long inclined plane, only 54 N of force are
required. Calculate the work done to push the
block up the plane. What PE does it have?
13. How does the amount of work done on an
automobile by its engine relate to the energy
content of the gasoline? (8.6)
14. In what
two
ways
can a machine alter an
input force? (8.7)
15. In what way is a machine subject to the law
of energy conservation? Is it possible for a
machine to multiply energy or work input? (8.7)
16. What does it mean to say that a machine has
a certain mechanical advantage? (8.7)
5. Calculate the change in potential energy of
8 million kg of water dropping 50 in over
Niagara Falls.
6. If 8 million kg of water flows over Niagara
Falls each second, calculate the power avail
able at the bottom of the falls.
7. a. Calculate the kinetic energy of a 3-kg toy
cart that moves at 4 rn/s.
17. In which type of lever is the output force
smaller than the input force? (8.7)
18. What is the efficiency of a machine that
requires 100 1 of input energy to do 35 1 of
useful work? (8.8)
19. I)istinguish between theoretical mechanical
advantage and actual mechanical advantage.
flow would these conpare if a machine were
100% efficient? (8.8)
b. Calculate the kinetic energy of the same
cart at twice the speed.
8. A lever is used to lift a heavy load. When a
50-N force pushes one end of the lever down
1.2 in, the load rises 0.2 m. Calculate the
weight of the load.
9.
20. What is the efficiency of her body when a
cyclist expends 100() W of power to deliver
mechanical energy to the bicycle at the rate
of 100W? (8.8)
1. Calculate the work done when a 20-N force
pushes a cart 3.5 rn.
b. If the workers actually pull with 2500 N of
force to lift the piano, what is the efficiency of
the pulley system?
10. Your European friends tell you they are pleased
because their new car gives them 15 kilome
ters per liter of fuel. Translate this into miles
per gallon so you can decide whether to be
impressed. One mile is 1.6 km, and one gallon
is 3.8 liters.
2. Calculate the work done in lifting a 500-N
barbell 2.2 in above the floor. What is the
potential energy of the barbell when it is
lifted to this height?
3. Calculate the power expended when the bar
bell above is lifted 2.2 m in 2 s.
4. a. Calculate the work needed to lift a 90-N
block of ice a vertical distance of 3 m. What
PE does it have?
b. When the same block of ice is raised the
same vertical distance by pushing it up a 5-rn
120
Chapter 8
Energy
a. In raising a 5000-N piano with a pulley
system, the workers note that for every 2 m
of rope p tilled down, the piano rises 0.4 m.
Ideally, how much force is required to lift the
piano?
1.
State two reasons why a rock projected with
a slingshot will go faster if the rubber is
stretched an extra distance.
2. Does an object with momentum always ha i
energy? Does an object with energy always
have momentum? Explain.
3.
4.