = R – mg = ma a < 0 Simulation: Apparent weight in a lift

7.3 Newton’s third law of motion That’s the end of Section 7.3 Check Point Key Ideas Previous Page Exit