Physics: Work Power and Energy

length before you stretch or compress; length of an object when no forces act on it

relaxed length

Work is a _______ quantity but it can be positive or negative.

scalar

Kinetic Energy is a ________ quantity and its units are _________

scalar (no direction), Joule

number that tells you how easily an object will stretch; how much force you need to stretch it a certain distance

spring constant

When is no work done 2?

there is no displacement

The force must be parallel to the displacement of the object for work to be done. Picking up a Football Force is ____ and displacement is ____ (work is done) Dropping a football Force is _____ and displacement is _____ (work is done by gravity) Running with a Football Force is _____ and displacement is ________. (no work is done because F and d are perpendicular) (force/displacement)

up, down, up, horizontal

done when a force causes an object to be displaced or moved (examples: push/moved)

work

total work done on an object is equal to the change in energy for that object

work change in energy theorem

the net work done on an object is equal to its change in kinetic and potential energy

work energy theorem

Mechanical Energy: Freefall Top: vi= ___ KEI= _____ PEi= _____ Heading down: Height ______ PE _________ V________ Mechanical Energy Halfway: 1/2 h PE= 1/2 KE= 1/2

0, 0, all, decrease, decrease, increase

What is the unit for work?

Nm, J

Mechanical Energy incline Heading down it is _____+______ Top: Vi= ____ PEI= ___ KeI= ____ Down: Hf= _______ PEF= _____ KEF= ____

PE, KE, 0, all, 0, 0, 0, all

What is the unit for power?

Watts, Horsepower (1 hp=746 Watts)

Is Work Done: 1) A man's car breaks down and he pushes it to the nearest gas station 2) A bully holds a younger kids backpack over his head, taunting him 3) A skydiver soars through the air, opens his parachute and slowly floats the ground.

Yes, No, Yes

Is Work Done: 4) The movers lift the heavy couch from the moving van. 5) Julie, leaving from her driveway, pulls her sister in a wagon once around the block and ends back in her driveway again. 6) An apple falls from a tree and hits Isaac Newton in the head

Yes, Yes, Yes

Mr. M holds his cat Fluffy over his head. As he holds the cat motionless in the air, a strict inteprtration of the laws of physics shows that Mr. M a. not doing any work nor expending power b. not doing any work but is expending power c. not doing work but not expending power d. doing work and expending power

a

A car accelerates from an initial speed to a higher final speed while the engine puts out constant power. During this time period, the magnitude of the cars's acceleration was a. increasing b. decreasing c. constant d. unpredictable

b

A pile driver is a device used to drive objects into the Earth by dropping a heavy weight on them. If the amount of weight dropped is doubled-without changing the height- the energy.... a. halved b. doubles c. quadruples d. squared

b

As an ideal pendulum swings back and forth.... a. energy is all kinetic at the highest pt in its swing b. its energy is all potential at the lowest point in its swing c. sum of potential and kinetic energy is constant d. its kinetic energy is always created than its potential energy

c

When the speed of an object doubles, its kinetic energy a. halved b. doubles c. quadraples d. squared

c

Conserved means the quantity remains _______; the quantity can change ____ (i.e. KE to PE and back) over a period of time but the amount will be the _______. Mechanical Energy is conserved but not in the presence of ___________. (mechanical energy)

constant, form, same, friction

A girl carries a heavy suitcase quickly up a flight of stairs. A boy of the same weight carries the same suitcase slowly up the flight of stairs. What statement is true? a. girl did less work and had less power than the boy b. girl had less power than boy c.. girl did more work and had more power than the boy d. girl had more power than the boy

d

Object must be __________ or moved for work to take place Work is only done when a force is __________ to displacement

displaced, parallel

What two things does elastic potential depend on?

distance stretched, compressed

potential energy in a stretched or compressed elastic object; spring, rubber band, etc.

elastic potential energy

ability of an object to do work or produce a change in itself or its surroundings

energy

If no friction: MEinitial= ME_______ FORMULA: KEI+PEI=KEF+PEF

final

When is positive work done?

force helps the motion of the object, applied force helps an object move. force and displacement are in the same direction.

When is no work done?

force is perpendicular to displacement

potential energy associated with an object due to its position relative to the Earth

gravitational potential energy

Mechanical Energy Freefall: Bottom: KE= ____ Hf= ____ PEF= _____ KEF= _____

increase, 0, 0, all

KE _________ as speed increases; KE __________ as mass increases

increases

energy of an object due to its motion/movement

kinetic energy

within a closed, isolated system, energy can change form, but the total amount of energy is constant

law of conservation of energy

As an object falls, it ___________ PE (height is smaller) and _________ KE (velocity is getting faster). At zero level, PE is _____ J.

loses, gains/increases, 0

In a spring constant Flexible spring: k is _____ Stiff Spring: k is _____

low, high

What two things does kinetic energy depend on?

mass (direct), velocity (quadratic: as one increases, it increases by the square)

What two things does gravitational potential energy depend on?

mass, height (above the "zero level") (must be measured relative to some zero level of reference)

sum of kinetic energy and potential energy

mechanical energy

More work= ________ energy will change

more

Work is _________ when it opposes the motion of the object. Friction does work to slow an object down; force and displacement are in opposite directions

negative

When is work done?

object must be displaced (moved) for work to take place

When do you use W=Fd?

only if both force and displacement are parallel to each other

When is negative work done?

opposes motion of the object (Friction does work to slow an object down, force/displacement are in opposite directions)

Work is only done when the force is __________ to the displacement. If the force on an object is not parallel to the object's motion, the parallel component does the work W=Fd

parallel

No work is done when: The force is ____________ to the displacement and there is ________ displacement.

perpendicular, no

PE describes an object that has the potential to move its __________ or __________.

position, location

Work is ______ when it helps the motion of the object. Ex) An applied force helps an object move; force and displacement are in the same direction

positive

energy associated with an object due to its position; stored energy

potential energy

rate at which work is done; depends on the time it takes

power