When you lift something, where does the work you do go? Today you’ll find out by lifting real masses, then follow that energy back down a ramp and see how much of it you can find again as motion.
🎯 Goals
- Show by measurement that the work you do lifting an object equals mgh.
- Calculate gravitational potential energy for everyday objects, and see how it grows with mass and height.
- Explain why only changes in height matter, and what choosing a “zero level” means.
- Compare the PE your ramp cart lost with the KE it gained, and explain where the missing energy went.
🧰 Materials
📏Spring scale (Newton meter), 10 N or 20 N
🏋️Three objects you can lift and mass: e.g. 200 g, 500 g, and 1 kg masses, or bags of washers
📐Meter stick or tape measure
⚖️A scale that reads in grams
🚗Your Lab A data from Activity 4-1-2 (or use the sample data on this page)
🧮Calculator
📓Physics notebook
Section 1 of 4
Lift It: Where Does the Work Go?
1
Find the mass of each of your three objects.
Mass each one in grams, then divide by 1000 to get kilograms. Write the masses in your table.
2
Lift each object straight up at a slow, steady speed to a height of 0.50 m.
Hook the object to the spring scale and lift it from the table to a measured height h. Read the scale while the object is moving at a steady speed. That reading is the force F your lift supplies.
⚠️ Lift slowly and keep your feet clear of the object. Don’t swing it.
3
Lift one object to 1.00 m as well.
Use your lightest or middle object for the extra row, so you have a case where only the height changed.
4
Calculate two numbers for each row.
The work of your lift is W = F × h (Activity 4-1-1). Then calculate m × 9.8 × h and compare.
PE = m × g × h
An object’s weight is the pull of gravity on it: m × g, with g = 9.8 N/kg. Lifting at a steady speed means your force just balances the weight, so the work you do is weight × height = mgh. That energy is now stored as gravitational potential energy.
📓 Physics Notebook
Compare your last two columns. How close are they? Where is the energy you put in now stored, and what would you have to do to get it back out?
Section 2 of 4
PE Builder: Who Has the Most Stored Energy?
5
Rank the five objects by gut feeling.
Look at the mass and height of each object below. Without calculating, write them in your notebook from least to most stored energy.
6
Calculate the PE of each object, then check.
Use PE = m × 9.8 × h. Enter your answers in joules. A correct answer is within 2%.
PE Builder
Rank first, then calculate.
7
Test how PE scales.
You found in 4-1-2 that speed is squared in KE. Does anything get squared in PE? Work out each case before you type.
Does PE Scale Like KE?
PE becomes how many times as big? Enter a number or a fraction like 1/2. Look at the formula PE = mgh and ask whether anything is squared.
📓 Physics Notebook
In your own words: how is the way PE grows with height different from the way KE grows with speed?
Section 3 of 4
Where Is Zero?
8
Take a 2.0 kg book on a shelf 1.5 m above the floor. A table sits 0.75 m above the floor.
Height always has to be measured from some starting level, called the zero level. Calculate the book’s PE with two different zero levels, then the energy released if it falls from the shelf to the table.
Zero Level Check
Use m = 2.0 kg and g = 9.8 N/kg.
📓 Physics Notebook
Two students pick different zero levels for the same falling book. Why can they still agree on how much energy the book turns into motion?
Section 4 of 4
Swap It: From PE to KE
9
Gather your Lab A data from Activity 4-1-2.
You need the cart’s mass in kilograms, the three release heights (5, 10, and 20 cm), and the speed you measured at each. If you don’t have your data, use the class sample below.
Sample data (use only if you don’t have your own)
Cart mass m =
0.120 kg. Release 5 cm → v =
0.85 m/s; 10 cm →
1.20 m/s; 20 cm →
1.70 m/s.
10
Calculate the PE the cart lost and the KE it gained at each height.
Convert each height to meters (cm ÷ 100). Then PE lost = mgh and KE gained = ½mv². Finish with the percentage: KE ÷ PE × 100.
11
Check one row.
Enter the mass, height, and speed from one of your rows below.
12
Find the missing energy.
If your percentages came out under 100%, some PE went somewhere other than the cart’s forward motion. Brainstorm where, then answer in your notebook.
📓 Physics Notebook
Name at least two places the missing energy could have gone. Which one do you think took the most, and why? (Think about the wheels, the ramp surface, the air, and your timing.)
13
Predict it: what if nothing went missing?
With no friction, all the PE becomes KE: mgh = ½mv². Solve for the speed. Watch what happens to the mass.
Predict It
Frictionless drops. Use g = 9.8 N/kg.
✅ Lab check: Section 1 table complete, one row verified with the Swap Checker, and all three Predict It rows green. Return the masses and spring scales.
Question 1
Use your Section 1 data to explain why the work you do lifting an object at a steady speed equals mgh, and where that energy is now.
Question 2
Compare how PE grows with height to how KE grows with speed. Use a specific example with numbers.
Question 3
Explain why the energy released by a falling object doesn’t depend on where you choose the zero level.
Question 4
In Section 4, the cart’s KE was less than the PE it lost. Where did the rest go, and which of those losses could you reduce when you build the pendulum in Project 4-1-4?
📤 Turn In
- Completed Section 1 lifting table, with your note comparing F × h to mgh
- Your gut ranking from Step 5 and your PE Builder results
- Your notebook answer on how PE scales differently from KE
- Completed Section 4 table (PE lost, KE gained, and percentage) with your list of where the missing energy went
- Written answers to the four Conclusion questions