The Scientific Meaning of Work What is work, and how do you calculate it?
The Scientific Meaning of Work One hot summer day, you are trying to move a heavy rock in a garden. No matter how hard you try, the rock refuses to budge. You eventually give up and go into the house for a cold glass of water. You raise the glass of water to your lips thinking of all the work you have done with so little to show for it. It might surprise you to know that you did more work on the glass of water than you did on the rock! “Work” has a special meaning in science. You do work only when the force you apply to an object causes the object to move. You definitely applied a force to the rock, but since it did not move any distance, you did no work on the rock. The force used on the glass of water was much smaller, but the glass moved, so some work was done on the glass.
Work or Not? According to the scientific definition, which is work and which is not? a teacher lecturing to her class? a mouse pushing a piece of cheese with its nose across the floor?
What’s work? A scientist delivers a speech to an audience of his peers. A body builder lifts 350 pounds above his head. A mother carries her baby from room to room. A father pushes a baby in a carriage. A woman carries a 20 kg grocery bag to her car?
What’s work? A scientist delivers a speech to an audience of his peers. No A body builder lifts 350 pounds above his head. Yes A mother carries her baby from room to room. No A father pushes a baby in a carriage. Yes A woman carries a 20 kg grocery bag to her car? No
Calculating Work In science and technology, work is calculated by multiplying the force you applied (in newtons) by the distance (in metres) the object moved. So, the formula for work is: Work = Force × distance W = F × d The units—newtons and metres—combine to form the newton- metre (Nm), the unit used as a measure of work. The newton-metre is more commonly called the joule (J). One newton-metre is equal to 1 joule (1 Nm = 1 J).
Calculating Work The force needed to lift the glass of water is about 4.0 N, and the distance raised is about 0.25 m. How much work is done in raising the glass? Given: force = 4.0 N distance = 0.25 m Required: work done (W) Analysis: W= F × d Solution: W = 4.0 N × 0.25 m W = 1.0 Nm W = 1.0 J (remember that 1 Nm = 1 J) Statement: 1.0 J of work is done when raising the glass.
Calculating Work The force applied to the rock in is about 800 N. The rock does not move. How much work is done on the rock? Given: force = 800 N distance = 0 m Required: work done (W) Analysis: W= F × d Solution: W = 800 N × 0 m W = 0 Nm W = 0J Statement: No work was done on the rock.
Work and Energy Energy is the ability to do work. You always use energy when you do work. Living things get energy from food. Not all of the energy your body uses is turned into useful work. When you tried to move the rock, you consumed energy, but the rock did not move. Energy was used as your muscles contracted while pushing on the rock. Your heart pumped more blood and your chest muscles moved air in and out of your lungs as you applied more force. In other words, work was done on your internal organs, but not on the rock. Energy, like work, is measured in joules. When thousands of joules are required, the kilojoule (kJ) is the unit of measurement. One kilojoule is equal to one thousand joules (1 kJ = 1000 J).
CHECK YOUR LEARNING 1. What is the formula for calculating work? 2. How does the newton-metre compare with the joule? 3. Calculate the work done in the following instances: pushing a car 15 m using a force of 500 N lifting a 100 N sewing machine from the floor to a tabletop 75 cm high 4. If you pushed a car 25 m and did 60 kJ of work, how much force did you use? 5. Imagine putting yourself in a push-up position and staying there. Assume that neither the floor nor your body moves. You will use a lot of energy but do no work. How can energy be consumed when no work is done?