HW2 Solutions
E1.34 You are doing work on the saw (positive work) both when pushing and pulling, since
the displacement of the saw is always in the same direction as the force that you are
applying.
E2.16 The opener pivots about the end resting on the bottle cap. The hook under the cap
moves a small distance and exerts a large force on the edge of the cap, while the other
end moves a longer distance and the user’s hand can apply a correspondingly smaller
force. In other words the two torques about the pivot point must be equal.
E2.22 The horse’s energy is converted into heat (thermal energy) by the friction in the wheel
hubs.
E2.26 The road surface exerts a horizontal force of static friction on the bicycle tires, pushing
them forward.
(There are some subtle aspects of this question and the question of how energy is
supplied to the system of you and the bike. You might want to search online to see what
is discussed about this situation. The friction force from the road does not do any work
on the bicycle (can you see why?) so the energy comes from the chemical energy of
your body, transferred by your muscles into the rotational and translational kinetic
energy of the bike and the kinetic energy of your legs moving up and down.)
E2.28 Since the sled is not accelerating the net force on it must be zero. Therefore, the force
that you exert on the sled must be equal and opposite to the force of sliding friction on
its runners.
P1.18 a) The gravitational potential energy is mgh = (1000 kg)(10 m/s2)(200 m) = 2 x 106
joules.
(I have approximated g = 9.8 m/s2 by 10 m/s2. Reasonable approximations of this kind
are always acceptable in this course for numerical problems. The important point is to
understand how to solve a problem in a logical manner and to get numbers that are
good approximations for the numerical answer.)
b) From the energy table we have that a 1g cookie has an energy content of 21,000
joules. So a 5 g cookie will have an energy content of 105,000 joules. Therefore, the
number of cookies required to match the energy of part (a) will be 2,000,000/105,000
or approximately 20 cookies. This is a rather remarkable result, don’t you think? Now
you can see why eating too many chocolate chip cookies will increase your body size
significantly unless you burn up the food calories with some exercise!!
P2.13 Conservation of momentum means that your friend’s momentum must be 450 kg m/s to
the Right. The system consisting of you and your friend had zero total momentum before
you pushed on one another and this cannot change unless an external force is applied to
