Download Practice Packet for Chapter 5: Work and Energy Name Read pages

Practice Packet for Chapter 5: Work and Energy
Name ____________________
Read pages 164 - 182 and answer the following questions using your notes and your book.
Introduction to Energy
1) When is energy of an object most obvious?
Energy is most obvious when it is being transformed from on type to another.
2) Can an object have work? Explain.
No, Work is something that an object does to other objects. An object can do work if it has energy.
3) A force sets an object in motion. What do we call the quantity force x distance, and what quantity does this
change?
Work, it changes the object’s energy.
4) A person lifts a 4.5 kg (this is mass not force) cement block a vertical distance of 1.2 m and then carries the block
horizontally a distance of 7.3 m. Determine the work done by the person on the book.
52.9 J
5) How many joules of work are done when a force of 1 N moves a book a 2 m?
2J
6) Which requires more work – lifting a 50 kg sack a vertical distance of 2 m or lifting a 25 kg sack a vertical
distance of 4 m? Explain.
The same energy is required. 980 J
7) Exactly what is it that an object having energy is capable of doing?
An object having energy is capable of doing work.
8) If you do 100 J of work to elevate a bucket of water, what is its gravitational potential energy relative to its
starting position?
100 J
9) In the above questions, what would the gravitational potential energy be if the bucket were raised twice as
high?
200 J
10) How many joules of gravitational potential energy does a 1 kg book gain when it is elevated 4 m?
39.2 J
11) How many joules of kinetic energy does a 1 kg book have when it is tossed across the room at a speed of 2 m/s?
2J
12) A car traveling at 50.0 km/hr skids a distance of 35 m after its brakes lock. How much more KE will the car have
at 100 km/hr?
4 times the KE
13) How far will the above car skid if its brakes lock when it is traveling 100 km/hr?
The car will travel 140 m.
14) Why does it require more work to stop a heavier truck than a light truck even if they have the same speed?
(Answer this in terms of KE and work)
The lighter truck has less KE so it requires less work to stop it.
15) Consider the kinetic energy of a person in a set on a fast moving train. Does it have the same or different kinetic
energies relative to the train and relative to the ground? Explain your answer.
The amount of KE is relative to a reference point like speed. The KE = 0 relative to the train but the KE is higher relative
to the ground.
Conservation of Energy
16) The largest watermelon ever grown had a mass of 118 kg. Suppose this watermelon were exhibited on a
platform 5.0 m above the ground. After the exhibition is was dropped to the ground. What is the speed of the
watermelon right before it hits the ground?
v = 9.9 m/s
17) Tarzan is in a tree 30 meters from the ground. He swings on a long vine to save Jane on the ground. What is his
linear speed at the bottom of the swing if he starts from rest? (Assume no air resistance)
v = 24.2 m/s
18) Complete the following energy trails.
Burning wood for heat
Sled riding down a hill
____Chemical______  ____thermal_____
__graviational____  ____translational_____
____EM____
_______sound______
____sound______
______thermal_______
Using a flashlight
____chemical____  __electrical______  ______EM________
_____thermal_______
Conservation of Mechanical Energy
19) For the diagram below, find the PE, KE, and linear speed (v) for the blanks. Assume that friction is negligible and
that at the bottom of the hill the height = 0 meters. The mass of the ball is 2 kg. Show your work for full credit.
A = 50 J
B = 7.07 m/s
I=0J
J = 100 J
K = 10m/s
20) What is the mechanical energy of the ball at each point in the process of the ball rolling down the hill?
100 J
Use the diagram below to answer questions 21 through 25.
A roller coaster rolls on a track as shown in the diagram below. The point B is assumed to be at ground level for the
purposes of the problem and you should also not consider any energy losses due to friction. The coaster car and the people
inside have a total mass of 1500 kg.
21) How much kinetic energy does the coaster have a point A?
KE = 18,750 J
22) How much gravitational potential energy does the coaster have at point A?
GPE = 73,500 J
23) What is the mechanical energy of the roller coaster?
92, 250 J
24) What is the speed of the coaster at point B?
v = 11.1 m/s
25) Will the coaster be able to reach point C? You must be able to prove your answer.
No, since the GPE at C = 117,600 J. There is not enough mechanical energy in the coaster.
Power: Rate of Energy Transfer
26) When doing a chin-up, a physics student lifts her 42.0-kg body a distance of 0.25 meters in 2 seconds. What is
the power delivered by the student's biceps?
P = 51.5 W
27) If a toy car is pulled up a hill and at the top it has 20,000 J of potential energy what power is provided by the
motor pulling it up if it took 45 seconds to get to the top?
P = 444 W
28) If an automobile engine delivers 50.0 hp of power, how much time will it take for the engine to do 640,000 J of
work? (1 hp = 746 Watts)
 t = 17.2 s
29) A motor driven pulley pulls a 75 kg student 6.0 meters up a rope at a constant speed of 1.5 m/s. How much
power does the motor use in raising the student?
P = 1102.5 W
30) If a runner ran 3.0 km down a mountain slope in 62.25 minutes, how much work was done if the power that the
runner used was 585.0 W? (Convert minutes to seconds)
W = 2,184,975 J
31) In words explain how work and power are related.
Power is the amount of work is a given time.
32) Car engines are often described as powerful. How does this relate this relate to the definition of power.
A powerful engine does more work is a given time.