Physics Class 2

Question 1

If circumference increases, what happens to:

1) Time?
2) Speed?

Answer 1

Time is the same

Speed increases too

Question 2

With uniform circular motion, is speed and velocity constant?

Answer 2

Speed is consttant

Velocity is not

Question 3

Centripetal acceleration

Answer 3

Ac = v^2 / r

Question 4

Centripetal Force

Answer 4

Fc = (mv^2)/ r

Question 5

Forces that act perpendicular to velocity _____ the speed, but ______.

Answer 5

Cannot change the speed

but can change direction

Question 6

Torque

Answer 6

Force’s effectiveness at making an object spin/rotate

Question 7

Torque formula

Answer 7

τ = rFsinθ
Units: N X m
Torque is maximized with sin90

OR (if you’re able to determine the lever arm):
τ = lF
l = rsinθ

Question 8

In static equilibrium, what is the net torque and net force?

Answer 8

0 because it is motionless

Question 9

What does ‘static equilibrium mean’?

Answer 9

Motionless, no net torque

Question 10

Lever arm

Answer 10

shortest distance from the pivot to the line of action of the force

Question 11

Fulcrum

Answer 11

Part that never moves

Question 12

Work

Answer 12

Measures how much force contributes to the displacement of an object

W = Fdcosθ
Units: N x m –> Joules
- can be positive, negative or 0

Question 13

When is work +, - or 0

Answer 13

Positive Work
0 < θ <90

Zero Work
θ = 90

Negative Work
90 < θ < 180

Question 14

Conservative Forces

Answer 14

Gravity

- what goes in, comes out

Question 15

What does work done by gravity depend on?

Answer 15

Initial and final height, does not matter about path taken to reach those heights

Question 16

Power

Answer 16

Rate at which work is done

Formula:
P = W/t
Units: J/s –> N x m / s –> W (watts)

Question 17

Power for a constant force parallel to a constant velocity

Answer 17

P = Fv

• v = d/t

Question 18

Kinetic Energy

Answer 18

Ability to do work

Formula: 1/2 mv^2
Units: kg x (m/s)^2 = J

Question 19

Total work done if no change in other forms of energy…

Answer 19

Wtotal = ΔKE

If work is positive = gains KE
If work is negative = loses KE
If work is 0 = speed is constant

Question 20

Work done by gravity

Answer 20

Wgravity = mgh

= -ΔPE (losing PE, gaining KE)

Question 21

Wgravity = -ΔPE (losing PE, gaining KE)

Answer 21

In any spontaneous process, potential energy decreases

Question 22

When only conservative forces are doing work…

Answer 22

KEi + PEi = KEf + PEf

Question 23

Conservative vs Non-Conservative forces

Answer 23

KEi + PEi + Wnc = KEf + PEf
Wnc = ΔE

Conservative:

• gravity, electrostatic force, spring force
• have PE associated with them
• if they’re the only forces doing work, mechanical energy is conserved

Non-conservative forces:

• kinetic friction, drag
• always do negative work
• if they’re doing work, mechanical energy is not conserved

Question 24

Simple machines

Answer 24

Reduce the amount of force required to do something

1. Pulleys
2. Inclined Planes

Question 25

Mechanical Advantage

Answer 25

Factor by which force can be reduced when using a simple machine

MA = Fresistance / Feffort
= normal / with machine > 1

Mechanical advantage > 1 because reducing amount of force by increasing displacement. Work stays the same

F = mg ---------------------> F = mgsinθ
d = h ------------------------> d = h / sinθ
W = mgh -------------------> W = mgh

Question 26

Efficiency

Answer 26

Measures how much useful work was accomplished with the energy input

Efficiency (%) = Woutput / Energy input <100% (in real world, something is always lost)