Download Physics 110 Homework Solutions Chapter 7

Physics 110 Homework Solutions
Chapter 7
(1) Force × distance is called work. Work, like force and impulse, produces a change in a quantity of motion of a body. In the case of work, the motion is describe by the kinetic energy.
(2) In order for a force to do work on a body, the body must undergo a displacement that is, at
least, partly in the direction of the force. Thus, if the body does not move (no displacement) no
work is done nor is work done if the displacement is perpendicular to the force. For instance,
pushing on a wall does no work nor does carrying a body horizontally with constant velocity.
(3) The gravitational potential energy of the car relative to the floor is proportional to the
height of the car above the floor. If the height of the car is doubled, its potential energy is
doubled.
(4) The gravitational potential energy of a body is proportional to the mass of the body. If two
cars are at the same height, the one with twice the mass will have twice the potential energy.
(5) If a body has no momentum in a given frame of reference, it must be at rest in that frame. It
may still have energy, however, such as gravitational potential energy due to its height above
the ground. Thus, a body may have energy if its momentum is zero. If the momentum of the
body is not zero, it must have velocity, which means that it must have kinetic energy. However,
it is possible for its potential energy to be negative. The sum of its kinetic and potential energies could be zero. Thus, it is possible for the energy of body to be zero even if it has momentum.
(6)(a) A bowling ball rolling along a bowling alley is moving horizontally. The force of gravity
on the bowling ball is vertical, that is, perpendicular to the direction of motion of the ball. A
force does no work on a body if the displacement of the body is perpendicular to the force. (b)
The force of gravity on a body in orbit about the Earth is toward the center of the Earth. For a
body in a circular orbit, the displacement of the body is tangential to the circular orbit, which is
perpendicular to the direction toward the center of the Earth. Thus, we again have a case in
which the force and displacement are perpendicular to each other an no work is done.