Newton’s Laws

1. Newton’s Laws Chapter 3 and 4

2. Newton’s First Law of Motion • An object moving at a constant velocity keeps moving at that velocity unless an unbalanced net force acts on it. • If an object is at rest it stays at rest unless an unbalanced net force acts on it. • Sometimes called the law of inertia. • Would a heavier or lighter object have more inertia?

3. Newton’s Second Law of Motion • The acceleration of an object is in the same direction as the net force on the object, and the acceleration can be calculated from this equation:

4. Newton’s Second Law of Motion • To find the net force: • The net force includes the resistance (example: friction) subtracted from the force exerted on the object • F = ma practice problems are on bottom of page 102 in text book

5. Newton’s Second Law of Motion • Amount of friction between the two surfaces depends on 2 factors: - the kinds of surfaces - the force pressing the surfaces together

6. Newton’s Second Law of Motion • As a skateboard slows down and comes to a stop there is a force that acts on it. • It is called friction. • Friction is the force that opposes the sliding motion of two surfaces that are touching each other.

7. Newton’s Second Law of Motion • Have you ever tried to push a box and were unable to move it? • This was due to static friction.

8. Newton’s Second Law of Motion • Static friction is the frictional force that prevents two surfaces from sliding past each other.

9. Newton’s Second Law of Motion • When an object falls to Earth it is pulled down by the force of gravity. • A friction-like force called air resistance opposes the motion of objects moving through the air.

10. Newton’s Second Law of Motion • Air resistance causes objects to fall with different accelerations and different speeds.

11. Newton’s Second Law of Motion • Air resistance acts in the opposite direction to the motion of the object through air. ↑ ↓ air object • The size of the air resistant force also depends on the size of the object.

12. Newton’s Second Law of Motion

13. Newton’s Second Law of Motion • When the upward force of air resistance equals the downward force of gravity the force of an object falls equals zero. • The acceleration of the object becomes zero and it falls at a constant speed. • This is known as terminal velocity.

14. Section 2Gravity • Gravity is an attractive force between any 2 objects. • It depends on the masses of the objects and the distance between them.

15. Section 2Gravity • Gravity is one of the 4 basic forces. • The other 3 are: 1. Electromagnetic force 2. Strong nuclear force 3. Weak nuclear force

16. Section 2Gravity • Issac Newton formulated the Law of Universal Gravitation in 1687. • It states that as the distance between 2 objects increases the gravitational forces between them decrease. It can never be 0.

17. Section 2Gravity • Example: Observed motion of Uranus was slightly off from what it was calculated to be, in the 1840’s. • Astronomer’s speculated that another undiscovered planet was affecting its motion. • After using the law of universal gravitation, astronomers calculated the orbit of the unknown planet.

18. Section 2Gravity • As a result, Neptune was found in 1846.

19. Section 2Gravity • Weight and mass are not the same • Why? • They are related. Weight increases as mass increases.

20. Section 2Gravity • The weight of an object can change, depending on the gravitational force of the object.

21. Section 2Gravity • What is weightlessness? • When does it occur? • Example: If you stand on a scale your weight can be calculated because there is a force pushing upward which balances your weight. You have a zero net force. • However, if you stand on the scale in an elevator that is falling you would no longer be pushing down on the scale.

22. Section 2Gravity • So the scale would show that you have a weight of zero even though the force of gravity on you has not changed. vs.

23. Section 2Gravity • The space shuttle uses the same concept. • It falls toward the Earth at the same acceleration as everything around it so it seems as if everything is floating.

24. Newton’s 3rd Law • When one object exerts a force on a second object, the second one exerts a force on the first that is equal in strength, but opposite in direction.

25. Newton’s 3rd Law • Example: When you jump on a trampoline you exert a downward force and the trampoline exerts an equal force upwards. • This is what sends you high into the air.

26. Newton’s 3rd Law • The forces are equal, but they are not balanced because they act on different objects. • Example: A swimmer “acts” on the water and the reaction of the water pushes the swimmer forward. • So an unbalanced force acts on the swimmer to move them.

27. Newton’s 3rd Law • A moving object has a property called momentum. • This is related to how much force is needed to change its motion. • Momentum is the product of an objects mass and velocity.

28. Newton’s 3rd Law • The momentum of an object does not change unless its mass and/or velocity changes. • The law of conservation of momentum states that if a group of objects exert forces on only each other, then their total momentum does not change.

29. Newton’s 3rd Law • Example: Hockey pucks • When the first hockey puck hits the second hockey puck, that is going in the same direction, it gives the first hockey puck momentum in the same direction.

30. Newton’s 3rd Law • If the hockey pucks move toward each other with the same speed, what would the momentum be?