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Flashcards in Work and Energy Deck (22):
1

Work


W = Fdcosθ 




F is force, d is the distance over which the force is applied, and θ is the angle between the force and distance.
 

2

Units of Work


Joule = N·m = kg·m/s2·m = kg·m2/s2
 

3

Sign Conventions of Work


If the force and the distance applied is in the same direction, work is positive. 



If the force and the distance applied is in opposite directions, work is negative.
 

4

When is no work done if a force is acting in one direction?


If the force is acting in one direction, but the object moves in a perpendicular direction, then no work is done.
 

5

Work of Gravitational Force


Unlike friction, gravity always acts downwards. Thus, it does not matter what detour you take because sideward motion perpendicular to the gravitational force involves no work.
 

6

Does the amount of work depend on the path?

No, work is path independent. Pushing an object at constant speed up a frictionless inclined plane involves the same amount of work as directly lifting the same object to the same height at constant speed. 

Sliding down a frictionless inclined plane involves the same gravitational work as doing a free fall at the same height. 





 

7

Mechanical Advantage


Mechanical advantage = little input force (effort) -> large output force.
 

8

Work-kinetic energy theorem

Work on an object can transform into kinetic energy and kinetic energy can do work. 

9

When you are pushing on an object, what is the expression for work?

Fd = ½mv2

10

When gravity does work on an object, what is the expression for work?

Fweighth = mgh = ½mv2

11

Power


Power is the rate of work, or work over time: P = W/t 




The unit for power is the Watt, or W (don't confuse this W with the shorthand of work). 






Watt = Joule / second
 

12

Kinetic Energy

½mv2

13

Units of Kinetic Energy

Joule (kg m2/s2)

14

Local Gravitational Potential Energy

mgh

15

Spring Potential Energy

(1/2)kx2

16

Is the k value larger or smaller for stiffer springs?


Stiff springs have a larger k because they are harder to stretch (it takes more energy to stretch them).
 

17

General Gravitational Potential Energy

18

Conservation of Energy


The total amount of energy before = the total amount of energy after.
 

19

Conservative Forces


If a force doesn't dissipate heat, sound or light, then it is a conservative force.
 

20

Work done by conservative forces are path (dependent/independent)


Work done by conservative forces are path independent.
 

21

Non-Conservative Forces


non-conservative include frictional forces and human exertion. When friction acts on an object, the heat and sound released cannot be recovered. When you flex your arm, you lose heat that cannot be recovered (you cannot re-absorb the heat you lost).
 

22

Power Units


The unit for power is the Watt, or Joule per second.