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WorksheetsMechanical Energy
Total questions: 47
Worksheet time: 48mins
What are the units for energy?
Watts
Joules
Newtons
meters
What has to be true about an object for it to have kinetic energy?
it must be moving
it must be elevated
it must be experiencing a net force
it must be accelerating
Does your desk have kinetic energy right now? Explain and justify your answer.
yes because it is sliding across the room right now
no because it is not moving
yes because it has someone sitting in it
no because it is here on earth
The equation for KINETIC ENERGY is KE = (½)mv^2. If a car with a mass of 2000 kg is traveling at 30 m/s, how much kinetic energy does it have?
(1/2)(2000)(30)
(1/2)(2000)(30)^2
(2000)(30)^2
(1/2)(30)(2000)^2
For the last question, what are the UNITS for the answer. (If a car with a mass of 2000 kg is traveling at 30 m/s, how much kinetic energy does it have?)
N
m
J
W
If a car with a mass of 1000 kg is traveling at 30 m/s, how much kinetic energy does it have?
(1/2)(1000)(30)
(1/2)(1000)(30)^2
(1000)(30)^2
(1/2)(30)(1000)^2
For the previous two questions, what are the magnitudes of the answers?
900,000 J and 450,000 J
90 J and 45 J
9 J and 4.5 J
For the previous two questions, the cars are traveling the same speed. When the MASS of the car was cut in half (2000 -> 1000), what happened to the KE?
cut in half
doubles
quadruples
cut to 1/4
If a car with a mass of 1000 kg is traveling at 60 m/s, how much kinetic energy does it have?
(1/2)(1000)(60)
(1/2)(1000)(60)^2
(1000)(60)^2
(1/2)(60)(1000)^2
The previous question works out to how much KE?
1,800,000 J
450,000 J
900,000 J
Looking at questions 2 and 3 in the notes OR looking at the equation, answer this: If the SPEED of a car DOUBLES, what happens to its KE?
doubles
cut in half
quadruples
cut to 1/4
If the MASS of an object doubles, what happens to its kinetic energy? 2m ⇒_____ KE
doubles
cut in half
quadruples
cut by 1/4
If the SPEED of an object doubles, what happens to its kinetic energy? 2v ⇒_____ KE
doubles
cut in half
quadruples
cut by 1/4
If an object has kinetic energy, does it necessarily have momentum?
yes
no
maybe
If an object has kinetic energy, does it necessarily have momentum? Explain.
yes because it is moving
yes because it is here on earth
yes because of Newton's Third Law
If an object has kinetic energy, does it necessarily have a force being exerted upon it?
yes
no
If an object has kinetic energy, does it necessarily have a force being exerted upon it? Explain
no because of Newton's Third Law
no because of Newton's First Law
no because of impulse causing a change in momentum
What has to be true about an object for it to have (gravitational) potential energy?
it must be moving
it must be elevated
it must be experiencing friction
it must be experiencing a net force
Does your desk have potential energy right now?
no because it is not elevated
no because it is not moving
no because there is friction
Give a justification that someone could use to explain that the opposite of your last answer was true. (Does your desk have potential energy right now?)
yes because it is moving
yes because we are on the second floor
yes because
(Gravitational) Potential Energy is calculated by PE = mgh. A 4 kg box sits on a table that is 1.5 m above the floor. How much potential energy does the box have?
(4) (10) (1.5)
4 (1.5)
4 (40) (1.5)
What are the units for the previous question? (A 4 kg box sits on a table that is 1.5 m above the floor. How much potential energy does the box have?)
N
m
J
W
The room in the above example is on the second floor. The second floor is 5 m above the ground. How much potential energy does the box have relative to the ground?
(4) (10) (6.5)
(4) (10) (5)
(4) (6.5)
(4) (40) (6.5)
A 2 kg box is sitting 3 m above the ground. It is then elevated to 7 m above the ground. How much does its potential energy change during this time?
140 J [because (2)(10)(7)]
60 J [because (2)(10)(3)]
80 J [because (2)(10)(7) - (2)(10)(3)]
In the previous question, the answer was 80 J [because (2)(10)(7) - (2)(10)(3)]. This could also be found by...
(2)(10)(3)
(2)(10)(4)
(2)(10)(7)
An alternate equation for work - called the Work-Energy Theorem: W = ΔE. Work is defined as…(3 words)
amount of friction
change in energy
action reaction force
mass times acceleration
Doing positive work means…
giving an object energy
taking energy away from an object
Doing negative work means…
giving an object energy
taking energy away from an object
Units for work:
We said the other day that units for work are Nm (because W=Fd) Now that we added that WORK = CHANGE IN ENERGY, what are other possible units for Work?
J
N
m
W
Two other equations for work (based on kinetic and potential energies): W= Δmgh W = Δ(½)mv^2
Do you see how these must be true?
yes
no
You lift a box. Are you doing positive work, negative work, or neither?
positive
negative
neither
You lower a box. Are you doing positive work, negative work, or neither?
positive
negative
neither
You lift a box, then lower it back to its original starting point. Did you do an overall net positive work, net negative work or neither?
positive
negative
neither
You slap a stationary puck, setting it into motion. Are you doing positive work, negative work, or neither?
positive
negative
neither
You hit a moving puck, stopping it from moving. Are you doing positive work, negative work, or neither?
positive
negative
neither
You start a puck moving, slide it across the floor, then stop it again. Did you do an overall net positive work, net negative work or neither?
positive
negative
neither
You carry a puck from one side of the room to the other. Did you do an overall net positive work, net negative work or neither?
positive
negative
neither
You kick a puck across asphalt and set it into motion. Are you doing positive work, negative work, or neither?
positive
negative
neither
The puck gliding along the asphalt stops after a moving a short distance. Overall, what kind of work was done on the puck? Positive work, net negative work or neither?
positive
negative
neither
Explain the discrepancy between the last two questions.
friction did negative work
the hockey puck stopped by magic
IS THIS FORCE DOING WORK? The box moves to the right. If the force you provide is shown for each situation, are you doing work on the box? (yes or no for each pic)
A. no B. yes C. no
A. yes B. yes C. no
A. yes B. yes C. yes
A. no B. no C. no
A 2 kg box is lifted from the ground to a shelf that is 3 m above the ground. How much work is done in the process?
6 J
60 J
600 J
What if you double the height (in other words elevate it 6 m)? How much work would be done if it had been lifted a full 6 m?
still 60 J
120 J (double the work)
240 J (quadruple the work.)
A 2 kg box is brought from rest to a speed of 3 m/s. How much work is done? (Ignore friction)
(1/2)(2)(3)^2 - 0
(2)(3)^2
(1/2)(2)(3)
What if you double the speed (in other words its speed is 6 m/s)? 2v ⇒_____W
4x the work, so 36 J
2x the work, so 18 J
same amount of work, so 9 J
If you are traveling twice as fast, your skid will be ___ times as long
2
4
9
1/2
Which equation shows why the previous relationship is true? (...hint...it stems off of W = change in energy)
Fd = change in (1/2)mv^2
W = change in PE
Power = W/t
