Pearson Physics Chapter 9 Work and KE

Work done by a force along a displacement

(COS for direction, SIN for y direction)

Find the work done by a varying force given area under a curve

(You could also integrate but you don't have to)

A car starts from rest and accelerates to 30 mph. Later, it gets on a highway and accelerates to 60 mph. Which takes more energy, the 0→30 mph, or the 30→60 mph?

30-60. The change in KE (1/2 mv2 ) involves the velocity squared. So in the first case, we have: 1/2 m (302 - 02) = 1/2 m (900) In the second case, we have: 1/2 m (602 - 302) = 1/2 m (2700) Thus, the bigger energy change occurs in the second case.

Work

A scalar quantity, it is the product of two vectors, measure of energy transfer that occurs when an object is moved over a distance by an external force at least part of which is applied in the direction of the displacement.

How does KE change

As work is done on it by a force

Multiply the vector using cross product

AxB->vector

A light plastic cart and a heavy steel cart are both pushed with the same force for a distance of 1.0 m, starting from rest. After the force is removed, the kinetic energy of the light plastic cart is ________ that of the heavy steel cart

Equal, Same force, same distance → same work done Same work → change of kinetic energy

By what factor does the kinetic energy of a car change when its speed is tripled?

Factor of 9, Since the kinetic energy is 1/2 mv2, if the speed increases by a factor of 3, then the KE will increase by a factor of 9.

Units for work

Joules or N-m

The work done by the earth on an apple as the apple falls towards the earth with increasing speed v is positive. Is there a change in KE?

KE increases

The work done by the earth on the moon as the moon makes one full revolution around the earth at constant speed v is zero. Is there a change in KE?

No change

Robert pushes the box to the left at constant speed. In doing so, Robert does ______ work on the box.

Positive

The work done divided by the time it takes to do the work

Power

Units of power

Watts

What is the transfer of energy to a system by the application of a force called?

Work

Describe the work and energy relationship

Work=energy expended

In a work and kinetic energy relationship what do we want

change in the kinetic energy to be exactly equal to the net work done on it.

What direction is friction

opposite of motion

the rate at which energy is transferred

power

The net work done on an object is equal to

the change in the object's kinetic energy

Power

the rate at which work is done

Work and kinetic energy must have what

the same units

Hooke's Law

the stress of a solid is directly proportional to the strain applied to it.

Is displacement scalar or vector?

vector

Is force a vector or a scalar?

vector

when is energy is exchanged or converted from one form to another

whenever work is done