Topic 2 of 8 · Form 3
Forces, mass, weight, density and momentum
Weight W = mg (g ≈ 9.8 N/kg on Earth). Mass is constant; weight depends on the gravitational field.
Notes for this topic 1 file, in order
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Mass and weight
Weight W = mg (g ≈ 9.8 N/kg on Earth). Mass is constant; weight depends on the gravitational field.
Density
ρ = m/V. Measuring density of regular solids, irregular solids (displacement) and liquids. Objects float if less dense than the fluid.
Forces
Effects of forces: change speed, direction or shape. Resultant force; Newton's laws; F = ma; friction and drag; terminal velocity.
Hooke's law
F = kx up to the limit of proportionality; load–extension graphs.
Momentum (p = mv)
Impulse = FΔt = Δ(mv); conservation of momentum in collisions and explosions.
Worked example: A 2 kg trolley moving at 3 m/s collides with and sticks to a stationary 1 kg trolley. Find their velocity.
2 × 3 = 3v → v = 2 m/s
Turning effects
Moment = force × perpendicular distance; principle of moments; centre of gravity; stability.
Worked example: A 30 N force acts 0.4 m from a pivot. Find the moment.
30 × 0.4 = 12 N m
Check yourself
- Find the weight of a 50 kg student on Earth (g = 9.8 N/kg).
- A block of volume 20 cm³ has mass 54 g. Find its density.
- A resultant force of 12 N acts on a 4 kg mass. Find the acceleration.
Show answers
- 490 N
- 2.7 g/cm³
- 3 m/s²
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