1. Chapter 5
Work and Energy

2. What is work?
List five examples of things you have done in the last year that you would consider work.
Based on these examples, how do you define work?

3. Kg*m2/s2 5-1 Work What is work?
In physics, work is the magnitude of the force (F) times the magnitude of the displacement (d) in the same direction as the force.
W = Fd
~ A force that causes a displacement of an object does work on that object
-A student holds a heavy chair at arm’s length for several minutes, was there work done?
~ Work is done only when components of a force are parallel to a displacement
- A man pushing a crate across a concrete floor
~ Sign of work can be positive or negative
Kg*m2/s2

4. How much work is 1 joule?
Lift an apple weighing about 1 N from the floor to the desk, a distance of about 1 m.

5. Pushing this car is work because F and d are in the same direction.
Why aren’t the following tasks considered work?
A student holds a heavy chair at arm’s length for several minutes.
A student carries a bucket of water along a horizontal path while walking at a constant velocity.

6. 5.1 Work Done by a Constant Force

7. 5.1 Work Done by a Constant Force
Example 1 Pulling a Suitcase-on-Wheels
Find the work done if the force is 45.0-N, the angle is 50.0
degrees, and the displacement is 75.0 m.

8. 5.1 Work Done by a Constant Force
W = ( F cos Ө) d = Fd
W = (F cosӨ) d = - Fd

9. 5.1 Work Done by a Constant Force
Example 3 Accelerating a Crate
The truck is accelerating at
a rate of m/s2. The mass
of the crate is 120-kg and it
does not slip. The magnitude of
the displacement is 65 m.
What is the total work done on
the crate by all of the forces
acting on it?

10. 5.1 Work Done by a Constant Force
The angle between the displacement
and the friction force is 0 degrees. ( ) N
180 s m 5 . 1 kg
120 2 = ma f

11. How would you calculate the work in this case?
What is the component of F in the direction of d?
F cos 
If the angle is 90°, what is the component of F in the direction of d?
F cos 90° = 0
If the angle is 0°, what is the component of F in the direction of d?
F cos 0° = F

12. Scalar or Vector
Work can be positive or negative but does not have a direction.
What is the angle between F and d in each case?
Work is a Scalar!!!!!

13. 5.1 Work Done by a Constant Force
Question #1
If a neighbor pushes a lawnmower four times as far as you do but exerts only half the force, which one of you does more work? By how much? W=Fd

14. 5.1 Work Done by a Constant Force
Question #1
W=Fd Neighbor W= (1/2F)4d → 2W You W = Fd → W The neighbor, does twice as much work

15. 5.1 Work Done by a Constant Force
Question #2
For each of the following cases, indicate whether the work done on the second object in each example will have a positive or a negative value.
The road exerts a friction force on a speeding car skidding to a stop.
NEGATIVE

16. 5.1 Work Done by a Constant Force
Question #2
For each of the following cases, indicate whether the work done on the second object in each example will have a positive or a negative value. B. A rope exerts a force on a bucket as the bucket is raised up a well POSITIVE

17. 5.1 Work Done by a Constant Force
Question #3
For each of the following cases, indicate whether the work done on the second object in each example will have a positive or a negative value. C. Air exerts a force on a parachute as the parachutist slowly falls to Earth. NEGATIVE

18. 5.1 Work Done by a Constant Force
Question #4
For each of the following cases, indicate whether the work done on the second object in each example will have a positive or a negative value. D. The Earth exerts a force on a bobsled as it moves down a track. POSITIVE

19. 5.1 Work Done by a Constant Force
Question #5
A worker pushes a 1.50 X103 N crate with a horizontal force of 345 N a distance of 24.0 m. Assume the coefficient of kinetic friction between the crate and the floor is How much work is done by the worker on the crate? 8.28 X 103 J

20. 5.1 Work Done by a Constant Force
Question #6
A worker pushes a 1.50 X103 N crate with a horizontal force of 345 N a distance of 24.0 m. Assume the coefficient of kinetic friction between the crate and the floor is How much work is done by the floor on the crate? X 103 J

21. 5.1 Work Done by a Constant Force
Question #7
A worker pushes a 1.50 X103 N crate with a horizontal force of 345 N a distance of 24.0 m. Assume the coefficient of kinetic friction between the crate and the floor is What is the net work done on the crate? 3.6X 102 J