Fundamentals 4

Question 1

When is the equation speed = distance / time valid?

Answer 1

when an object moves at constant speed.

Question 2

A lump of clay is stuck to a meterstick, which rotates about the meterstick’s center of mass. The rotational inertia of the meterstick is I, and the rotational inertia of the clay is MR^2. What is the rotational inertia of the meterstick-clay system?

Answer 2

I + MR^2. (Rotational inertias are additive.)

Question 3

In a satellite’s circular orbit, is the gravitational potential energy of the planet-satellite system constant or changing throughout the orbital path? Explain very briefly why.

Answer 3

constant. GPE here depends on the distance from the planet’s center, which doesn’t change in a circular orbit.

Question 4

A coconut falls from a tree onto a person’s head, where it stops. The equation J = Ft is used to calculate the force of the coconut on the person. Describe in words what the variable t represents.

Answer 4

the time for the collision to happen. (Not the time the coconut was in the air; nor just “time”, because “time” can mean a lot of things.)

Question 5

What are the units of acceleration?

Answer 5

m/s/s. (Again, please see the 2 Apr 2023 post.)

Question 6

I stand at rest on a platform scale. What is the Newton’s third law companion force to my weight?

Answer 6

the force of me on the earth. (Weight is the force of the earth on me, so the force pair is the force of me on the earth. The force of the scale on me is equal and opposite to my weight, but that has nothing whatsoever to do with Newton’s third law; that’s due to balanced forces and equilibrium.)

Question 7

(a) Can angular momentum be converted to linear momentum? (b) Can rotational kinetic energy be converted to translational kinetic energy?

Answer 7

(a) no. (b) yes.

Question 8

Define angular acceleration.

Answer 8

the change in angular speed every second. (Not “radians per second per second” or “rad/s/s” - these are the units of angular acceleration. Say what angular acceleration actually means. See the post from 2 Apr 2023.)

Question 9

Write the formula to find the gravitational field at a planet’s surface.

Answer 9

g = GM/r^2. (If you said “F = GM/r^2”, you’re wrong - this is a gravitational field, NOT a force!)

Question 10

What are the units of power?

Answer 10

watts, or joules per second.