Work, Energy, Power, and Momentum

Back to simpler stuff.

In it’s simplest form, work = F * d

Work can be done by you, as well as on you.

Work is a measure of expended ENERGY

Machines make work easier (ramps, levers, etc) by allowing for the application of less force over larger distances.

Work carries a specific meaning in physics

Often, people are limited by the amount of

force they can apply. Putting ‘full weight’ or force into turning a

wrench is limited by the amount of mg that an individual can apply.

Ramps, levers, and pulleys allow you to do the same amount of work but by applying it over a larger distance the force required will be lessened.

Working at an Advantage

In this case, the distance being traveled is equal to the height

the box is raised.

vs

The box raises the same height in each image, but increasing the distance of movement significantly reduces the force required to move it

Ramps

Work = Force * distance

W = Fd Δh so ΔW = mg * Δh

Gravitational Potential Energy

A ramp is 10 m long and 1 m high Lifting 100 kg up a mere 1 m would require

980 N (220 lb) of brute force However, extended over a 10m ramp, only 98

N, which is 22 lb, is needed. This is something we can do.

A Ramp Example

How much work does it take to lift a 30 kg

suitcase onto the table, 1 meter high?

The unit of work (ENERGY) is the N*m, or the Joule (J). One joule is equivalent to approximately .000239 Calories of food energy.

How much work does gravity do to a 10 N book that it has ‘pulled’ off of a 2 meter shelf?

Work Examples

Energy is the capacity to do work.

Two main categories of energy:

Kinetic energy is the energy of motion. A moving baseball can do work A falling leaf can do work

Work is the Exchange of Energy

Potential Energy is stored or latent capacity to do

work. Gravitational potential energy (person on high-dive) Mechanical potential energy (a compressed spring) Chemical potential energy (stored in bonds) Nuclear potential energy (like in…. Nuclear bonds.)

Energy can be converted between the two types

Work is the Exchange of Energy

Falling objects convert gravitational potential energy

into kinetic energy.

Friction converts kinetic energy into vibrations which indicate thermal energy. As it sounds, thermal energy makes items hot. This type of energy is unable to be retrieved.

Doing work on something changes that object’s energy by amount of work done, transferring energy from whatever is doing the work.

Conversion of Energy

The total energy (in all forms) in a closed

system remains constant. Remember the half-pipe example?

This is one of nature’s “conservation laws” Conservation applies to energy, momentum,

angular momentum, and electric charge Conservation laws are fundamental in physics

and stem from symmetries in our space and time.

Energy is Conserved!

Power is simply energy exchanged per unit

time, or how fast you get work done. The unit of power is the watt which is equal to

Joules/seconds. Therefore, power = w/t How much power is needed to do 2300 J of

work in 3 seconds?

Power

Conserved just like regular momentum but

deals with rotational inertia. A spinning wheel wants to keep on spinning,

and a stationary wheel wants to keep still unless acted on by an outside force.

Newton’s laws for linear motion have direct application toward rotational motion.

Angular Momentum

Angular momentum (L) is proportional to

rotation, or angular speed (abbreviated omega, ) times rotational inertia (I)

Rotational inertia is found by m(r^2). (mass x radius squared)

This is how ice skaters speed up as they tuck their arms in. I( ) = I( ) If the inertia (based on the radius) is decreased, then the angular speed will increase.

Angular Momentum