1. Unit 7: Work and Energy

2. Section A: Work Corresponding Book Sections: PA Assessment Anchors:
7.1
PA Assessment Anchors:
S11.C.3.1

3. What is “Work” ? Work occurs when three conditions are met:
A force is applied to an object
The object moves
At least some of the force being applied is in the direction of the motion of the object

4. General Equation for Work
W = Fd
Unit: Joule (J)

5. Practice Problem
Find the work necessary to accomplish what is shown in the picture.
m = 98 kg

6. Am I doing work? Let’s say I go shopping at Weis:
Picking out items from the shelf
Placing the groceries on the belt
Holding the bag of groceries
Carrying the bag of groceries to my car

7. Work, version 2.0 What happens in this situation?
Does our equation for work “work” ?

8. The best equation for Work
W = Fd cos θ

9. Practice Problem Find the work done by gravity in this situation:
mass = 4970 kg
distance = 5 m

10. Positive, Negative, Zero Work
Work is positive
if the force has a
component in
the direction of
motion
Work is zero
if the force has
no component in
the direction of
motion
Work is negative
if the force has a
component opposite
the direction of motion

11. Finding Total Work Work can be added together, just like forces:
Wtotal = W1 + W2 + W3 + … = ∑W
Wtotal = Ftotald cos θ
Sum of the Work

12. Practice Problem
Find the work done in this situation:

13. Section B: Work & Energy
Corresponding Book Sections:
7.2
PA Assessment Anchors:
S11.C.3.1

14. Work-Energy Theorem
The total work done on an object is equal to the change in its kinetic energy.
Wtotal = ΔK =

15. Practice Problem
A truck moving at 15 m/s has a kinetic energy of 140,000 J. What is the mass of the truck?

16. Practice Problem #2
How much work is required for a 74 kg sprinkler to accelerate from rest to 2.2m/s ?

17. Pratice Problem #3
A boy pulls a sled as shown. Find the work done by the boy and the final speed of the sled after it moves 2 m, assuming initial speed of 0.5 m/s.

18. Let’s take another look at PP#3
Could we solve this using the kinematics equations and Newton’s 2nd Law?
The answer is YES.
Should we try?

19. Work on a Spring “k” is referred to a the spring constant
Remember…from the last unit…

20. Practice Problem
In the chase scene from Toy Story the Slinky Dog is stretched 1m, which requires 2J of work. Find the spring constant.

21. Practice Problem, Part 2
How much work is required to stretch the dog from 1m to 2m?

22. Power A measure of how quickly work is done Units: Joule / second: J/s
Watt: W (preferred unit)

23. Typical values of power

24. Practice Problem #1
Calculate the power needed to accelerate from m/s to 17.9 m/s in 3.00 s if your car has a mass of 1,300 kg.

25. Practice Problem #2
What is the average power needed to accelerate a 950 kg car from 0 m/s to 26.8 m/s (60 mph) in 6 s. Ignore friction.

26. Section C: Energy Corresponding Book Sections: PA Assessment Anchors:
8.1, 8.2, 8.3
PA Assessment Anchors:
S11.C.3.1

27. Two main types of energy
Kinetic Energy
Energy an object has while it’s in motion
Potential Energy
Energy an object has while it’s not moving

28. Kinetic Energy
Energy an object has while in motion
Unit: Joule (J)

29. Practice Problem #1
A truck moving at 15 m/s has KE of 14,000 J. Find the mass.

30. Potential Energy
Energy available to be converted to kinetic energy (energy of non-motion)
Unit: Joule (J)

31. Gravitational Potential Energy
Your book uses “U” to represent Potential Energy -- I’ll use “PE”

32. Two types of forces: Conservative
The work done by a conservative force is stored as energy that can be released later
Example: Lifting a box from the floor
As you lift the box, you exert force and do work
If you let go of the box, gravity exerts a force and does work

33. Two types of forces: Nonconservative
The work done by a nonconservative force cannot be recovered later as KE
Example: Sliding box across floor
The work done to slide the box can’t be restored as KE
Instead, the energy changes forms into heat

34. Examples of Conservative & Nonconservative Forces
Springs
Gravity
Nonconservative
Friction
Tension

35. Sections D & E: Momentum
Corresponding Book Sections:
9.1, 9.2, 9.3
PA Assessment Anchors:
S11.C.3.1

36. What is momentum? Linear momentum Units: kg m/s
The product of an object’s mass and velocity
Units: kg m/s

37. So, this means… If mass increases, momentum increases
If speed increases, momentum increases
Vice-versa if speed or mass decrease

38. Sample Problem #1
A 1180 kg car drives along a street at 13.4 m/s. Find the momentum.

39. Sample Problem #2
A major league pitcher can throw a kg baseball at 45.1 m/s. Find the momentum.

40. Change in Momentum Just like the change in speed, distance, etc.
Final - initial
Equation:

41. Adding momentum
Since momentum is a vector quantity, it will add like vectors add
We’ll keep it simple and say that:

42. Practice Problem #1
Two 4.00 kg ducks and 9.00 kg goose swim toward some bread that was thrown in the pond. The ducks each have a speed of 1.10 m/s while the goose has a speed of 1.30 m/s. Find the total momentum.

43. Momentum and Newton’s 2nd Law
Remember that Newton’s 2nd Law is ƩF=ma
We can relate this to momentum:

44. Impulse
Relationship between applied force and time

45. What is impulse? Vector quantity Units: kg m/s
Points in same direction as average force

46. Another way to represent Impulse:
If:
Then:
And if:

47. Practice Problem #1
A kg baseball is moving toward home plate at m/s when it is hit. The bat exerts a force of 6,500 N for s. Find the final speed of the ball.

48. Practice Problem #2
After winning a prize on a game show, a 72 kg contestant jumps for joy with a speed of 2.1 m/s. Find the impulse experienced.

49. Rain vs. Hail
As you’re holding an umbrella, does it require more force, less force, or the same force to hold up the umbrella if the raindrops turn to hail?

50. Conservation of momentum
If the net force acting on an object is zero, its momentum is conserved
In other words, the momentum before a collision is the same as the momentum after a collision
pf = pi

51. Practice Problem #1
A honeybee with a mass of 0.150g lands on a 4.75g popsicle stick. The bee runs toward the opposite end of the stick. The stick moves with a speed of cm/s relative to the water. Find the speed of the bee.

52. Elastic vs. Inelastic Collsions
Momentum is conserved
Kinetic energy is conserved
In other words:
Objects bounce off each other
Inelastic
Momentum is conserved
Kinetic Energy is NOT conserved
In other words:
Objects either stick or stop