1. Momentum & Energy
Chapter 5 & 6

2. What is momentum? Inertia in motion
A moving object can have a large momentum if it has a large mass, a high speed, or both.
Equation:
Momentum = mass x velocity
p = mv

3. What would the units be?
p = mv
Units = kg·m/s

4. Examples
It is harder to stop a large truck than a small car when both are moving at the same speed.
A fast car can have more momentum than a slow truck.
A truck at rest has no momentum at all.

5. Think…
Can you think of a case where a roller skate and a truck would have the same momentum?
(1000)(0.01) = (1)(10)

6. Change in Momentum What factors affect an object’s momentum?
If the momentum of an object changes, either the mass or the velocity change.
The change in momentum depends on the force that acts and the length of time it acts.
The greater the force acting on an object, the greater its change in velocity and the greater its change in momentum.

7. Impulse Impulse is a change in momentum Impulse = force x time
To increase the momentum of an object, apply the greatest force possible for as long as possible.
Examples:
Athletes (baseball, volleyball, golf, etc…)
Shotgun, Cannon, ect…

8. Impulse in action!

9. Choose your car crash…
If you were in a car that was out of control and had to choose between hitting a haystack or a concrete wall, which would you choose? Why?

12. Haystack = Still alive
The same impulse does not mean the same amount of force or the same amount of time.
It means the same product of force and time.
To keep the force small, we extend the time.

13. Think…
When a dish falls, will the impulse be less if it lands on carpet than if it lands on a hard floor?
No. The impulse would be the same for either surface because the same momentum change occurs for each. It is the force that is less for the impulse on the carpet because of the greater time of momentum change.

14. Conservation of Momentum
The law of conservation of momentum states:
“In the absence of an external force, the momentum of a system remains unchanged.”

15. Conservation of Momentum
Before the firing, the momentum is zero.
After the firing, the net momentum is still zero.
Net momentum is neither gained nor lost.

16. Equation Conservation of momentum equation
Initial (m1v1 + m2v2) = Final (m1v1 + m2v2)
p1 = p2

17. Collisions
elastic collision - When objects collide without being permanently deformed and without generating heat
Ex. Billiards

18. Elastic Collisions

19. Collisions
inelastic collision - the colliding objects become distorted and generate heat during the collision.
Ex. car/train wreck
Conservation:
m1v1 + m2v2 = (m1+ m2) v

20. Inelastic Collision

21. Energy – The big idea
Energy can change from one form to another without a net loss or gain.

22. Work
Work is done when a net force acts on an object and the object moves in the direction of the net force.
Work = Force x distance
W = F · d
Greater distance or greater force = more work!
The unit of work is the newton-meter (N·m), also called the joule.

23. Practice
Suppose that you apply a 60-N horizontal force to a 32-kg package, which pushes it 4 meters across a mailroom floor. How much work do you do on the package?
Answer:
W = Fd = 60 N × 4 m = 240 J

24. Power
When carrying a box up some stairs, you do the same amount of work whether you walk or run up the stairs.
Power is the rate at which work is done.

25. Power
The unit of power is the joule per second, also known as the watt. (kilo = 1,000)
One horsepower (hp) = 0.75 kW
so an engine rated at 134 hp is a 100-kW engine.

26. Practice
Mr. Fisher can do 660-J of work for 1.8 seconds. How much power does he have in watts?
Mr. Bourquin can lift a 105-kg dumbbell a distance of 2.1 m in 3.6 s. How much power does Mr. Bourquin have in watts?
In horsepower?

27. Energy
The property of an object or system that enables it to do work is energy.
The two forms of mechanical energy:
kinetic energy
potential energy

28. Potential Energy
potential energy (PE) - Energy that is stored and held in readiness
it has the potential for doing work
3 types:
Elastic
Chemical
Gravitational

29. Examples Elastic: Chemical: Gravitational: PE = mgh Bow and arrow
Compressed spring
Chemical:
Gasoline
Sugar / ATP
Gravitational:
Rock on cliff
Waterfall
PE = mgh

30. Practice You lift a 100-N boulder 1 m.
a. How much work is done on the boulder?
b. What power is expended if you lift the boulder in a time of 2 s?
c. What is the gravitational potential energy of the boulder in the lifted position?

31. Kinetic Energy kinetic energy (KE) - energy of motion
If an object is moving, then it is capable of doing work

32. Kinetic Energy Kinetic energy can do work!
Note that the speed is squared, so if the speed of an object is doubled, its kinetic energy is quadrupled (22 = 4).

33. Don’t Break the Law
The law of conservation of energy states that energy cannot be created or destroyed. It can be transformed from one form into another, but the total amount of energy never changes.

34. Conservation of Energy
Potential energy will become the kinetic energy of the arrow.
Before = After
PE + KE = PE + KE

35. Conservation of Energy
Everywhere along the path of the pendulum, the sum of PE and KE is the same. Because of the work done against friction, this energy will eventually be transformed into heat.

36. The End! Review Problems Pg 92 – Momentum RQ’s # 1-4, 7-9, 18-23
Probs #1-5
Pg 111 – Energy
RQ’s # 1-11, 13-17
Probs # 1-4