Work, Energy, and Momentum

Question 1

Energy definition

Answer 1

• Energy is a property or characteristic of a system to do work
• Energy measured in Joules J=kg*m²/s²

Question 2

Kinetic Energy definition

Answer 2

Energy of motion

any obj that is moving has KE

K=½mv²

Question 3

Potential energy definition

Answer 3

different types:

1. gravitational
2. electrostatic
3. elastic (ie. compressed spring)

Question 4

Gravitational Potential Energy

Answer 4

U=mgh

• NOTE: U directly proportional to all three variables
• used when close to earth’s surface

Question 5

Total Mechanical Energy

Answer 5

E= U + K

E_f=E_i

KE_i + U_i = KE_f + U_f

Question 6

1st Law of Thermodynamics

Answer 6

Energy is never created or destroyed, merely transfered from one system to another

Question 7

Fill out this table:

Answer 7

• non conservative forces= make richer or poorer
• conservative forces=move money from one account to another

Question 8

How can you determine if a force is conservative based on the net work needed to move a particle on a round trip?

Answer 8

• if the net work done to move a particle in any round trip path is zero= conservative
• if the net work done to move a particle b/w 2 points is the same regardless of path taken= conservative

Question 9

Definition of work

Answer 9

• A process by which energy is transferred from one form to another (scalar) (J)
• if the force and the distance applied are in the same direction, work is positive
• If the force and the distance applied are in opposite directions, work is negative

Question 10

Work = ?

Answer 10

W=FdcosØ

• where ø is the angle b/w the force and displacement vectors
• therefore only forces parallel or antiparallel to d vector do work b/c cos 0 = 1, cos 90= zero
• cos #<90=positive and cos 90-180=negative
• classic example is that no work is done by your arms when you carry a bucket of water for a mile b/c you’re lifting bucket vertically while its motion is horizontal

Question 11

Power = ?

Answer 11

P=Work/time

• measured in Watts (J/s)
• rate at which energy is transferred from one system to another

Question 12

Work Energy Theorem = ?

Answer 12

W_net= ∆KE = K_f- K_i

W_{cons. force}= ∆KE = -∆PE

NOTE: ∆ means final-initial

-∆ means initial-final

Question 13

How much work is done by the force of gravity on a satellite which moves in a circular orbit?

Answer 13

Zero. Because uniform cicular motion = constant velocity= no net work

Question 14

What is the work done by gravity as a ball rises? As the ball falls?

Answer 14

As the ball rises, is losing speed so work is -

As the ball falls is gaining speed so work is +

(work_{air resistance} is always - )

Question 15

gravitational potential energy (far from earth) = ?

Answer 15

U= (-GMm)/r

NOTE: negative sign in there to keep relationship that as r increases, U decreases

Question 16

Which is a greater potential energy -10J or -100J?

Answer 16

-10J b/c it is scalar (in vector world -100J would have been bigger)

Question 17

Pulleys

Answer 17

The distance of pulling increases by the same factor that the effort decreases

Question 18

Stationary pulleys: how much force and how far do you have to pull to move 100N box 1m?

Answer 18

• If the weight of the box is 100 N, you have to pull with a force of 100 N
• For every 1 meter you pull, the box goes up 1 meter

Question 19

One Moving Pulley: how much force and how far do you have to pull to move 100N box 1m?

Answer 19

• Force needed to pull is halved because strings on both side of the pulley contribute equally. Therefore you supply 50 N (which is transmitted to the right-hand rope) while the left-hand rope contributes the other 50 N
• Because effort here is halved, the distance required to pull the box is doubled

Question 20

Two moving pulleys: how much force and how far do you have to pull to move 100N box 1m

Answer 20

• Counting the ropes reveal that when we tug on one rope, it gets transmitted to a system where 4 ropes pull on the load. Thus, you can pull the 100 N box with only 25 N.
• However, for every 4 m you pull, the box only goes up 1 m.

Question 21

Answer 21

Question 22

momentum = ?

Answer 22

p=mv

(vector) (kg*m/s)
* for more than two objects, total momentum is vector sum of indiv momentums

Question 23

Inertia def.

Answer 23

tendancy of objets to resist changes in their motion and momentum

Question 24

Impulse = ?

Answer 24

I=F∆t=∆p=m(v_f-v_i)

(vector) (kg*m/s)

where ∆t is time of collision

• Definition: force applied to an object over time causes a ∆ objcet’s momentum=impulse
• if impulse happens over a longer period of time, the force decreases (ie. car safety measures)

Question 25

elastic collisions

Answer 25

both momentum and KE are conserved

Question 26

inellastic collisions

Answer 26

momentum is conserved, KE is not

(∆KE lost is amount of energy released from system ie as light, heat, sound)

Question 27

totally inellastic collisions

Answer 27

momentum is conservced, KE is not

(∆KE lost is amount of energy released from system ie as light, heat, sound)

Question 28

Answer 28

area under graph = impulse= ∆p = m∆v

area of triangle= 1/2 bh=500=(10kg)∆v

∆v=50 m/s

ANSWER: B

Question 29

Mechanical advantage = ?

Answer 29

Mechanical advantage = F_out/F_in

Question 30

Two ropes are holding up a 100N block, what is the tension in each rope?

Answer 30

T₁ + T₂ = mg

therefore tension in each is 1/2 the block

i.e. a lone moving pulley

Question 31

Efficiency

Answer 31

Efficiency= W_out/W_in

=(load*load distance)/(effort*effortdistance)

often given as percentage

Question 32

Center of mass

Answer 32

the point within an object that follows a parabolic path of flight

Question 33

PE of a spring

Answer 33

PE= ½ kx²

where x= dist from equilib position