Everyday Einstein looks at how scientists define work, how work is related to force, and how to find out the amount of work you’re really doing each day.
A well-known proverb teaches us “Work while you work, play while you play.”
Well I’ve got bad news for you. It turns out that even when you’re playing, you’re doing work. What an outrage! How could work have snuck into our playtime? Is nothing sacred? Let’s take a look at how scientists define work, and how it’s related to Newton’s second law of motion.
Work While You Work
People have different ways of measuring work. Some of us might measure work by how long it takes us to complete it, “I have about an hour of work to do.” Others might measure it with currency, “That’s about $100 worth of work.”
Scientists don’t use either of those methods to measure work. Instead, they measure work using something called joules. Not the kind of jewel you give your fiancée when you propose, but joule as in James Prescott Joule, a scientist who studied the relationship between heat, work, and energy.
But how much work is one joule’s worth? There are lots of ways to define a joule, but for our purposes we’re going to say that one joule is the amount of work done when one newton of force is used to move an object one meter. That’s a great definition if you know what a newton of force is, but pretty worthless if you don’t.
So What’s a Newton?
Back in the episode on Newton’s 3 Laws of Motion, we learned that the second law of motion states that force is equal to mass times acceleration. What I didn’t mention in that episode is that force is typically measured in newtons, and that one newton is the amount of force required to take an object with a mass of one kilogram and accelerate it one meter per second squared. Wait. What?
Play While You Play
Let’s imagine that one day your mother asks you to clean your bedroom. You walk into the room and think, “Before I start working, I’m going to grab my iPhone off of the floor and see if I have any Twitter messages.” You’re standing there, holding your iPhone in your hand, reading your messages, when your mother comes in and asks, “Why haven’t you started working yet?”
Now, ordinarily you would be at a complete loss as to how to answer this question, but because you’ve listened to today’s episode, you’ll be able to look at her innocently and proclaim, “But mother I have done work.”
Let’s imagine that your iPhone weighs about 100 grams. The amount of force that Earth’s gravity exerts on a mass of 100 grams is about 1 newton. So to lift that iPhone off the floor, you have to use over 1 newton of force. Let’s say you use 1.25 newtons of force to lift that iPhone about 1.5 meters off the floor so that you can read your Twitter messages. Remembering that Work equals Force times Distance, we have:
Work = Force x Distance
Work = 1.25 newtons x 1.5 meters
Work = 1.875 joules
So you can calmly tell your mother, that just by picking up your iPhone, you’ve already done 1.875 Joules of work. Of course she might not be using the scientific definition of work and read you the riot act, so use this strategy at your own risk.