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PHYSICS: CHAPTER 4.1 NOTE PACKET
Chapter 4.1: Work and Machines
I.
Vocabulary: Define the following terms from Chapter 4.1.
a. Work-when you exert a force on an object and it moves
b. Joule-the amount of work you do when you exert a force of 1 Newton to move an object a
distance of 1 meter
c. Power-the rate at which work is done.
Chapter 4.1: Work and Machines
II.
Notes:
a. What is WORK?
i. Work is done on an object when the OBJECT MOVES IN THE SAME
DIRECTION IN WHICH THE FORCE IS EXERTED.
1. The object must move some DISTANCE as a result of your FORCE.
2. Only the force acting is IN THE SAME DIRECTION as the object’s
motion is performing work.
a. Fig. 2, page 109:
QUESTION: IF YOU PULL AN OBJECT HORIZONTALLY, WHAT PART OF YOUR FORCE
DOES WORK? _____All of your forces contributes to work__________ _________.
b. Calculating Work
i. The amount of work you do depends on both the AMOUNT OF FORCE you
exert and the DISTANCE THE OBJECT MOVES.
1. The amount of WORK done on an object can be determined by
MULTIPLYING FORCE times DISTANCE:
a. WORK = FORCE x DISTANCE
b. SI Unit for WORK:
i. When force = NEWTONS (N)
ii. When distance = METERS (m)
1. The SI Unit = (N*m) or a joule ( J )
iii. Ex:
1. How much work is performed if you move a 50 N
object .5m?
2. What if the object was 100 N?
QUESTION: WHAT IS THE SI UNIT FOR WORK? ________Joule__________________________.
c. POWER
i. Work is UNAFFECTED BY TIME.
ii. Power is the RATE AT WHICH WORK IS DONE.
1. Power equals the AMOUNT OF WORK DONE on an object in a
UNIT OF TIME.
a. With MORE POWER, you can do the same amount of WORK
in LESS time.
iii. Calculating Power
1. POWER = WORK/TIME
2. POWER = FORCE x DISTANCE/TIME
a. Ex:
i. A tow truck exerts a force of 11,000 N to pull a car out of a
ditch. It moves the car a distance of 5 m in 25 seconds.
What is the power of the tow truck? (Set Up, Substitute,
Solve).
iv. Power Units
1. When work is measured in joules and time in seconds, the SI unit of power
is the joule per second (J/s).
a. Also known as the WATT (W).
i. In honor of JAMES WATT
1. Steam engine engineer.
b. One watt is a very small amount of power.
i. Power is more often expressed in a KILOWATT.
1. 1000 watts
a. One horsepower = 746 watts.
QUESTION: WHAT IS A KILOWATT? ________1,000 watts_______________________.
Chapter 4.1: Work and Machines
III.
Assessment Questions
1.
What is WORK?
Work is when you exert force on an object and it causes it to move some distance.
2.
In order for work to be done on an object, what must happen to the object?
A force must be applied to the object and the object must move at least partly in the
direction in which the force is applied.
3.
In which of the following situations is work being done?
a. Rolling a bowling ball
b. Pushing on a tree for ten minutes
c. Kicking a football
4.
What is a JOULE?
A joule is the SI unit for work.
5.
How can you determine the amount of work done on an object?
You can determine the amount of work performed by moving an object by multiplying the
force applied by the distance the object moved.
6.
Is more work done when a force of 2 N moves an object 3 m or when a force of 3 N
moves an object 2 m? Explain
2N x 3m = 6J ; 3N x 2m = 6J
The work is equal in both situations.
7.
What is power?
Power equals the amount of work done on an object in a unit of time.
8.
How are power and work related?
Power is calculated by dividing the amount of work done by the time required to do the
work.
9.
Your laundry basket weighs 22 N and your room is 3 m above you on the second floor. It
takes you 6 seconds to carry the laundry basket up. What is your power?
22N x 3m = 11W
6
If you take only 4.4 seconds to carry the basket upstairs, what is your power?
10.
22N x 3m =
4.4
15W
11.
A bulldozer does 72,000 J of work in 48 seconds. How much power does the bulldozer
use?
72,000 J / 48sec =1500W
12.
The mythical god Atlas was believed to hold the weight of the sky on his shoulders. Was
Atlas performing any work? Explain.
No, because he does not move the sky, he only holds it. There is no work without motion.