Q: What does IP Combined Science Notes (Lower Sec, Year 1-2): 08) Work, Energy, Power & Pressure cover? A: Apply work-energy relationships, efficiency, and pressure calculations in solids and fluids.
Energy never disappears - it changes form. Track these transformations with equations, careful unit usage, and thought-out assumptions.
Learning targets
Calculate work done, kinetic energy, and gravitational potential energy in SI units.
Relate power to rate of energy conversion and discuss efficiency improvements.
Apply pressure formulas in solids (force over area) and fluids (depth dependence).
Interpret hydraulic system diagrams and solve transmission-of-force problems.
1. Work & energy
Quantity
Formula
Unit
Work done
W=Fscosθ
J
Kinetic energy
Ek=21mv2
J
Gravitational potential energy
Ep=mgh
J
Assume θ=0∘ for force parallel to displacement unless stated otherwise.
Worked example - Roller coaster drop
A 450kg car descends 25m.
Loss in Ep: 450×9.81×25=1.10×105,J.
Neglecting losses, this becomes kinetic energy. Solve for v:
21×450×v2=1.10×105⇒v=22.2,m⋅s−1.
Discuss real-world losses (air resistance, friction) reducing final speed.
2. Power & efficiency
P=tW=Fv.
Efficiency:
η=Input energyUseful output energy×100%.
Appliance example
An electric kettle rated 2.4kW heats 1.2kg of water from 22∘C to 100∘C in 4.0min.
Energy absorbed by water:
Q=mcΔT=1.2×4.18×(100−22)=392,kJ.
Electrical energy input:
E=Pt=2.4×103×(4.0×60)=576,kJ.
Efficiency =576392×100=68.1%.
3. Pressure in solids
P=AF.
Example - Snowshoes
A person of mass 68kg stands on snowshoes with total area 0.32m2.
F=mg=68×9.81=667,N.
P=0.32667=2.08×103,Pa.
Compare with regular shoes area 0.08m2 to highlight fourfold increase in pressure.
4. Pressure in fluids
Hydrostatic pressure at depth h:
P=ρgh.
Example: Water density 1000kg⋅m−3, depth 1.5m:
P=1000×9.81×1.5=1.47×104,Pa.
Hydraulic lift
If master piston area 50cm2 and slave piston area 500cm2:
Mechanical advantage of 10 achieved assuming negligible losses.
Try it yourself
A 65kg hiker climbs 420m in 35min. Determine change in gravitational potential energy and average useful power output.
Two people push a crate with force 210N over 12m, with 5% of work lost to heat. Calculate useful work delivered to the crate.
A hydraulic press has a large piston diameter twice that of the small piston. If 80N is applied on the small piston, find the output force (ignore losses) and describe two practical limitations of this model.
