Module 9 · Answers
Answers & explanations
Section A — Easy
01
speed = distance ÷ time
02
(a) 5 m/s (b) 30 km/h (c) 150 m
03
pressure = force ÷ area, in Pa (= N/m²)
04
The object is stationary — its distance from the start isn't changing.
05
moment = force × perpendicular distance from pivot (Nm)
Section B — Medium
06
72 km/h, which is 20 m/s
144 ÷ 2 = 72 km/h.
72 km/h × 1000 ÷ 3600 = 20 m/s.
72 km/h × 1000 ÷ 3600 = 20 m/s.
07
200 000 Pa (200 kPa) per foot
Force per foot = 40 000 ÷ 4 = 10 000 N.
Pressure = 10 000 ÷ 0.05 = 200 000 Pa.
Pressure = 10 000 ÷ 0.05 = 200 000 Pa.
08
A sharp knife has a smaller blade area, so the same force is concentrated into a much higher pressure on the food, allowing it to cut.
09
1 m
25 × 1.6 = 40 × d → 40 = 40d → d = 1.
10
When warmed, the gas particles inside move faster. They hit the walls more often and harder, so pressure inside the bottle rises.
Section C — Hard
11
0–10s: rising line from (0,0) to (10,20) — walking at 2 m/s.
10–20s: flat line at distance 20m — stationary.
20–30s: steeper rising line from (20,20) to (30,50) — running at 3 m/s.
30–40s: line going down from (30,50) to (40,40) — walking back at 1 m/s.
10–20s: flat line at distance 20m — stationary.
20–30s: steeper rising line from (20,20) to (30,50) — running at 3 m/s.
30–40s: line going down from (30,50) to (40,40) — walking back at 1 m/s.
12
Pressure in a liquid increases with depth, so the water at the bottom of the dam pushes much harder than the water near the top. The wall must be thicker at the bottom to withstand the higher pressure.
13
20 Nm
25 cm = 0.25 m. Moment = 80 × 0.25.
14
Moment = force × distance from pivot. Pushing close to the hinge means a small distance, so for the same force there's a small moment — not enough turning effect to overcome friction in the hinge. Pushing at the handle gives a much bigger moment for the same force.