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Newton’s Laws of Motion

Learn about Newton's three laws of motion, including inertia, force and acceleration, and action-reaction pairs. Understand how these laws apply to everyday situations.

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Newton’s Laws of Motion

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  1. Newton’s Laws of Motion 1st Law of Inertia 2nd F=ma 3rd Action-Reaction 3.2 and 3.3 combined

  2. Review Vocabulary acceleration – rate of change of velocity a = vf – vi t SI Units acceleration – m/s2

  3. momentum – property of a moving object that equals its mass times its velocity

  4. Newton’s 1st Law of Motion • An object in motion tends to stay in motion and an object at rest tends to stay at rest unless acted upon by an unbalanced force. • also called Law of Inertia

  5. If objects in motion tend to stay in motion, why don’t moving objects keep moving forever? Things don’t keep moving forever because there’s almost always an unbalanced force acting upon them. A book sliding across a table slows down and stops because of the force of friction. If you throw a ball upwards it will eventually slow down and fall because of the force of gravity.

  6. What happens in a crash? Newton’s first law of motion can explain what happens in a car crash. According to Newton’s first law, the passengers will continue to travel at the same velocity that the car was moving unless a force acts on them.

  7. The crash dummies have inertia and resist changes in motion.Air bags and seat belts save lives.

  8. Newton’s Second Law of Motion Newton’s first law of motion states that the motion of an object changes only if an unbalanced force acts on the object. Newton’s second law of motion describes how the forces exerted on an object, its mass, and its acceleration are related. • Which ball player will throw the ball with more force? • More acceleration?

  9. Newton’s 2nd Law of Motion • Acceleration is produced when a force acts on a mass. • The greater the mass of the object being accelerated, the amount of force needed to accelerate the object. F = ma

  10. Or you can solve for net force: Fnet = ma

  11. Solve for Acceleration: You push a wagon that has a mass of 12 kg. If the net force on the wagon is 6 N south, what is the wagon’s acceleration? m = 12 kg • What is given? • What is unknown? • Which formula? • Set up problem and show work. • Does your final answer have the correct unit? Fnet = 6 N south wagon’s acceleration a = Fnet / m a = 6 N south 12 kg a = 0.5 m/s2

  12. If a helicopter's mass is 4,500 kg and the net force on it is 18,000 N upward, what is its acceleration? • What is the net force on a dragster with a mass of 900 kg if its acceleration is 32.0 m/s2 west? • A car pulled by a tow truck has an acceleration of 2.0 m/s2 east. What is the mass of the car if the net force on the car is 3,000 N east?

  13. If a helicopter's mass is 4,500 kg and the net force on it is 18,000 N upward, what is its acceleration?a = Fnet/m • a = 18,000 N upward/4,500 kg • a = 4.0 m/s2 upward • What is the net force on a dragster with a mass of 900 kg if its acceleration is 32.0 m/s2 west?Fnet = ma • Fnet = 900 kg × 32.0 m/s2 west • Fnet = 28,800 N west • A car pulled by a tow truck has an acceleration of 2.0 m/s2 east. What is the mass of the car if the net force on the car is 3,000 N east?m = Fnet/a • m = 3,000 N east / 2.0 m/s2 east • m = 1,500 kg

  14. Newton’s Third Law of Motion Newton’s third law of motion states that when one object exerts a force on a second object, the second object exerts a force on the first that is equal in strength and opposite in direction.

  15. Newton’s 3rd Law of Motion For every action there is an equal and opposite reaction. Book to earth Table to book

  16. What does this mean? For every force acting on an object, there is an equal force acting in the opposite direction. Right now, gravity is pulling you down in your seat, but Newton’s Third Law says your seat is pushing up against you with equal force. This is why you are not moving. There is a balanced force acting on you– gravity pulling down, your seat pushing up.

  17. Think about it . . . What happens if you are standing on a skateboard or a slippery floor and push against a wall? You slide in the opposite direction (away from the wall), because you pushed on the wall but the wall pushed back on you with equal and opposite force. Why does it hurt so much when you stub your toe? When your toe exerts a force on a rock, the rock exerts an equal force back on your toe. The harder you hit your toe against it, the more force the rock exerts back on your toe (and the more your toe hurts).

  18. Newton’s 3rd Law • A bug with a mass of 5 grams flies into the windshield of a moving 1000kg bus. • Which will have the most force? • The bug on the bus • The bus on the bug

  19. The force would be the same. • Force (bug) = m x A • Force (bus) = M x a Think I look bad? You should see the other guy!

  20. Action and Reaction on Different Masses Consider you and the earth Action: earth pulls on you Reaction: you pull on earth

  21. Reaction: road pushes on tire Action: tire pushes on road

  22. Reaction: gases push on rocket Action: rocket pushes on gases

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