1. Motion

2. Unit 1: Motion Chapter 4: Conservation Laws
4.1 Newton’s Third Law and Momentum
4.2 Energy and the Conservation of Energy
4.3 Collisions

3. 4.1 Investigation: Conservation of Energy
Key Question:
What happens when equal and opposite forces are exerted on a pair of  Energy Cars?
Objectives:
Explain the meaning of action–reaction forces.
Apply knowledge of Newton’s first and second laws to explain the resulting force when objects experience equal and opposite forces.
Use different numbers of marbles in each Energy Car to see how motion is affected.

4. Newton’s Third Law
Newton’s Third Law (action- reaction) applies when a force is placed on any object, such as a book on a table.
The second law says the acceleration of an object is directly proportional to force and inversely proportional to the mass (a = F ÷ m).

5. Newton’s Third Law Forces always come in pairs.
Since the action and reaction forces are equal in strength, Newton accelerates more because his mass is smaller.

6. 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.

7. 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 _______________.

8. Action and reaction forces don’t cancel
The forces do not cancel because they act on different objects.

10. Action and reaction
A woman with a weight of 500 newtons is sitting on a chair. Describe one action-reaction pair of forces in this situation.
Looking for: You are asked for a pair of action and reaction forces.
Given: You are given Fw = 500 N.
Relationships: Action-reaction forces are equal and opposite, and act on different objects.
Solution: The downward contact force the woman exerts on the chair is an action. This action force happens to equal her weight, which is 500 N. The reaction force is the chair acting on the woman with an upward force of 500 N.
Fw = -500 N
Fc = 500 N

11. Momentum Momentum is the mass of a object times its velocity.
The units for momentum are kilogram•meters per second (kg·m/s).

12. Momentum
Stopping a fast-moving object is harder than stopping a slow-moving one.

13. Momentum
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.

14. Calculating momentum
Momentum is calculated with velocity instead of speed because the direction of momentum is always important.

15. Impulse The change in momentum is often referred to as impulse.
We can use this relationship to solve equations using changes in mass or velocity.

16. Impulse
The tennis ball’s momentum changes after it is hit by the racquet with a force of 80 N for 0.1 seconds.
Both the change of momentum and the impulse are +8 N·s.

17. Force and momentum
A net force of 100 N is applied for 5 seconds to a 10-kg car that is initially at rest. What is the speed of the car at the end of the 5 s?
Looking for: … the speed.
Given: … the net force (100N), the time the force acts (5s), and the mass of the car (10 kg).
Relationships: Use: impulse (Δp) = force (F) × time (t)
momentum = mass (m) × velocity (v)
Solution: Δp = 100 N × 5 s = 500 kg·m/s
Velocity is momentum divided by mass, or
v = (500 kg·m/s) ÷ 10 kg = 50 m/s

18. Momentum and the third law
If we combine Newton’s third law with the relationship between force and momentum, the result is a powerful new tool for understanding motion.
If you stand on a skateboard and throw a 5 kg ball, with a velocity of 4 m/s, you apply a force to the ball.
That force changes the momentum of the ball.
If the ball gains +20 kg·m/s of forward momentum, you must gain –20 kg·m/s of backward momentum assuming there is no friction.

19. Momentum
The result of throwing a 5- kg ball at a speed of 4 m/sec is that person and the skateboard with a total mass of 40 kg move backward at a speed of m/sec (if you ignore friction).
We use positive and negative numbers to show opposite directions.

20. Law of momentum conservation
The law says the total momentum in a system of interacting objects cannot change as long as all forces act only between the objects in the system.

21. Using the momentum relationship
An astronaut floating in space throws a 2-kilogram hammer to the left at 15 m/s. If the astronaut’s mass is 60 kilograms, how fast does the astronaut move to the right after throwing the hammer?
Looking for: …the velocity of the astronaut after throwing the hammer.
Given: … the mass of the hammer (2 kg) and the velocity of the hammer (15 m/s) and the mass of the astronaut (60 kg).
Relationships: The total momentum before the hammer is thrown must be the same as the total momentum after it is thrown.
momentum (p) = mass (m) × velocity (v)
A negative sign indicates the
velocity is to the left.

22. Using the momentum relationship
Solution: Both the astronaut and hammer were initially at rest, so the initial momentum was zero. Use subscripts a and h to distinguish between the astronaut and the hammer.
momentum after + momentum before = 0
mava + mhvh = 0
Substitute the known quantities:
(60 kg)(va) + (2 kg)(–15 m/s) = 0
Solve:
(60 kg)(va) = +30 kg·m/s
va = +0.5 m/s
The astronaut moves to the right at a velocity of 0.5 m/s.

