Lifting a box takes the same energy whether you take one second or ten. So what’s different about the fast lift? Day 1 finds the answer by measurement. Day 2 puts it to work on stairs, electric bills, and a roller coaster motor.
🎯 Goals
Show by measurement that lifting a load takes the same work at any speed, but the power depends on how fast you do it.
Calculate power in watts from mass, height, and time.
Measure your own power on a flight of stairs and compare it to everyday devices.
Convert power and time into energy in joules and kilowatt-hours, and use power to size a roller coaster motor.
🧰 Materials
🏋️A hooked mass of about 1 kg (any known mass works) and a string to lift it with
📐Meter stick, and masking tape to mark the start and 1.00 m finish
⏱️Stopwatch or phone timer
🪜Day 2: one flight of stairs with a handrail, and a tape measure
⚖️A scale that reads in grams for the hooked mass
🧮Calculator
📓Physics notebook
DAY 1Same work, different power
Section 1 of 5
Lift Fast, Lift Slow
1
Set up your lift.
Hang the mass from a string. Mass it in kilograms. Stand a meter stick upright and tape a start mark at the table or floor and a finish mark exactly 1.00 m above it.
2
Lift the mass straight up 1.00 m at three different speeds.
One partner lifts, one times. Start and end the lift with the mass at rest. Aim for a slow lift of about 4 s, a medium lift of about 2 s, and a fast lift of about 1 s. Do each one twice and use the time that looks most steady.
⚠️ Keep the mass low and under control. Don’t swing it, and keep feet and the lifter’s face clear of it.
3
Calculate the work and the power for each lift.
The work of each lift is W = mgh (Activity 4-1-3). Power is P = W ÷ t. Watch which column changes and which doesn’t.
P = W ÷ t Power is the rate of doing work, or the rate of transferring energy. It’s measured in watts (W), where 1 W = 1 joule per second.
Lift speed
Load m (kg)
Height h (m)
Work W = mgh (J)
Time t (s)
Power P = W ÷ t (W)
Power Checker
Calculate a row by hand first. Then enter it here to check your work and your power.
📓 Physics Notebook
Look at your Work column and your Power column. Which stayed the same, and which changed? Use your table to explain why it takes the same energy to lift a box slowly or quickly, but a different amount of power.
Section 2 of 5
Power Practice
4
Practice the power formula.
Use P = W ÷ t, and remember that W = mgh for a lift. Enter each answer in watts. A correct answer is within 2%.
Power Practice
Use g = 9.8 N/kg where needed.
A lamp uses 60 J of energy every second. What is its power?
You lift a 2.0 kg book 1.5 m in 3.0 s. What is the power of your lift?g = 9.8 N/kg
A motor does 1,200 J of work in 4.0 s. What is its power?
✅ End of Day 1 check: lifting table complete, one row verified with the Power Checker, and all three Power Practice rows green. Keep your mass and string for later.
DAY 2Use the idea: stairs, bills, and a coaster
Section 3 of 5
Stair Climb: Your Power
5
Choose a climber mass.
Use your own mass in kilograms if you’re comfortable sharing it, or use the standard 50 kg climber so nobody has to share their weight. Everyone in the group uses the same number.
6
Measure the stair height.
Measure the vertical rise of one step, then multiply by the number of steps to get the total height h in meters. A typical step rises about 0.18 m.
7
Climb the flight at two paces and time each one.
One person climbs at a normal walking pace, then at a brisk walking pace. A partner times from the first step to the top. Take turns so each person climbs once at each pace.
⚠️ Use the handrail, go one at a time, and keep the stairwell clear. Walk only. No running.
Sample data (use only if stairs aren’t available)
Climber mass 50 kg, stair height 2.9 m. Normal pace 8.5 s, brisk pace 5.0 s.
8
Calculate your work and your power at each pace.
The work is the same both times, because you rose the same height. Compare the power.
Pace
Climber mass m (kg)
Stair height h (m)
Work W = mgh (J)
Time t (s)
Power P = W ÷ t (W)
Power Checker
Calculate a row by hand first. Then enter it here to check your work and your power.
📓 Physics Notebook
Both climbs did the same work. Why was your power higher at the brisk pace? Compare your brisk power with a 60 W light bulb. What does that tell you about how much power a person can produce for a few seconds?
Section 4 of 5
Power × Time = Energy
9
Turn power and time back into energy.
Rearranging P = W ÷ t gives energy = P × t. With power in watts and time in seconds, the answer is in joules. Electric companies use a bigger unit, the kilowatt-hour (kWh): power in kilowatts times time in hours. One kWh is 3,600,000 J.
Energy Practice
Joules for the short jobs, kilowatt-hours for the long ones.
A 750 W microwave runs for 2.0 minutes. How much energy in joules?Convert minutes to seconds first
A 1,000 W heater runs for 3.0 hours. How much energy in kilowatt-hours?
A 100 W bulb stays on for 10 hours. How much energy in kilowatt-hours?
A 1,500 W hair dryer runs 6 minutes a day for 30 days. Total energy in kilowatt-hours?6 min = 0.1 h
Section 5 of 5
Design Check: A Coaster Lift Motor
10
Size the motor for a lift hill.
A coaster car with riders has a mass of 500 kg. The lift hill raises it 30 m, and you want it to finish in 40 s. Find the ideal energy and the minimum power.
Lift Hill Numbers
Ideal case: no friction, no motor losses.
Energy needed to lift the car 30 m (ideal, no losses)m = 500 kg, g = 9.8 N/kg
Minimum power to finish the lift in 40 s
11
Pick a motor.
Real motors and chains lose some energy to friction, so you need more than the ideal minimum. Choose the smallest motor that works with a little margin.
Motor Picker
Your lift hill needs at least 3,675 W. Which motor would you order?
📓 Physics Notebook
The lift would use the same energy at any speed, but a faster lift needs a bigger motor. Explain why, using the words energy and power. You’ll face this same design choice in Project 4-2-3.
Question 1
Use your Day 1 data to explain why the work stayed the same while the power changed.
Question 2
Using your brisk-pace stair power, how many 60 W bulbs could you light, and what does that number tell you?
Question 3
A 500 kg coaster car must be lifted 30 m. Explain why finishing in 20 s instead of 40 s needs twice the motor power, even though the energy is the same.
Question 4
Real lift motors need more power than the ideal number you calculated. Use what you learned in Project 4-1-4 about missing energy to explain why.
📤 Turn In
Completed Day 1 lifting table, with your notebook explanation of work vs. power
Your Power Practice and Energy Practice results
Completed stair-climb table, with your notebook comparison to a 60 W bulb
Your lift-hill energy and power, and the motor you chose with your reasoning