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Physics X: Work

Author: oLahav

This series of lessons is designed to help you learn, or review, the fundamentals of physics. This part focuses on work.

All work and no play make Timmy a dull boy

No, that's a joke. But seriously now- in physics, work means more than doing your job (unless your a physicist… in which case work and work wouldn't be the same).

Work, or mechanical work to be more precise, is really measuring the force applied over a distance. It makes sense actually- if you push a box 1 m, you're doing less work than if you push it 1 km, right? So that's the whole concept really.

In equation terms, W=Fd . Work is a scalar, as it's always done in the direction of the motion d. It's measured in units called Joules (J). $J=Nm=\frac{m ^2kg}{s ^2}$

All that work and that's it?

Nope. There's still an important thing- the question of direction. Work only relates to the force that is applied in the direction of the motion. Therefore, if $\theta$ is the angle between F and d, then $W=F\cos{\theta}d$ .

In real terms, say I'm pulling a box like so:

As you can see, only the horizontal component of the force I'm using is actually moving the box. The vertical component isn't really doing anything at all. Therefore, I'm wasting energy.

It's possible to do negative work. For example, friction always causes negative work to be done, since it always acts in the direction opposite to the motion. Another example would be catching a ball- the ball moves forward, but you're using a force to stop it, which means the force is going backwards, and work is negative.

It's even possible to do zero work with motion. For example, say I'm holding an apply in my open palm and I walk forward. The only force exerted on the apple is the normal force from my hand, yet the apple is moving forward. Since the angle is 90 degrees, the equation would be $W=F\cos{90}d=0$ - no work.

Working overtime takes power

Power, in physics, has nothing to do with your strength or how tired you are. It's defined as the rate at which work is performed- very simple, $P=\frac{W}{t}$

Power is measured is Watts (W), and $W=\frac{J}{s}$ . Power can also be measured in horsepower and other non-metric units which are less cool.

There's not a lot we can do with power other than apply the formula. However, there are some useful applications in monetary terms- for example, you pay your electricity bill in $/kW, dollars (or cents) per kilowatt. Power has further uses in electrical concepts, but these are beyond the scope of this series.

And that's all for now, a short lesson. Isn't it annoying though how physicists take words we like using every day and put them in a whole new context? Don't confuse physics' power and work with regular power and work, otherwise you may end up exerting forces over time through a distance instead of doing your job, and wasting all of your power doing so.

Physics word games aren't very fun

Next time, we'll define and look at some types of energy. Yay energy.