Ch. 8, pg. 120. Review Questions. Nrs. 14 – 20
14. In what two ways can a machine alter an input force? Ans. It can change the force’s magnitude
and its direction.
15. In what way is a machine subject to the law of conservation? Is it possible for a machine to
multiply energy or work input? Ans. Everything is subject to the law of conservation. That is
why it is a law, and has no exceptions. A machine can multiply force at the expense of distance
but cannot multiply work.
16. What does it mean to say that a machine has a certain mechanical advantage? Ans. A machine
can give you an advantage by allowing you to put in less force and get out a greater force.
17. In which type of lever is the output force smaller than the input force? Ans. Type 3, always.
18. What is the efficiency of a machine that requires 100 J of input energy to do 35 Joules of useful
work? Eff.= Useful Work Output/Total Energy Input x 100% = 35 J/100 J x 100% = 35%
19. Distinguish between theoretical mechanical advantage and actual mechanical advantage. How
would these compare if a machine were 100% efficient? Theoretical mechanical advantage is
without friction, in the ideal world. Actual mechanical advantage is less than TMA because
friction uses some of the input energy.
20. What is the efficiency of her body when a cyclist expends 1000 W of power to deliver
mechanical energy to the bicycle at the rate of 100 W? Eff.= Power Output/Total Power Input x
100% = 100 J/1000 J x 100% = 10%
Ch. 8, pg. 121. Think & Explain. Nrs. 8 – 10
8. What is the theoretical mechanical advantage for each of the lever systems shown?
The first lever has no mechanical advantage (in other words MA = 1) because the lever only changes
the direction of the force. The distance the applied force () is from the fulcrum is the same as the
distance is from the output force () to the fulcrum.
The second lever has a mechanical advantage of 2:1 because the input force (arrow) is twice as far
from the fulcrum as the load (output force, ()) is.
The third lever has a mechanical advantage of 1:2 because the input force (arrow) is only half the
distance away from the fulcrum than the load ().
9. You tell your friend that no machine can possibly put out more energy than is put into it and your
friend states that a nuclear reactor puts out more energy than is put into it. What do you say?
The energy in uranium is locked in as Nuclear Potential Energy and can be released according to
the formula E = mc2. Uranium subatomic particles themselves are converted into energy and
there is a broader version of the law of conservation of energy which is called the law of
conservation of mass and energy.
10. The energy we require to live comes from the chemically stored potential energy in food, which
is transformed into other energy forms during the digestion process. What happens to a person
whose combined work and heat output is less than the energy consumed? This person will gain
weight, s/he will retain some of the calories. What happens when the person’s work and heat
output is greater than the energy consumed? This person will lose weight because they will burn
reserves after getting done with the energy the food provided. Can an undernourished person
perform extra work without extra food? An undernourished purpose can only perform extra
work if he is willing to give up his reserves and lose weight. Defend your answers.