1. LAWS OF MOTION

2. Chapter Six: Laws of Motion  6.1 Newton’s First Law  6.2 Newton’s Second Law  6.3 Newton’s Third Law and Momentum

3. Chapter 6.1 Learning Goals  Describe how forces cause changes in motion.  Demonstrate and describe Newton’s first law.  Explain the meaning of net force.

4. 6.1 Force changes motion  A force is a push or pull, or any action that is able to change motion.

5. 6.1 Law of inertia  Newton’s first law says that objects continue the motion they already have unless they are acted on by a net force.  If the net force is zero, an object at rest will stay at rest.  If an object is acted upon by unbalanced forces, its motion will change.

6. 6.1 Net force  Newton’s first law is often written in terms of the net force:  “An object at rest will stay at rest and an object in motion will continue in motion at constant velocity UNLESS there is a net force.” According to these vectors, in what direction is the net force?

7. 6.1 Force changes motion  Forces can be used to increase or decrease the speed of an object, or to change the direction an object is moving.

8. 6.1 Law of inertia  Inertia is the property of an object that resists changes in motion.  Objects with more mass have more inertia and are more resistant to changes in their motion. Which ball has more inertia?

10. Chapter Six: Laws of Motion  6.1 Newton’s First Law  6.2 Newton’s Second Law  6.3 Newton’s Third Law and Momentum

11. Chapter 6.2 Learning Goals  Define Newton’s second law by relating force, mass, and acceleration.  Apply Newton’s second law quantitatively.  Describe the relationship between net force and acceleration.

12. Investigation 6A  Key Question:  What is the relationship between force and motion? Newton’s First and Second Laws

13. 6.2 Newton’s second law  Newton’s first law tells us that motion cannot change without a net force.  According to Newton’s second law, the amount of acceleration depends on both the force and the mass.

14. 6.2 The newton  The S.I. unit of force (newton) is defined by the second law.  A newton is the amount of force needed to accelerate a 1 kg object by 1m/s.

15. 6.2 Newton’s second law  There are three main ideas related to Newton’s Second Law: 1.Acceleration is the result of unbalanced forces. 2.A larger force makes a proportionally larger acceleration. 3.Acceleration is inversely proportional to mass.

16. 6.2 Newton’s second law  Unbalanced forces cause changes in speed, direction, or both.

18. 6.2 Acceleration and mass  The greater the mass, the smaller the acceleration for a given force.  This means acceleration is inversely proportional to mass.

19. 6.2 Acceleration, force and mass  The acceleration caused by a force is proportional to force and inversely proportional to mass.

21. 6.2 Applying the second law  Keep the following important ideas in mind: 1. The net force is what causes acceleration. 2. If there is no acceleration, the net force must be zero. 3. If there is acceleration, there must also be a net force. 4. The force unit of newtons is based on kilograms, meters, and seconds.

22.  A car has a mass of 1,000 kilograms. If a net force of 2,000 N is exerted on the car, what is its acceleration? 1.Looking for:  …car’s acceleration 2.Given  …mass = 1,000 kg; net force = 2,000 N 3.Relationships:  a = F / m 4.Solution:  2, 000 N ÷ 1,000 kg = 2 N/kg = 2 m/s 2 Solving Problems

23.  Your bicycle has a mass of 9.1 kilograms. You accelerate at a rate of 1.79 m/s 2. Calculate the net force that is accelerating the bicycle.  Calculate the acceleration of a car if the force on the car is 450 Newtons and the mass is 1300 kilograms. Solving Problems

24. Chapter Six: Laws of Motion  6.1 Newton’s First Law  6.2 Newton’s Second Law  6.3 Newton’s Third Law and Momentum

25. Chapter 6.3 Learning Goals  Describe action-reaction force pairs.  Explain what happens when objects collide in terms of Newton’s third law.  Apply the law of conservation of momentum when describing the motion of colliding objects.

26. Investigation 6B  Key Question: What happens when equal and opposite forces are exerted on a pair of Energy Cars? Newton’s Third Law

27. 6.3 Newton’s Third Law  Newton’s Third Law (action- reaction) applies when a force is placed on any object, such as a basketball.

29. 6.3 The Third Law: Action/Reaction  Newton’s Third Law states that every action force creates a reaction force that is equal in strength and opposite in direction.  There can never be a single force, alone, without its action- reaction partner.

30. 6.3 The Third Law: Action/Reaction  It doesn’t matter which force you call the action and which the reaction.  The forces do not cancel because we can only cancel forces acting on the same object. One force acts on the ball, and the other force acts on the hand.

31. 6.3 Action and reaction  When sorting out action and reaction forces it is helpful to examine or draw diagrams. Here the action force is on the ________________, and the reaction force is on the _______________.

33. 6.3 Collisions  Newton’s third law tells us that any time two objects hit each other, they exert equal and opposite forces on each other.  The effect of the force is not always the same.

34. 6.3 Momentum  Momentum is the mass of a object times its velocity.  The units for momentum are kilogram- meter per second (kg·m/s).

36. 6.3 Collisions  When a large truck hits a small car, the forces are equal.  The small car experiences a much greater change in velocity much more rapidly than the big truck. Which vehicle ends up with more damage?

37. Solving Problems  If an astronaut in space were to release a 2-kilogram wrench at a speed of 10 m/s, the astronaut would move backward at what speed?  The astronaut’s mass is 100 kilograms.

38. 1.Looking for:  … the velocity of the astronaut (backward) 2.Given  …velocity 1 = 10 m/s; mass 1 = 2 kg; ...mass 2 = 100 kg; 3.Relationships:  m 1 v 1 = m 2 v 2 4.Solution  (10 m/s)(2 kg) = (100kg) v 2  20(kg·m/s) = (100kg) v 2  0.2 m/s Solving Problems

39.  A rocket engine expels 875 kg of exhaust gasses at a speed of 3,200 m/s. As a result, the space ship increases its speed by 930 m/s. What is the mass of the spaceship?  A cannon (m = 550 kg) fires a cannon ball (m = 4.5 kg) at a speed of 420 m/s. What is the recoil speed of the cannon? Solving Problems