Physics 100 Energy in Today’s World Homework Chs. 2-3
Prof. Menningen
p. 1 of 3
Name _________________________________
1.
3.
A car travels at 25 mph. About how fast must it travel to double its kinetic energy?
a. 35 mph
b. 50 mph
c. 313 mph
d. 625 mph
A bowler lifts a bowling ball from the floor and places it on a rack. If you know the mass
of the ball, what else must you know in order to calculate the total work she does on the
ball?
a. The height of the rack.
b. The way she grips the ball.
c. The mass of the rack.
d. The speed of the lift.
Is it possible for an object's kinetic energy to be negative? Yes No
4.
Is it possible for an object's gravitational potential energy to be negative? Yes No
5.
Which, if any, of the following is conserved as a ball falls freely in a vacuum? The ______
of the ball (check all that apply):
a. kinetic energy
b. gravitational potential energy
c. mechanical energy
d. none of these
2.
6.
A ball dropped from a height of 10 meters only bounces to a height of 5 meters. Which, if
any, of the following conserved in this situation? The ______ of the ball (check all that apply):
a. kinetic energy
b. gravitational potential energy
c. mechanical energy
d. none of these
7. Describe the energy transformations that occur when a skateboarder
rolls up a ramp from the initial position (i), becomes airborne, and
momentarily comes to rest at (f ).
The kinetic energy of the skateboarder is transformed into gravitational
potential energy as the skateboarder gains altitude. It is also partially
transformed into thermal energy by air friction and rolling friction.
8.
The pendulum of a clock swings back and forth. At what position will its kinetic energy be
the greatest?
a. At cos−1(1/g) = 84.1°
b. Halfway between its lowest and highest positions.
c. At its highest position.
d. At its lowest position.
At what position will its kinetic energy equal its potential energy?
a. At cos−1(1/g) = 84.1°
b. Halfway between its lowest and highest positions.
c. At its highest position.
d. At its lowest position.
Physics 100 Energy in Today’s World Homework Chs. 2-3
9.
Prof. Menningen
p. 2 of 3
Categorize the following units as those of work or power.
(a) joules
Work
_________
(b) watts
Power
_________
(c) kilowatt-hours
Work
_________
(d) ft-lb
Work
_________
(e) calories
Work
_________
(f) kW
Power
_________
(g) ft-lb/min
Power
_________
10. Calculate the work done in lifting a 2.5 lb book to a height of 9.0 ft. (Use g = 10 m/s2.)
W  F  d   2.5 lb  9.0 ft   22.5 ft-lb 
1.356 J
 30.5 J
1.0 ft-lb
11. A toy car has a kinetic energy of 6.00 J. What is its kinetic energy after a frictional force of
0.400 N has acted on it for 3.00 m?
W   F  d    0.400 N  3.00 m   1.20 J
KE final  KEinitial  W  6.00 J   1.20 J   4.80 J
12. (a) If a 0.500-kg ball is dropped from a height of 5.50 m, what is its kinetic energy just
before it hits the ground?
ME A  ME B
mghA  12 mvA2  mghB  12 mvB2
mghA  0  0  12 mvB2
mghA  KE B   0.500 kg  10 m/s 2   5.50 m   27.5 J
(b) What is the speed of the falling ball at the instant it is at a height of 2.75 m? Try to use
conservation of energy to find the answer.
mghA  12 mvA2  mghB  12 mvB2
mghA  0  mghB  12 mvB2
2 g  hA  hB   vB  2 10 m/s 2   5.50  2.75 m   7.42 m/s
Physics 100 Energy in Today’s World Homework Chs. 2-3
Prof. Menningen
p. 3 of 3
13. A pendulum bob changes height by a total of 71.0 cm from one end of its swing to its
lowest point. What is the speed of the pendulum bob at the lowest point?
mghA  12 mvA2  mghB  12 mvB2
mghA  0  mghB  12 mvB2
2 g  hA  hB   vB  2 10 m/s 2   0.71 m  0   3.77 m/s
14. An engine performs 4600 joules of work in 9.0 seconds. What is its power output in
kilowatts and in horsepower?
P
W 4600 J 1 kW
1.0 hp


 0.511 kW  511 W 
 0.685 hp
t
9.0 s 1000 W
746 W
15. A 100-W light bulb is accidentally left on for three days in a basement. If electricity costs
10¢/kWh, how much did this oversight cost?
E  Pt   0.100 kW  3.0 d  24 h/d   7.2 kWh  $0.10 /kWh  $0.72