The method and the results are on the resultant force lab. Do the analysis yourself first, then check the answers below.
Quick answer: A resultant force is the single force that has the same effect as all the forces acting on an object. These answers go with the Physics 9-10 lab on resultant force: the graph, the calculations, the uncertainties and the conclusion explained, for all 8 analysis questions.
The model answers use the sample table. Your own values will differ a little, but friction should be roughly proportional to the normal reaction, giving a straight line through the origin.
Level: Physics grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: GCSE P5
Question 1 Why the pull equals friction
At constant velocity the resultant force on the block is zero. The pull forward is therefore balanced by an equal friction force backwards, so the reading equals the friction.
Question 2 Normal reaction
The block is at rest vertically, so the reaction equals the weight: R = m g = 0.60 × 9.8 = 5.88 N.
Question 3 Friction divided by reaction
0.8 ÷ 1.96 = 0.408; 1.6 ÷ 3.92 = 0.408; 2.4 ÷ 5.88 = 0.408; 3.1 ÷ 7.84 = 0.395; 4.0 ÷ 9.80 = 0.408. Mean = 2.028 ÷ 5 = 0.406.
Question 4 Graph
The points lie on a straight line through the origin, so friction is directly proportional to the normal reaction. Gradient = 4.0 ÷ 9.80 = 0.41.
Question 5 Prediction
R = 1.20 × 9.8 = 11.76 N. Friction = 0.406 × 11.76 = 4.77, so about 4.8 N.
Question 6 The 0.80 kg result
The expected friction is 0.406 × 7.84 = 3.18 N, so a reading of 3.1 N differs by about 0.1 N. That equals the resolution of the newton meter, so it is not a real difference.
Question 7 Uncertainty in the reading
0.8 N: 0.05 ÷ 0.8 × 100 = 6.3 %. 4.0 N: 0.05 ÷ 4.0 × 100 = 1.3 %.
Question 8 Better friction readings
1. Repeat each reading three times and average, to reduce random error caused by uneven pulling.
2. Use a data-logging force sensor, or a newton meter with finer divisions, and pull with a motor at steady speed to lower the uncertainty.
