The method and the results are on the forces lab. Do the analysis yourself first, then check the answers below.
Quick answer: A force is a push or a pull, measured in newtons with a spring scale. These answers go with the grade 7 science lab on forces: the graph, the calculations, the uncertainties and the conclusion explained, for all 8 analysis questions.
These model answers use the sample table. Your own readings will differ, but the ranking of the surfaces should be the same and doubling the load should roughly double the friction.
Level: Grade 7 science (Year 8 · Class 7) · Curriculum: MS-PS2-1, MS-PS2-2
Question 1 Variables
1. The type of surface and the load on the block.
2. The force read on the scale (the friction force).
3. The pulling speed (steady and slow), or the contact area of the block.
Question 2 Why steady speed?
At a steady speed the net force is zero, so the pull and the friction are balanced and equal. If the block speeds up, the pull is bigger than friction and the reading would be too high.
Question 3 Ranking surfaces
Plastic (1.2 N), wood (2.0 N), carpet (2.8 N), sandpaper (3.3 N). The ranking from least to most friction is plastic, wood, carpet, sandpaper.
Question 4 Effect of load
Ratio for sandpaper = 6.5 ÷ 3.3 = 1.97, about 2. For every surface, doubling the load roughly doubles the friction.
Question 5 Friction compared with weight
0.50 kg: 2.0 ÷ 4.9 = 0.41. 1.00 kg: 4.1 ÷ 9.8 = 0.42.
The ratio is almost the same, so friction is proportional to the pressing force for a given surface.
Question 6 Prediction
1.5 kg is three times 0.50 kg, so friction ≈ 3 × 1.2 = 3.6 N.
Question 7 Uncertainty
0.1 ÷ 1.2 × 100 = 8 % (7.8 % unrounded).
The small readings have the largest percentage uncertainty, so differences between close values should be treated with care.
Question 8 Better friction measurements
Use a scale with finer divisions, check its zero before every reading and repeat more times. A motor-driven runner would keep the pulling speed the same each time.
