Class 3

Question 1

What is the equation for work? What is really going here?

Answer 1

W = Fd cos(theta)

Fcos(theta) is looking for the component of force parallel to d

Question 2

What is the unit of work?

Answer 2

N x m = J

1 Newton-meter or a Joule

Question 3

What kind of work is kinetic friction?

Answer 3

Negative work because friction always acts against motion

Question 4

What kind of work is centripetal force?

Answer 4

Zero because the centripetal force is ALWAYS perpendicular to the instantaneous velocity.

Question 5

What kind of work is the normal force?

Answer 5

Usually zero, because the normal force is perpendicular to motion if the object is not moving.

One example where it does do work is on an elevator. Displacement is up/down not left/right!

Question 6

What kind of force is gravity? What does this mean?

Answer 6

Gravity is a conservative force, meaning work done by gravity only depends on the initial and final heights of the object, not the path the object follows.

Question 7

What is the equation for calculating work done by gravity?

Answer 7

Wg = mgh

Question 8

What is the equation for power?

Answer 8

P = Watts/time

Question 9

What is power?

Answer 9

The rate at which work is done

Question 10

What is the equation for power for a constant force parallel to a constant velocity?

Answer 10

P = Fv

Question 11

What is the equation for kinetic energy?

Answer 11

KE = 1/2mv^2

Question 12

Total work done on something is equal to what?

Answer 12

It’s change in kinetic energy

Question 13

Change in potential energy = ?

Answer 13

mg(delta)h

Question 14

KEi + PEi = ?

Answer 14

KEf + PEf

Question 15

What are the two ways you can calculate mechanical advantage?

Answer 15

MA = F resistance/F effort

or MA = d effort/d resistance

Question 16

Does mechanical advantage ever change the amount of work you have to do?

Answer 16

No because you can’t create energy from using a ramp, tool, etc. You gain an advantage by not needing as much FORCE.

Question 17

Mechanical advantage will always be ______ than one.

Answer 17

Greater

Question 18

Is momentum a vector or scalar?

Answer 18

Vector

Question 19

What is the equation for momentum?

Answer 19

p = mv

mass x velocity

Question 20

What is the equation for change in momentum?

Answer 20

(delta)p = Favg x (delta)t

note that delta p is sometimes called “J”

Question 21

If you throw a ball at a padded wall versus a hard wall, what happens to momentum, time, average force, etc.

Answer 21

Change in momentum WILL NOT change, but the time to change momentum INCREASES.
Since change in momentum = Favg/delta t, increasing the time, decreases the average force!

This is why we use airbags.

Question 22

Is momentum conserved in [free] collisions conserved?

Answer 22

YES! Momentum before collision will be the same as the momentum after the collision.

Question 23

What is the difference between perfectly inelastic and perfectly elastic collisions?

Answer 23

A collision is perfectly INELASTIC if objects STICK TOGETHER after the collision.

Question 24

Do inelastic or elastic collisions result in the greatest loss of kinetic energy?

Answer 24

Inelastic