23. Unit 1: Motion Chapter 4: Conservation Laws
4.1 Newton’s Third Law and Momentum
4.2 Energy and the Conservation of Energy
4.3 Collisions

24. 4.2 Investigation: Conservation of Energy
Key Question:
How can you predict the maximum velocity of a pendulum?
Objectives:
Describe the relationship between potential energy and kinetic energy in a system.
Apply the law of conservation of energy to predict the maximum velocity of a pendulum when it is released from different heights.
Use the time required for a swinging bob to break the photogate’s beam to calculate the velocity of the pendulum at different heights.
Compare the measured and predicted velocities.

25. What is energy?
Energy measures the ability for things to change themselves or to cause change in other things.
A gust of wind has energy because it can move objects in its path.
A piece of wood in a fireplace has energy because it can produce heat and light.
You have energy because you can change the motion of your own body.
Batteries have energy because they can be used in a radio to make sound.
Gasoline has energy because it can be burned in an engine to move a car.
A ball at the top of a hill has energy because it can roll down the hill and move objects in its path.

26. Units of energy
A joule (J) is the S.I. unit of measurement for energy.
Pushing a 1-kilogram object with a force of one newton for a distance of one meter uses one joule of energy.

27. Joules One joule is a small amount of energy.
An ordinary 100 watt electric light bulb uses 100 joules of energy every second!

28. Work
In science, work is a form of energy you either use or get when a force is applied over a distance.
You do 1 joule of work if you push with a force of 1 newton for a distance of 1 meter.

29. Work
When thinking about work, remember that work is done by forces that cause movement.
If nothing moves (distance is zero), then no work is done.

30. Work
Force (N)
W = F x d
Work (joules)
Distance (m)

31. Potential energy
Systems or objects with potential energy are able to exert forces (exchange energy) as they change.
Potential energy is energy due to position.

32. Potential energy A stretched spring has potential energy.
If released, the spring will use this energy to move itself and anything attached to it back to its original length.

33. height object raised (m)
Potential Energy
mass of object (g)
height object raised (m)
PE (joules)
EP = mgh
gravity (9.8 m/sec2)

34. Kinetic energy Energy of motion is called kinetic energy.
A skateboard and rider have kinetic energy because they can hit other objects and cause change.
The amount of kinetic energy is equal to the amount of work the moving board and rider do as they come to a stop.

35. Kinetic Energy EK = ½ mv2 KE (joules) mass of object (kg)
velocity (m/sec)

36. Kinetic Energy Kinetic energy increases as the square of the speed.
A car going twice as fast has four times the kinetic energy and needs four times the stopping distance.

37. Potential and kinetic energy
A 2-kg rock is at the edge of a cliff 20 m. above a lake. It becomes loose and falls toward the water below. Calculate its potential and kinetic energy when it is on the cliff edge and when it is halfway down. Its speed is 14 m/s halfway down.
Looking for: … the potential energy (Ep) and kinetic energy (Ek) at two locations.
Given: … the mass (2 kg), the height at cliff edge (20 m), and the speed halfway down (14 m/s). You can assume the initial speed is 0 m/s because the rock starts from rest.
Relationships: Use: Ep = mgh and Ek = ½mv2

38. Potential and kinetic energy
Solution:
Potential energy at the top:
Ep= (2 kg)(9.8 N/kg)(20 m) = 392 J
Potential energy halfway down:
Ep= (2 kg)(9.8 N/kg)(10 m) = 196 J
Kinetic energy at the top:
Ek= (½)(2 kg)(0 m/s)2 = 0 J
Kinetic energy halfway down:
Ek= (½)(2 kg)(14 m/s)2 = 196 J

39. Conservation of Energy
The law of conservation of energy states that energy can never be created or destroyed, just converted from one form into another.
If gravity is the only force acting on the ball, it returns to your hand with exactly the same speed and kinetic energy it started with, except that it returns to your hand from the opposite direction.

40. Conservation of Energy
The law of energy conservation says the total energy before the change equals the total energy after it.
The law of energy conservation says the total energy before the change equals the total energy after it.

