Work/Energy

Question 1

Energy & corresponding units

Answer 1

Our system’s ability to do work; (kg*m^2)/s^2

Question 2

Units for work

Answer 2

SI unit: J
Other unit: 1 Pa * m^3

Question 3

Relationship between kinetic energy and speed

Answer 3

v^2 so if v doubles, kinetic energy quadruples

Question 4

What is potential energy

Answer 4

Energy associated with a given object’s position in space

Question 5

what is gravitational potential energy

Answer 5

Stored energy between the object and Earth

Question 6

What is thermal energy ∆Eth?

Answer 6

Energy associated with friction
∆Eth = - Wfriction

Question 7

kinetic energy

Answer 7

Energy associated with motion/acceleration
K = 1/2mv^2

Question 8

Calculating gravitational potential energy:

Answer 8

U = mg∆y

Question 9

Calculating elastic potential energy of a spring

Answer 9

∆Usp = (1/2)k(x^2)
*k is the spring constant
*x is the magnitude of displacement of the spring from equilibrium
∆Usp = - Wspring

Question 10

Relationship between spring potential energy and displacement

Answer 10

There is a square relationship.
If the spring is stretched/compressed double than before, the Usp is quadrupled.
∆x increases by 4
Usp increases by 16 and so on…

Question 11

Conservation of energy equation when given a problem of one type of energy being converted into another

Answer 11

Einitial = Efinal
Example question using the equation shown in picture: Spring potential energy –> kinetic energy

Question 12

Calculating Fsp (spring force)

Answer 12

Fsp = k∆x
∆x is how much extra distance is covered when the spring stretches from equilibrium
Fsp is the force of the spring required to go back to its equilibrium position (the recoil)

Question 13

The larger the spring constant, the (more elastic/more stiff) the spring.

Answer 13

More stiff

Question 14

In a diagram of a person pushing a spring in, what direction is the displacement and what direction is the spring force? What is the relationship between them?

Answer 14

Displacement is always going the opposite direction of spring force (Fsp).
As Fsp increases, the displacement increases by the same ratio.
Fsp always points towards the equilibrium point (the position at which the spring originally started)

Question 15

Spring force vs. displacement graph

Answer 15

Linear relationship, slope = k (spring constant)

Question 16

Springs in parallel:

Answer 16

∆x decreases by 1/2
k (spring constant) increases by 2

Question 17

Springs in a series

Answer 17

∆x increases by 2
k (spring constant) decreases by 1/2

Question 18

If two springs are put in parallel, the springs become (more soft/more stiff).

Answer 18

More stiff, because the spring constant k increases

Question 19

Equation for work

Answer 19

W = |F||d|cosθ
θ is the angle between the applied force vector & the displacement vector
*Note sometimes the force and displacement vectors go in the same direction and are parallel, so in that case θ is 0

Question 20

Equation for power

Answer 20

P = Work/∆t or ∆E/t or Fvelocity
Units: Watts, J/s, (kgm2)/s3, (ft*lb)/s

Question 21

Work-energy theorem

Answer 21

W = ∆K = 1/2m((Vfinal)^2 - (Vinitial)^2)
∆K is the change in kinetic energy

Question 22

If the displacement vector and force vector are at a 90° angle from each other, the work will always be _____.

Answer 22

Zero; cos90° is 0 so that makes the work for that particular force to be 0.

Question 23

Can work be negative?

Answer 23

Yes it can, in particular when the angle between the displacement and force vector is greater than 90°, work is NEGATIVE.

Question 24

Give 2 examples of when work is 0.

Answer 24

1. If d=0, object isn’t moving –> work is 0.
2. If the angle between the force & displacement of the object is 90°. For example, a person carrying a suitcase with their hand across a straight surface.

Question 25

If a person is traveling down a slope at a constant speed, what is their kinetic energy? What energy transformation takes place?

Answer 25

1. Kinetic energy = 0
   *anytime it says constant speed assume that it is 0
2. Ug (potentialE) –> Eth (thermal/friction)
   *The person is sliding which produces frictional energy

Question 26

Tip: If anything is rising or gaining distance in the upward direction, potential energy always increases.

Question 27

What is the work done by gravity when a person pushes a 2kg file cabinet 10m up a ramp which is 10° tilted off the ground.

Answer 27

W = mgdcos100
W = 2010cos100
cos is 100 because when drawing the gravity vector in relation to displacement vector, the 10° has to be included in addition to the 90°

Question 28

How does velocity affect gravitational potential energy?

Answer 28

It doesn’t affect ∆Ug. It doesn’t matter whether it is moving up or down, the object with the highest ∆Ug is the one with the highest height.

Question 29

Equation for conservation of energy problems:

Answer 29

∆E = ∆K + ∆Ug + ∆Us + ∆Eth + ∆Echem
∆K = Kf - Ki
∆Ug = mgyfinal - mgyinitial
∆Eth = - Wfriction
∆Usp = Uspfinal - Uspinitial

Question 30

What is torque

Answer 30

It is a rotational force that makes an object spin about a pivot point

Question 31

Cardiac output formula

Answer 31

CO = Stroke volume * Heart rate
CO is vol of blood pumped by the heart in 1 min
Stroke volume = vol of blood pumped out in a single heartbeat

Question 32

When do you use the conservation of energy formulas?

Answer 32

-When you/object are going from a motionless position to moving
-In general, when you see one form of energy is changing to another
Potential –> kinetic
Chemical –> kinetic

Question 33

Digestion is an example of what type of energy change?

Answer 33

Chemical energy –> thermal energy

Question 34

2 ways to calculate work

Answer 34

1. W = Fdcostheta
2. Pressure * volume on a PV diagram

Question 35

Work done by a potential field is path independent. Work done for conservative forces is independent on path. Name some conservative forces:

Answer 35

Gravitational force, elastic spring force, electrostatic, force, buoyancy force