Quick answer: A resultant force is the single force that has the same effect as all the forces acting on an object. This Physics 9-10 lab on resultant force gives the aim, the equipment, the safety points, a step-by-step method, sample results and 8 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
Friction is the force that makes a sliding block need a steady pull. By dragging a block at constant speed, you use balanced forces to measure friction, and you test how it changes as the block gets heavier.
Level: Physics grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: GCSE P5
Aim
To find how the sliding friction force depends on the normal reaction, using the fact that the resultant force is zero when a block slides at constant speed.
Equipment
- Wooden block with a hook or loop of string
- Newton meter (0 to 10 N, resolution 0.1 N)
- Slotted masses of 0.20 kg each
- Flat bench with a smooth surface
- Balance, pencil and graph paper
Safety
Keep your feet clear of the masses. Place a tray or cloth on the floor under the bench edge so that a dropped mass cannot hit feet or break.
Method
- Weigh the block and note its mass. In this sample the block with its first mass is 0.20 kg in total.
- Attach the newton meter and pull the block horizontally along the bench at the slowest steady speed you can manage.
- Read the newton meter while the block moves at constant speed. This reading equals the friction force.
- Add 0.20 kg to the block, repeat, and continue up to 1.00 kg total. Calculate the weight of the block and masses as W = m g with g = 9.8 N/kg.
Results
Sample measurements:
| Total mass (kg) | Normal reaction R (N) | Pulling force at constant speed (N) |
|---|---|---|
| 0.20 | 1.96 | 0.8 |
| 0.40 | 3.92 | 1.6 |
| 0.60 | 5.88 | 2.4 |
| 0.80 | 7.84 | 3.1 |
| 1.00 | 9.80 | 4.0 |
Analysis questions
Question 1 Why the pull equals friction
Explain why the newton meter reading equals the friction force when the block moves at constant speed.Question 2 Normal reaction
The block rests on a level bench before it is pulled. State the normal reaction for the 0.60 kg total mass and explain how you know.Question 3 Friction divided by reaction
Calculate friction ÷ normal reaction for each row, to 3 decimal places, and find the mean.Question 4 Graph
Describe the graph of friction (y-axis) against normal reaction (x-axis) and find its gradient from the last point.Question 5 Prediction
Use your mean ratio to predict the friction force for a total mass of 1.20 kg.Question 6 The 0.80 kg result
The 0.80 kg result gives a lower ratio than the others. Decide whether this is a real difference.Question 7 Uncertainty in the reading
The newton meter can be read to ±0.05 N. Calculate the percentage uncertainty in the 0.8 N and the 4.0 N readings.Question 8 Better friction readings
Propose two changes to the procedure that would give friction values closer to the true ones and with less scatter.
