Potential Energy and Energy Conservation

Question 1

Capacity to do work

Answer 1

Energy

Question 2

Energy transferred to the object by applying a force along a displacement

Answer 2

Work

Question 3

Unit for Work

Answer 3

Joule

Question 4

Energy stores in an object associated with position; scalar quantity; possibility for work to be done

Answer 4

Potential Energy

Question 5

Is Potential Energy a property of a single object?

Answer 5

No. It is a property of interacting systems.

e.g. ball-earth system, block-spring system

Question 6

T or F. There is more potential for work to be done on the person by the gravitational force.

Answer 6

T

Question 7

Potential energy associated with a body’s weight and its height above the ground

Answer 7

Gravitational Potential Energy

Question 8

Two ways to describe freely falling bodies

Answer 8

1. Force of gravity does work on the body so KE increases
2. GPE decreases as KE increases

Question 9

What happens when a block is raised to a height?

Answer 9

Negative Work by weight → storing of energy into GPE

Question 10

What happens when a block falls from a height?

Answer 10

Positive Work by weight → change in Kinetic Energy

Question 11

How to solve for GPE, U_grav

Answer 11

U_grav = mgy

product of weight mg and height y above the origin of coordinates

Question 12

Formula when raising an object to a height y_2

Answer 12

W_grav = -ΔU_grav

this holds true in both cases if the body is moving upwards or falling

the negative (-) sign is essential

Question 13

W if the body moves up

Answer 13

negative

Question 14

W if the body moves down

Answer 14

positive

Question 15

Formula/equation for conservation of ME if gravity is the only force acting on the object

Answer 15

1/2mv1^2 + mgy1 = 1/2mv2^2 + mgy2

numbers after v and y are subscript

Question 16

What is the total mechanical energy of a system (E)?

Answer 16

sum of kinetic and potenetial energy

E = K + U_grav

Question 17

“system” – ________ + ________

Answer 17

“system” – body + earth

Question 18

When only the force of gravity does work, the total mechanical energy is ________, i.e. conserved

Answer 18

constant

ΔE_mechanical = 0

Question 19

A quantity that always has the same value

Answer 19

“conserved”

Question 20

ΔK and ΔU_grav of a ball thrown up

Answer 20

ΔK < 0 ; ΔU_grav > 0

Question 21

ΔK and ΔU_grav of a ball dropped

Answer 21

ΔK > 0 ; ΔU_grav < 0

Question 22

A body that can return to its original shape and size after being deformed

Answer 22

“elastic”

Question 23

Basis of formula for elastic potential energy

Answer 23

Newton’s Third Law

For every action (force) there is an equal and opposite reaction

Question 24

Formula/equation for work done on a spring

Answer 24

W = 1/2kx2^2 - 1/2kx1^2

numbers beside x are subscripts

Question 25

Formula/equation for work done **by** a spring

Answer 25

W_el = 1/2kx1^2 - 1/2kx2^2 | numbers beside *x* are subscripts

Question 26

Work done by the spring can be defined in terms of ______________.

Answer 26

**elastic potential energy**

Question 27

In a spring system, what is always the position where the spring is neither stretched nor compressed?

Answer 27

x = 0 ## Footnote In a spring system, x = 0 is always the position where the spring is neither stretched nor compressed.

Question 28

Formula for elastic potential energy

Answer 28

U_el = 1/2kx^2

Question 29

W_el formula

Answer 29

W_el = ΔU_el

Question 30

value of W_el as the spring stretches

Answer 30

negative

Question 31

value of W_el as the spring relaxes

Answer 31

positive

Question 32

T or F. The work done by the spring can be defined in terms of elastic potential energy.

Answer 32

T. Formula is K_1 + U_el,1 = K_2 + U_el,2

Question 33

Force that offers a two-way conversion between kinetic and potential energy

Answer 33

Conservative force

Question 34

Examples of conservative force

Answer 34

* Gravitational Force * Force due to a spring

Question 35

Force that doesn't have a two-way conversion between kinetic and potential energy

Answer 35

Non-conservative force

Question 36

Examples of non-conservative force

Answer 36

* Friction force * Force due to fluid resistance

Question 37

Characteristics of a conservative force

Answer 37

* path-independent * reversible * when starting and ending points are the same, total work is zero

Question 38

Characteristics of a non-conservative force

Answer 38

* path-dependent * irreversible * when starting and ending points are the same, total work is NOT zero

Question 39

Energy associated with the change in the state of the materials

Answer 39

internal energy | ΔK + ΔU + ΔU_int = 0 ## Footnote KE, PE, and internal energy of a system may all change but the *sum* of those changes is **always zero.**