Name: _____________________________ Date: _______________ Hr: ________
UNIT 4 Homework Packet # 2: ENERGY
KE = ½ m v2
PE
w = weight in N (can be found by mass in kg x 9.8 m/s2)
w
h
Part 1: Conservation of Energy
A bull drops a rock off the edge of a cliff. The rock is 2 kg and
the cliff is 125 m tall.
1. What is the potential energy of the rock before it begins to fall?
2. What is the kinetic energy before it begins to fall?
3. 3 seconds after the rock is dropped, what is its new height off the
ground below? Hint: Use this formula for distance d = 4.9 (t) ²
4. So what is its new P.E.?
5. And how much P.E. has it “lost” since starting to fall?
6. What is the new velocity of the rock 3 seconds after it got dropped?
7. So what is its new kinetic energy?
8. And how much KE has it gained?
9. Now compare the amount of PE lost and KE gained. Explain
10. Now do the same calculations (3 through 9) but consider the rock 5 seconds after it was dropped.
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Part 2: Potential and Kinetic Energy Calculations
1. What is the kinetic energy of a 1000 kg rollercoaster moving at a speed of 20 m/s?
2. If the rollercoaster above was moving twice as fast, what is the new speed?
3. If the rollercoaster in number 1 had double the mass and the same speed, what would the kinetic energy
be?
4. Missy Diwater, the former platform diver for the Ringling Bros Circus, had a kinetic energy of 15,000 J just
before she hit the water. If her mass is 50 kg, what is her speed?
5. A 750 kg compact car travels at 100 km/hr and has 3,750,000 J of kinetic energy. What is the KE of the
car if it travels at 50 km/hr?
6. John has a 50 kg object suspended in air, 50 m above the ground. If the object is dropped, how much
work will be done?
7. Mrs. Jacobs dropped an object from a height of 10 m. What was the weight of the object if it did 50 J of
work?
8. A cart is loaded with a brick and pulled at a constant speed along an inclined plane to the height of a seat
top. If the mass of the brick and cart is 3.0 kg and the seat is 0.45 m high, what is the potential energy of
the brick and cart when they reach the top?
Part 3: More Kinetic and Potential Energy Calculations
1. A moving car has kinetic energy. If it speeds up until it is going 4 times the original speed, how much
kinetic energy does it have compared to the original?
2. When the mass of a moving object is doubled with no change in speed, by how much will the kinetic
energy change?
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3. When the velocity of an object is doubled, by what factor is its kinetic energy changed?
4. Car A is lifted a certain distance in a service station and therefore has potential energy relative to the floor.
If car B were lifted twice as high, how much potential energy would Car B have compared to Car A?
5. Two cars are lifted to the same height in a service station. If car A is twice as massive as the Car B, which
has more potential energy? How much more? Why?
6. How many joules of potential energy does a 1 N book gain when it is elevated 4 m? 8 m?
Part 4: Energy Efficiency
We all use devices every day that use energy - or more accurately, transfer energy from one form to another.
Everything we use wastes energy - some of the energy transfers into forms that are not useful to us. For
example when driving a car, energy from burning fuel is transferred into kinetic energy of the car. However,
more than half the energy is lost as heat and sound. How effectively devices transfer energy - i.e. how much
of the energy used is useful - is called its efficiency. Efficiency is normally calculated as a percentage something 90% efficient is considered good at its job. Devices that transfer only 5% of the energy they use into
something useful are inefficient (very wasteful). We can calculate efficiency if we know the total energy used,
and how much is transferred into useful forms. To solve for efficiency think of it this way, the resulting work
that a machine does for you (work output) divided by the net work you do (work input). Example: If you do
80 Joules of work (work input) on a something and get 40 Joules of work out of it (work output), you have 50
% efficiency. 40 J / 80 J = .50 or 50% IT WILL ALWAYS BE PERCENTAGE!!!!
1. A student pushes on a lever with a force 175 N through a height of 5 m. As a result a 450 N crate raises 1
m upward. Calculate the efficiency of this machine.
2. What is the efficiency of a pulley that can lift a 250N treasure chest 13 m when 150 N of force is used to
pull the rope 25 m?
3. A wedge is calculated to be 82% efficient. If the wedge produces 210 J of work, how much work was put
into the wedge?
4. A box weighing 100 N is pushed up an inclined plane that is 5 meters long. It takes a force of 75 N to push
it to the top, which has a height of 3 meters. Calculate the efficiency of this machine.
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