What is work?
What is “work” in science?
Work is when a force moves an object over a distance.
What is the formula for work?
Work = Force × Distance (W = F × d)
You lift a book with 20 N of force over 3.7 meters. How much work is done?
W = 20N x 3.7m
W = 74 J
What does efficiency measure?
How well a machine uses energy (or how much useful work it produces).
What is the formula for efficiency?
Efficiency = (Output Work ÷ Input Work) × 100%
What two things must happen for work to occur?
A force must be applied AND the object must move in the same direction.
What units are used for work, force, and distance?
Joules (J)
Newtons (N)
Meters (m)
A chair is lifted with 50 N of force to a height of 0.4 meters. Calculate the work
W = 50N x 0.4m
W = 20 J
What is the difference between input work and output work?
Input = work put into the machine,
Output = useful work produced.
A machine has 50 J of input work and 25 J of output work. What is its efficiency?
Eff = (25J ÷ 50J) × 100%
Eff = 50%
If you push a wall and it doesn’t move, is work done? Why?
No, because there is no movement (distance = 0).
How much work is done if you apply 10.9 N of force over 2.2 meters?
W = 10.9N x 2.2m
W = 23.98 J
Jane applies 50.3 N of force over 3 meters on a lever. How much work is done?
W = 50.3N x 3m
W = 150.9 J
If a machine has high efficiency, what does that mean?
Most of the input work becomes useful output work.
Higher input work.
A machine does 80 J of output work from 113.9 J input. Calculate efficiency.
Eff = (80 J ÷ 113.9 J) × 100%
Eff = 70.2%
If you carry a backpack across a room at constant height, are you doing work on it? Explain.
No, because the force is upward but the motion is horizontal (not same direction).
A force of 25 N moves an object 40 meters. Calculate the work.
W = 25N x 40m
W = 1000 J
A 200 N load is lifted 0.25 meters. How much work is done?
W = 200N x 0.25m
W = 50 J
Why are most machines NOT 100% efficient?
Energy is lost (usually as heat or friction).
A machine has 203 J input and 149 J output. What is the efficiency?
Eff = (149 J ÷ 203 J) × 100%
Eff = 73.4%
A student pushes a box forward, but the box moves sideways. Is all the force doing work? Explain.
No, only the force in the direction of motion does work.
An object is moved 5 m using 60 N of force. Then it is moved another 5 m using 20 N. What is the total work done?
W = (60N×5m) + (20N×5m)
W = 300J + 100J
W = 400 J
A student pushes a box with 80 N of force for 3 meters, then pushes it back 2 meters with the same force. What is the total work done?
W = (80N×3m) + (80N×2m)
W = 240 J + 160 J
W = 400 J
If a machine’s output work is much less than its input work, what can you say about its efficiency?
It has low efficiency.
A machine’s efficiency is 60%. If the input work is 500 J, how much output work is produced?
Output work = 300 J