Two days, one question: when a force acts on an object, how much does it change the object’s motion? On Day 1 you build the idea of work and measure it. On Day 2 you use it to work out how fast a sliding block started, just from how far it slid.
🎯 Goals
Decide whether a force is doing work by checking whether the object moves along (not across) that force.
Measure a force and a distance, then calculate work in joules.
Use the angle between a force and the motion to find how much of a pull actually does work.
Use friction’s work on a sliding block to find the energy it started with, then its starting speed. That is the work-energy theorem at work.
🧰 Materials
📏Spring scale (Newton meter), 10 N or 20 N
🧱A wooden block or hardcover textbook with a loop of string attached
📐Meter stick or tape measure, plus masking tape for start and finish lines
⚖️A scale to find the block’s mass in kg (or mass provided by your teacher)
🧮Calculator
📓Physics notebook
DAY 1Build the idea of work
Section 1 of 5
Work or No Work?
1
Read the rule before you sort.
In physics, a force does work on an object only when the object moves and part of the force points along the motion (with it or against it).
Positive work: force and motion point the same way. The object speeds up. Negative work: force points against the motion. The object slows down. Zero work: no movement, or the force is at 90° to the motion.
2
Sort all seven situations.
For each one, decide whether the named force does positive, negative, or zero work. You get feedback right away. If you miss one, fix it before moving on.
1.You lift a backpack straight up onto a shelf.
Work done by: your lifting force
2.You carry that backpack across the room at a steady height.
Work done by: your lifting force
3.You push on a wall as hard as you can. The wall doesn’t move.
Work done by: your push on the wall
4.A book slides across a table and slows to a stop.
Work done by: friction
5.A ball falls from your hand to the floor.
Work done by: gravity
6.You slowly lower a heavy box to the ground.
Work done by: your lifting force
7.A pendulum bob swings along its curved path.
Work done by: the string’s tension
Correct so far: 0 of 7 · 7 still to answer
📓 Physics Notebook
Pick the situation that surprised you most. Explain in two or three sentences why the answer is what it is, using the words force and motion.
Section 2 of 5
Measure Work: Drag the Block
3
Set up a straight track.
Tape a start line on a smooth floor or lab table. Tape finish lines 1.0 m, 2.0 m, and 3.0 m from the start. Hook the spring scale to the block’s string.
4
Pull the block slowly and steadily from the start line to the 1.0 m line.
Keep the scale parallel to the floor. Read the scale while the block is moving at a steady speed. That reading is your force F. If the needle bounces, use the middle of the bounce.
5
Repeat for 2.0 m and 3.0 m with the same block, then add a load.
Put a second book on top of the block (the load), pull it 2.0 m, and record the new force. Calculate the work for every trial: W = F × d. One newton-meter is one joule (J).
Trial
Load on block?
Force F (N)
Distance d (m)
Work W = F × d (J)
📓 Physics Notebook
Look at your table. What happened to the work when you doubled the distance with the same force? What happened when the force went up and the distance stayed the same? Write the pattern as a sentence.
Section 3 of 5
Does the Angle Matter?
6
Feel the difference.
Drag the block a short way with the string flat along the floor. Then drag it with the string angled steeply upward. In which case does more of your pull go into moving the block forward? Why?
7
Meet the angle rule.
When the pull is at an angle θ to the motion, only part of it points along the motion. The work is W = F × d × cosθ. Use these values: cos 0° = 1.00, cos 30° = 0.87, cos 60° = 0.50, cos 90° = 0.
Angle Challenge: same pull, different angle
A wagon is pulled 5.0 m with a 20 N force. Only the angle of the handle changes. Use W = F × d × cosθ and enter the work in joules.
θ = 0°, cosθ = 1.00
θ = 30°, cosθ = 0.87
θ = 60°, cosθ = 0.50
θ = 90°, cosθ = 0.00
📓 Physics Notebook
At which angle did the pull do zero work? Connect that result to carrying a backpack across the room in Section 1.
✅ End of Day 1 check: sorter finished, Drag-the-Block table complete, Angle Challenge all green. Keep your block, scale, and masking tape lines for tomorrow.
DAY 2Use the idea: the work-energy theorem
Section 4 of 5
Slide & Stop Lab
8
Measure the friction force and the mass.
Pull the block at a slow, steady speed along the floor and read the scale three times. Average them. That average is the friction force F_f, because at steady speed your pull equals friction. Then find the block’s mass m in kilograms.
9
Give the block three pushes: gentle, medium, and firm.
Push the block along the floor so it slides across a start line and stops on its own. Measure from the start line to the front edge of the block. Record the stopping distance d for each push. Use the same floor surface the whole time.
⚠️ Slide the block along the floor. Never throw it. Keep feet and fingers clear and make sure nobody is standing in the sliding path.
10
Calculate the work friction did, and the energy the block started with.
Friction points against the motion, so its work is negative: W_f = −F_f × d. The block ends at rest, and only friction does net work (gravity and the floor push straight across the motion, so their work is zero). The theorem says W_net = KE_final − KE_start, which is −F_f × d = 0 − KE_start. So the energy it started with is KE_start = F_f × d.
Push
Stop distance d (m)
Friction force F_f (N)
Work by friction (J)
Starting KE (J)
11
Turn starting energy into starting speed.
Kinetic energy is KE = ½ m v², so v = √(2 × KE ÷ m). Calculate the starting speed for each push. Then check one row below.
Speed Checker
Do the math by hand first. Then enter one row from your table to see if your speed checks out.
12
Sense-check your speeds.
A brisk walk is about 1.5 m/s. Do your speeds match how hard you pushed? If one looks way off, look for a measurement slip before you blame the physics.
Section 5 of 5
Connect It: Who Does the Work?
13
Account for every force on the sliding block.
Sort the three forces that act on the block while it slides. Then explain in your notebook why friction alone sets the change in kinetic energy.
1.The block slides across the floor and stops.
Work done by: friction on the block
2.The block slides across the level floor.
Work done by: gravity on the block
3.The block slides across the level floor.
Work done by: the floor’s upward (normal) force
Correct so far: 0 of 3 · 3 still to answer
14
Look ahead: two Unit 4 builds.
Project 4-1-4 (Pendulum): you just sorted the string’s tension as zero work. If tension does nothing, which force speeds the bob up? Problem 4-3-1 (Emergency Slide): a rider slides down. Which forces do positive work, which do negative work, and which do none? Jot down your first guess.
Question 1
Explain why carrying a backpack across a room at a steady height is zero work, even though your arms get tired.
Question 2
Use your Drag-the-Block data: what happens to the work when the distance doubles at the same force? When the force doubles at the same distance?
Question 3
Use the work-energy theorem to explain how the distance the block slid is connected to the energy it started with.
Question 4
A car going twice as fast as another needs far more room to stop when both brake with the same force. Use KE = ½ m v² and W = F × d to explain why.
📤 Turn In
Notebook explanation for the Work-or-No-Work situation that surprised you most
Completed Drag-the-Block table, with your doubling-pattern sentence
Angle Challenge results and your note on the zero-work angle
Completed Slide & Stop table, including starting KE and starting speed for each push