Download KEY - Mrs. Wendorf

Physics Fix 20 Name: ______________________________ Date: __________________ Period: _____
Information: Potential Energy
What is energy? In a loose sense, energy is the ability to do work. There are many types of energy. For now,
we will look at mechanical energy, of which there are two types. The two types of mechanical energy are
potential and kinetic.
Potential energy is a kind of energy that is stored or held ready. A common example of potential energy is
gravitational potential energy. For example, on the ground, a rock has no real potential to do anything. But if
the rock is lifted a meter off the ground it has the potential to fall and do some damage to someone’s toe. If
the rock is lifted 30 meters off the ground, it will have even more potential energy and could potentially hurt
someone if it fell.
Gravitational potential energy, then, depends on the height of an object off the ground. The equation is:
PE = mgh
PE is potential energy, “m” stands for the mass of the object in kg, g is the constant 9.8 m/s2, and h is the
height in meters. Please note that m times g equals the weight of the object in Newtons (N) so that the
equation is sometimes PE = wh, where w is weight.
Critical Thinking Questions
1. A 24 kg rock is resting on the floor. How much potential energy does it have? Explain.
0 Joules
2. Calculate the potential energy of a 1200 kg boulder on a cliff 45 m about the ground.
529740 J
Information: Kinetic Energy
Kinetic energy is the energy of motion. For an object to have kinetic energy it must be moving. An object
that is moving has the potential to cause work to be done. The equation for kinetic energy is:
KE = ½ mv2
KE stands for kinetic energy and is measured in Joules, v is velocity in m/s and m is mass in kg.
Critical Thinking Questions
3. A young man heaves a 6.5 kg rock at a velocity of 6.9 m/s. What is the kinetic energy of the rock?
154.7 J
4. An object was thrown with a velocity of 4.3 m/s. If the kinetic energy of the object was 225 J, what was
the mass of the object?
24.3 kg
For questions 5-9, consider the following scenario: A 145-g ball was thrown straight up into the air at an
initial velocity of 16 m/s (assume the ball started at ground level.) and it reached a maximum height of 13 m.
5. What was the kinetic energy immediately after it was thrown?
18.56 J
6. What was the kinetic energy when the ball reached the maximum height (before it started traveling
downward)?
0J
7. What was the potential energy when the ball reached the maximum height?
18.5 J
8. What was the kinetic energy of the ball just before it hit the ground (on its return)?
18.56 J
9. What was the potential energy when the ball landed on the ground?
0J
10. The total energy of a substance is the sum of its potential and kinetic energy. The sum of your answers to
questions 6 and 7 will give you the total energy of the ball when it was at its peak. Find this sum.
18.5 J
11. Just as question 10 gave you the total energy at the ball’s peak, so the sums of questions 8 and 9 will give
you the energy of the ball at the end of its journey. Find the sum of questions 8 and 9.
18.56 J
12. Compare your answers to questions 10 and 11.
Essentially Equal
13. Hopefully in question 12 you noted that questions 10 and 11 were the same. Which of the following
statements are true. (There may be more than one.)
a) An object’s kinetic energy remains the same.
b) An object’s potential energy remains the same.
c) An object’s total energy remains the same.
d) An object’s potential energy is sometimes transformed into kinetic energy.
Information: Conservation of Energy
The law of conservation of energy states that within a closed, isolated system the total amount of energy is
constant. In the last few questions you saw that the total amount of potential and kinetic energy remained the
same. We can summarize our results with an equation:
KEi + PEi = KEf +PEf
Critical Thinking Questions
1. Consider a 4.0 kg piece of ice that falls from a roof 4 m above the ground.
a) Calculate the initial kinetic energy, KEi, just before the ice fell. (Use KE = ½ mv2 and ask yourself,
“What is the velocity right now?”)
0J
b) Calculate the initial potential energy, PEi, just before the ice fell. (Use the equation PE = mgh)
157 J
2. We are now going to find the speed of the ice at the very moment it reaches the ground. Because the total
kinetic and potential energy remains the same, we can use the following equation:
157 J
= KEf + PEf
add answers from 1a and 1b
a) Why does PEf equal zero? (Hint: use PE = mgh)
h=0m
b) Calculate KEf. using the equation given in question 2.
157 J
c) Finally, find the speed of the ice as it hits the ground using the equation KEf = ½ mv2.
8.86 m/s
3. A 3.5 kg ball is rolling toward a hill at a speed of 12 m/s. The ball reaches the hill and rolls up the hill to a
certain height. How high up the hill will the ball go? (Use the approach from questions 1 & 2)
7.33 m
4. Calculate the potential energy of a 93 kg person standing on a bridge 4.2 meters off the ground.
3831 J
5. A young man heaves a 10 kg shot put at a velocity of 12.9 m/s. What is the kinetic energy of the shot put?
832.05 J
6. What does it mean to say that energy is “conserved”?
7. A boy holds a 0.8 kg rock over a 100 m cliff.
a. Calculate both the potential and the kinetic energy of the rock as the boy holds it over the cliff.
PE = 784.8 J KE = 0 J
b. The boy drops the rock. Halfway down the cliff, calculate the rock’s potential and kinetic energy.
PE = 392 J KE = 392 J
c. What is the kinetic and potential energy of the rock just as it hits the ground?
PE = 0 J KE = 784.8 J
8. An object was thrown with a velocity of 14.7 m/s. If the kinetic energy of the object was 825 J, what was
the mass of the object?
7.6 kg
9. A 19.5 kg cart is rolling toward a hill at a speed of 13 m/s. The cart reaches the hill and rolls up the hill to
a certain height. How high up the hill will the ball go?
8.61 m