Newtons 3rd Law of Motion.ppt
- 1. Newton’s 3rd Law For every action….. (The “Action Reaction Law”)
- 2. Action and Reaction • Newton’s third law describes something else that happens when one object exerts a force on another object. 3 Newton’s Third Law • According to Newton’s third law of motion, forces always act in equal but opposite pairs.
- 3. Action and Reaction • Another way of saying this *is for every action, there is an equal but opposite reaction!* 3 Newton’s Third Law • .
- 4. Action and Reaction Forces Don’t Cancel • The forces exerted by two objects on each other are often called and action-reaction force pair. 3 Newton’s Third Law • Action and reaction force pairs don’t cancel because they act on different objects.
- 5. • You constantly use action- reaction force pairs as you move about. 3 Newton’s Third Law • When you jump, you push down on the ground. • The ground then pushes up on you. It is this upward force that pushes you into the air. Action and Reaction Forces Don’t Cancel
- 6. Action and Reaction Forces Don’t Cancel • When a bird flies, its wings push air in a downward and a backward direction. 3 Newton’s Third Law • . • By Newton’s third law, the air resistance pushes back on the bird in the opposite directions—upward and forward. • This force keeps a bird in the air and propels it forward – an OPPOSITE BUT EQUAL FORCE! Action Reaction!!!
- 7. Large and Small Objects • When you walk forward, you push backward on the ground. 3 Newton’s Third Law • Your shoe pushes Earth backward, and Earth pushes your shoe forward.
- 8. Large and Small Objects…why don’t I always feel this “push” back?!? Earth has sooo much mass compared to you, that it does not move noticeably when you push it. 3 Newton’s Third Law • If you step on something that has less mass than you do, like a skateboard, you can see it being pushed back.
- 9. A Rocket Launch • (Don’t write) • When the rocket fuel is ignited, a hot gas is produced. 3 Newton’s Third Law • As the gas molecules collide with the inside engine walls, the walls exert a force that pushes them out of the bottom of the engine.
- 10. A Rocket Launch • (Don’t write) • This downward push is the action force. 3 Newton’s Third Law • The reaction force is the upward push on the rocket engine by the gas molecules. • This is the thrust that propels the rocket upward.
- 11. Weightlessness (Don’t write) • You might have seen pictures of astronauts floating inside a space shuttle as it orbits Earth. 3 Newton’s Third Law • The astronauts are said to be weightless. • Yet the force of gravity on the shuttle is almost 90 percent as large as at Earth’s surface. • Newton’s laws of motion can explain why the astronauts float as if there were no forces acting on them.
- 12. Measuring Weight (Don’t write) • When you stand on a scale, your weight pushes down on the scale. 3 Newton’s Third Law • This causes the scale pointer to point to your weight.
- 13. Measuring Weight (Don’t write) • At the same time, by Newton’s third law the scale pushes up on you with a force equal to your weight. 3 Newton’s Third Law • This force balances the downward pull of gravity on you.
- 14. Free Fall and Weightlessness • (Don’t write) Now suppose you were standing on a scale in an elevator that is falling. 3 Newton’s Third Law • A falling object is in free fall when the only force acting on the force is gravity. • You and the scale are both in free fall.
- 15. Free Fall and Weightlessness • (Don’t write) • Because the only force acting on you is gravity, the scale no longer is pushing up on you. 3 Newton’s Third Law • According to Newton’s third law, you no longer push down on the scale.
- 16. Free Fall and Weightlessness • (Don’t write) • So the scale pointer stays at zero and you seem to be weightless. 3 Newton’s Third Law • Weightlessness is the condition that occurs in free fall when the weight of an object seems to be zero.
- 17. This is known as the “Action-Reaction” Law
- 18. So far Newton has explained… • Speed… • Velocity… • Acceleration… • Force… • Newton also went on to explain the “quantity of motion” called MOMENTUM!
- 19. Momentum = Mass x Velocity • Objects with a lot of momentum, have a lot of motion. • They have a high amount of a push or pull (force) going on. • An objects momentum is dependent upon its mass and its velocity (speed). • Momentum = mass x velocity(units in kg x m/s)
- 20. • Objects with a greater mass have greater momentum. Objects with a greater velocity have greater momentum. And vise-versa for both concepts. • If you know the mass and velocity of two different objects, you can determine their momentum by using: Momentum = mass x velocity
- 21. Conservation of Momentum: • If someone bumps in to you from behind, you gain momentum & move forward. • When two objects collide, in the absence of friction, momentum is not lost. • The Law of Conservation of Momentum says that momentum does not change when objects interact, it remains the same before and after the interaction.
- 22. •The Law of Conservation of Momentum states that the total momentum of any group of objects remains the same unless an outside force acts on the objects!
- 23. Guided PrAcTiCe! • Which has more momentum, a 3 kg sledgehammer (a) swung at 1.5 m/s, or a 4 kg sledgehammer (b) swung at 0.9 m/s? • (a) 3 kg x 1.5 m/s = kg x m/s • (b) 4kg x 0.9 m/s = kg x m/s • A has more momentum than the heavier one, because it is swung with a greater velocity – almost twice as fast!
- 24. Independent PrAcTiCe! • 1. A golf ball travels at 16 m/s, while a baseball moves at 7 m/s. The mass of the golf ball is 0.045 kg and the mass of the baseball is 0.14 kg. Which had greater momentum? • Golf ball = = kg x m/s • Baseball = = kg x m/s • 2. What is the momentum of a bird with a mass of 0.018 kg flying at 15 m/s? • =