# Warm-Up Exercise for lecture 7

## Due 8:00 am, Tues, Sep 23

Physics 105, Fall 2008

The force required to maintain an object at a constant speed in free space is equal to
☐ the mass of the object
☐ the weight of the object
☑ zero
☐ the force required to stop it
☐ none of the above

Two teams are having a tug-of-war, using a nylon rope with practically no mass. Team A (on the left) is winning--both teams are accelerating to the left. What can you say about the tension in the rope?
☐ It is higher on the left than on the right.
☐ It is higher on the right than on the left.
☑ It is constant throughout the rope.

You slide a block of ice down a 45° ramp. There is no friction between the ice and the ramp. The acceleration of the ice block down the ramp will be
☐ greater than
☑ less than
☐ still equal to
the normal acceleration of gravity, 9.8 m/s2.

Ralph told me he was thinking about the penny and feather falling in a vacuum. The penny and feather both fell at the same rate. He asked, "Does this mean that the force on the penny and the force on the feather are equal?" How would you answer his question? Be sure to explain your answer.

No, the forces are not equal. The *accelerations* are equal, but since the net force = mass x acceleration, the net forces will not be equal. In this case, each object's net force is simply provided by its weight, w = mg, so you can see that the heavier object will have a larger net force.