41. Potential and kinetic energy
A 2-kg car moving with a speed of 2 m/s starts up a hill. How high does the car roll before it stops?
Looking for: … the height.
Given: …the car’s mass (2 kg), and initial speed (2 m/s)
Relationships: The law of conservation of energy states that the sum of the kinetic and potential energy is constant. The ball keeps going uphill until all of its kinetic energy has been turned into potential energy.

42. Potential and kinetic energy
Solution:
Find the kinetic energy at the start:
EK = (½)(2 kg)(2 m/s)2 = 4 J
Use the potential energy to find the height:
EP = 4 J = mgh
Therefore:
h = (4 J) ÷ (2 kg)(9.8 N/kg) = 0.2 m
The car rolls to a height of 0.2 m above where it started.

43. Using and conserving energy
When you “use” energy by turning on a light, you are really converting energy from one form (electricity) to other forms (light and heat).
In the “physics” sense, the energy is not “used up” but converted into other forms of energy.
The total amount of energy stays constant.

44. Unit 1: Motion Chapter 4: Conservation Laws
4.1 Newton’s Third Law and Momentum
4.2 Energy and the Conservation of Energy
4.3 Collisions

45. 4.2 Investigation: Collisions
Key Question:
Why do things bounce back when they collide?
Objectives:
Explore collisions and explain how they obey the law of conservation of momentum.
Describe how Newton’s laws explain collisions.

46. Collisions
There are two main types of collisions, elastic and inelastic.
When an elastic collision occurs, objects bounce off each other with no loss in the total kinetic energy of the system.

47. Collisions
In an inelastic collision, objects change shape or stick together, and the total kinetic energy of the system decreases.
An egg hitting the floor is one example of an inelastic collision.

48. Collisions
When two billiard balls collide, it looks like they bounce without a loss of kinetic energy.
But the sound of the collision tells you a small amount of kinetic energy is being changed into sound energy.

49. Momentum
As long as there are no outside forces (such as friction), momentum is conserved in both elastic and inelastic collisions.
Conservation of momentum makes it possible to determine the motion of objects before or after colliding.

50. Momentum Using momentum to analyze collision problems takes practice.
Draw a diagram.
Decide whether the collision is elastic or inelastic.
Let variables represent the masses and velocities of the objects before and after the collision.
Use momentum conservation to write an equation stating that the total momentum before the collision equals the total after. Then solve it.

51. Momentum and collisions
An 8,000-kg train car moves to the right at 10 m/s. It collides with a 2,000-kg parked train car. The cars get stuck together and roll along the track. How fast do they move after the collision?
Looking for: … the velocity of the train cars after the collision.
Given: … both masses (m1= 8,000 kg; m2= 2000 kg) and the initial velocity of the moving car (10 m/s). You know the collision is inelastic because the cars are stuck together.

52. Momentum and collisions
Relationships: Apply the law of conservation of momentum. Because the two cars are stuck together, consider them to be a single larger train car after the collision (m3). The final mass is the sum of the two individual masses: initial momentum of car1 + initial momentum of car2 = final momentum of combined cars.
m1v1 + m2v2 = (m1+ m2)v3
Solution:
(8,000 kg)(10 m/s) + (2,000 kg)(0 m/s) = (8,000 kg + 2,000 kg)v3
v3 = 8 m/s.
The train cars move together to the right at 8 m/s.

53. Forces in collisions
Collisions create forces because the colliding objects’ motion changes.
Since most collisions take place quickly, the forces change rapidly and are hard to measure directly.
The total change in momentum is equal to the force multiplied by the time during which the force acts.

54. Solving impulse problems
Impulse can be used to solve many practical problems.

55. Impulse
A 1 kg clay ball hits the floor with a velocity of –5 m/s and comes to a stop in 0.1 second. What force did the floor exert on it?
Looking for: …upward force exerted by floor.
Given: …ball’s mass (1 kg) final velocity (-5 m/s) and final time (0.1 s)
Relationships:.Use: Ft = mv2 − mv1 ,then solve for F
Solution: F = (1 kg)(0 m/s) – (1 kg) (-5 m/s) = 50 N
(0.1s)

56. Car crash safety
Air bags work together with seat belts to make cars safer.
An air bag inflates when the force applied to the front of a car reaches a specific level.
Automakers use crash test dummies to study the effects of collisions on passengers.
Crash test dummies contain electronic sensors to measure the forces exerted at various places on the body.

57. Forensic Engineering
We usually think of engineering as a science focused on designing and constructing things—like bridges, computers, automobiles, or sneakers.
However, there is one branch of engineering that focuses on how things fail, collapse, or crash.