5.1-5.3 Review KEY

CLASS ______ DATE ____________________________
You push on a desk with a force of 120 N to the right.
Your friend pushes on the same desk with a force of 90 N to the left.
What is the net force on the desk?
The net force is 30 N to the right
Your friend increases her force on the desk by 30 N. She doesn’t change the direction of her
push. What happens to the net force on the desk? Will the desk accelerate?
The forces on the desk would be equal on each side. The net force is zero and there is no acceleration.
3. The net force determines how and if an object will
4. When two forces act in opposite directions, the object will accelerate in the same direction as the larger
5. A force is described by its strength and by the direction in which it acts.
6. What is work? Work is when you exert a force on an object and that object moves a certain distance in
the same direction of the force.
7. Do simple machines reduce work? Explain. No. Simple machines do not lessen the amount of work
done, they just change how the work is being done.
You’re holding your dog’s leash and trying to stand still as he pulls on the leash at an angle.
You move forward. (All of/Some of/None of) his force does work on you. Circle one.
9. CALCULATE How much work do you do when you push a shopping cart with a force of 50 N for a
distance of 5 m? 50N x 5m = 250 Joules
10. Work divided by time equals Power
11. The unit of work is the Joule
12. The unit of power is the Watt
13. Force multiplied by distance equals Work
14. A force of 100 N is applied to a simple machine in order to lift a box that weighs 300 N. The input force
is applied over a distance of 4 meters, causing the box to rise 1 meter.
a. What is the input force? What is the input distance?
The input force is 100 N. The input distance is 4m.
b. What is the output force? What is the output distance?
The output force 300 N. The output distance is 1m.
c. Calculate the input work.
100N x 4m = 400 J
d. Calculate the output work.
300N x 1m = 300 J
e. Determine the mechanical advantage for the machine.
300N/100N = 3
15. PREDICT What happens to the efficiency of a bicycle as it gets rusty? It decreases.
What must you do to maintain the same amount of output work? Use grease or oil to prevent rusting
and to keep the chain smooth.
16. Describe the three ways a machine can help you do work and tell why machines are not 100 percent
Increasing Force: The output force is greater than the input force. This machine makes it feel easier for
us to complete the job. (Ex: faucet handle) Mechanical advantage is greater than one.
Increasing Distance: The machine increases the distance desired. The input force is higher than the
output force. (Ex: chop sticks) Mechanical advantage is less than one.
Changing Direction: The machine changes the direction of the motion. The input and output work are the
same, and the input and output distances are the same. (Ex: Pulley) Mechanical advantage is one.
Machines are not 100% efficient because friction must always be considered.
DIRECTIONS: If the statement is true, write true. If the statement is false, change the underlined word
or words to make the statement true.
17. False A machine’s mechanical advantage is the output force Divided by the input force.
18. True If the output force is greater than the input force, the mechanical advantage of the machine is greater
than one.
19. False If the machine increases distance, the output force is
greater than the input force.
20. False If a machine changes the direction, but not the
amount of the input force, the mechanical advantage is equal to 1.
21. True An ideal machine has no friction.
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