1. Which of the following is not an example of friction? a. Air resistance. c. a desk at rest.
b. a ball rolling down a hill. d. getting warm by a campfire.
2. Which is an example of helpful friction?
a. getting a static shock c. sliding across a gym floor on your
b. wearing batting gloves d. All of the above
3. The gravitational force between two objects depends on masses of objects and
a. accelerations of objects. c. speeds of objects.
b. distance between objects. d. sizes of objects.
4. Which of the following objects exerts the largest gravitational force?
a. a 10 kg bowling ball c. a 0.2 kg feather
b. a 1kg book d. All are the same

The ultracentrifuge is an important tool for separating and analyzing proteins. because of the enormous centripetal accelerations, the centrifuge must be carefully balanced, with each sample matched by a sample of identical mass on the opposite side. any difference in the masses of opposing samples creates a net force on the shaft of the rotor, potentially leading to a catastrophic failure of the apparatus. suppose a scientist makes a slight error in sample preparation and one sample has a mass 10 mg larger than the opposing sample. if the samples are 12 cm from the axis of the rotor and the ultracentrifuge spins at 70,000 rpm, what is the magnitude of the net force on the rotor due to the unbalanced samples? ( be thorough on your answer)

Aspring has a natural length of 8 m. if a 12-n force is required to keep it stretched to a length of 10 m, how much work w is required to stretch it from 8 m to 16 m? (round your answer to two decimal places.)

What equation is used to calculate the weight of an object on a plant ?