Topic 2 of 8 · Form 5
Forces, energy, deformation of solids
Moments, couples and torque; equilibrium conditions; centre of gravity; upthrust (Archimedes' principle); drag and viscous forces.
Notes for this topic 1 file, in order
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Forces and turning effects
Moments, couples and torque; equilibrium conditions; centre of gravity; upthrust (Archimedes' principle); drag and viscous forces.
Work, energy and power
W = Fs cos θ; Ek = ½mv²; ΔEp = mgΔh; efficiency; P = Fv.
Worked example: A car of mass 900 kg reaches 20 m s⁻¹ from rest in 10 s on a level road. Find the average output power if 20% of the energy is lost.
Ek = ½ × 900 × 400 = 180 000 J; useful power = 18 000 W; input = 18 000 ÷ 0.8 = 22 500 W
Pressure
p = F/A; p = ρgh for fluids.
Deformation of solids
Hooke's law F = kx; springs in series and parallel; stress σ = F/A; strain ε = x/L; Young modulus E = σ/ε; elastic and plastic deformation; elastic potential energy = ½Fx (area under the force–extension graph).
Worked example: A wire of length 2.0 m and diameter 0.50 mm extends by 1.6 mm under 40 N. Find the Young modulus.
A = π(0.25 × 10⁻³)² = 1.96 × 10⁻⁷ m²; σ = 2.04 × 10⁸ Pa; ε = 8.0 × 10⁻⁴ → E = 2.55 × 10¹¹ Pa
Check yourself
- Two identical springs (k = 200 N m⁻¹) are connected in parallel. Find the combined spring constant.
- What is the condition for a body to be in equilibrium?
- A 50 N load stretches a spring by 0.04 m. Find the elastic potential energy.
Show answers
- 400 N m⁻¹
- Zero resultant force and zero resultant torque.
- 1.0 J
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