Physics 2210 (Schroeder) Name
spring 2006
Problem Set 4
(due Thursday, Feb. 2)
1. For each of the following objects, draw a qualitatively correct force diagram, being
sure that the net force points in the direction of the acceleration (if any). For each
force, list the type of force (e.g., tension, gravity) and the agent of the force (i.e., the
nearby object that exerts the force).
(a) A box sitting at rest on an inclined ramp.
(b) A car braking to a stop as it travels in a straight line on a horizontal road.
(c) An unpowered roller coaster car at the very top of a “loop-de-loop”, where the
car is exactly upside-down.
2. If an object is at rest (for more than an instant), can there still be a force acting on
it? Explain.
3. A hockey puck is sliding on frictionless ice. With a single swing of your stick, you
want to make the puck turn a 90◦angle without changing speed. In what direction
should you aim your swing? (Please draw a picture showing the path of the puck and
the direction of the swing, and explain carefully.)
4. Calculate your weight in newtons and your mass in kilograms.
5. If gold were sold by weight, would you rather buy it in Denver or in Death Valley?
What if it were sold by mass? Explain carefully.
6. The force of gravity is twice as great on a 20-N rock as on a 10-N rock. Why doesn’t
the 20-N rock accelerate more rapidly when dropped? Explain carefully.
7. A standard kilogram mass (rolling on small frictionless wheels) is pulled horizontally
with a standard spring scale, and is found to accelerate at a rate of 5.0 m/s2. (a)
What is the force exerted by the spring scale? (b) Suppose that an unknown mass is
pulled with this same force, and found to accelerate at a rate of 2.0 m/s2. What is
the mass of this object?
8. An elevator in a skyscraper takes 4 seconds to accelerate from rest to its cruising speed
of 10 m/s, and another 4 seconds to come to a stop. A 60-kg passenger boards the
elevator on the ground floor. What is the passenger’s apparent weight (a) before the
elevator starts moving? (b) while the elevator is speeding up? (c) while the elevator
is moving at cruising speed? and (d) while the elevator is slowing down? Draw a
qualitatively accurate force diagram for the passenger in each of these four situations.
9. A crate is sitting in in the middle of the bed of a pickup truck, stopped at a red
light, on a horizontal road. After the light turns green, the truck accelerates and the
crate remains in the middle of the truck bed, without slipping. Draw a qualitatively
accurate force diagram for the crate, indicating the type and agent of each force. Also
indicate the direction of the truck’s motion on your diagram.
