Fundamentals 12

Question 1

Write the equation relating angular momentum to torque.

Answer 1

change in angular momentum equals torque times time.

Question 2

How do you find acceleration from a velocity-time graph?

Answer 2

take the slope.

Question 3

Give an example of an object that experiences nonzero acceleration, but is not speeding up.

Answer 3

a car slowing down on the highway; a ball on a string moving in a circle at constant speed. (Anything slowing down, or moving in a circle, is acceptable.)

Question 4

Under what conditions is a system’s momentum conserved?

Answer 4

when the system does not experience a net external force.

Question 5

Two people wearing ice skates are initially at rest. They push off each other, and head off with different speeds. Why do the two people have the same magnitude of momentum as each other after the push?

Answer 5

their total momentum was zero before they push, so is also zero after they push. That means their momentums have to subtract to zero after the push. The only way to subtract to zero is for both momentums to have the same magnitude.

Question 6

Write the equation for the force exerted by a spring.

Answer 6

F = kx.

Question 7

Moses runs up a staircase; Pav, who has the same mass as Moses, walks up the same staircase. Who uses more energy to reach the top of the stairs? Who generates more power reaching the top of the stairs?

Answer 7

they use the same energy; Moses generates more power. (The change in energy to get to the top of the stairs is mgh, and all values are the same for both people; power is energy per time, and Moses uses less time so more power.)

Question 8

Satellite A is in low-earth orbit; satellite B orbits much farther from Earth’s surface. Which has greater gravitational potential energy: the satellite A-earth system, or the satellite B-earth system?

Answer 8

the satellite B-earth system. (Gravitational energy of a system increases as objects move farther from one another.)

Question 9

Under what conditions are the kinematics equations valid?

Answer 9

when acceleration is constant.

Question 10

A string pulls a block to the left while the block moves to the right. Is the work done by the string on the block positive, negative, or zero?

Answer 10

negative.