Will two children of different masses reach the bottom of the same slide with the same velocity?
Key idea
Work Done by a Constant Force
Work done = force applied × displacement in the direction of the force.
W = F × s
The SI unit of work is the joule (J). 1 joule = 1 newton of force displacing an object by 1 metre in the force's direction. Lifting 3 bags, or lifting 1 bag 3× as high, both do 3× the work.
Key idea
When Work Isn't Simply "Effort"
1
Zero work
No force (F = 0), no displacement (s = 0), or a force perpendicular to displacement — like carrying a box while walking — all give zero work, however tiring it feels.
→
2
Positive or negative
Force and displacement in the same direction: positive work (pushing a wheelchair forward). Opposite directions: negative work (a goalkeeper stopping a ball).
Key idea
The Work-Energy Theorem
An object with the capacity to do work is said to possess energy.
Work done on an object = change in its energy
A thrown ball gains energy from the work done throwing it — then transfers that energy, knocking over the wicket it hits. The SI unit of energy is also the joule (J).
Key idea
Forms of Energy
MechanicalMotion or position
ThermalMakes things warm or hot
SoundVibrations of air or other matter
ElectricalMotion or position of charges
NuclearStored in atomic nuclei
ChemicalStored in fuels and food
Key idea
Kinetic Energy
1
K = ½mv²
The energy an object has because it's moving. Doubling velocity quadruples kinetic energy — since v is squared.
→
2
Worked example
A 0.2 kg cricket ball bowled at 43 m/s: K = ½ × 0.2 × 43² = ½ × 0.2 × 1849 ≈ 184.9 J
Key idea
Potential Energy
Stored by deformation or positionA stretched slingshot, a bent bow, a compressed spring, separated magnets, or a raised ball above the Earth — all store energy that can later become motion.
U = mghGravitational potential energy at height h. Example: a 200 g ball 10 m up has U = 0.2 × 10 × 10 = 20 J (taking g = 10 m/s²).
Key idea
Conservation of Mechanical Energy
Mechanical energy = kinetic energy + potential energy. As an object falls freely, one converts into the other — their sum stays constant.
Pendulum position | Potential energy | Kinetic energy
P
released from the side — Maximum (mgh) — Zero
Q
swinging through the bottom — Zero — Maximum
R
rising on the far side — Maximum again — Zero
Key idea
Power
1
P = W ÷ t
Power is the rate of doing work. Running up the stairs and walking up slowly do the same work — but running needs far more power. SI unit: watt (W) = 1 J/s.
→
2
Worked example
A weightlifter lifts 75 kg by 2 m in 5 s: W = mgh = 75 × 10 × 2 = 1500 J P = 1500 J ÷ 5 s = 300 W
Key idea
Simple Machines and Mechanical Advantage
A machine can't reduce the total work needed
but it can change the size or direction of the force you apply.
Mechanical advantage = Load ÷ Effort
The load is the force to be overcome; the effort is the force you apply. A pulley, an inclined plane, and a lever are three of the simplest machines.
Key idea
The Pulley and the Inclined Plane
1
Fixed pulley
Changes the direction of your effort (pulling down instead of lifting up), but doesn't reduce it — mechanical advantage = 1. A movable pulley system can do better.
→
2
Inclined plane
Mechanical advantage = L ÷ h (ramp length ÷ height). A longer, gentler ramp needs less force — but you push it over a longer distance, so total work stays the same.
Key idea
The Lever
F₁ × d₁ = F₂ × d₂. A longer effort arm lets a small force move a much larger load — the total work done stays the same either way.
Chapter · Key terms to remember
Key Terms
Work (W = F × s)
Force times displacement in the force's direction.
Work-energy theorem
Work done on an object equals its change in energy.
Kinetic energy (K = ½mv²)
The energy of motion.
Chapter · Key terms to remember
More Key Terms
Potential energy (U = mgh)
Energy of position or deformation.
Mechanical energy
Kinetic energy + potential energy.
Power (P = W ÷ t)
The rate at which work is done.
Chapter · Key terms to remember
More Key Terms
Simple machine
A device changing the size/direction of a force.
Mechanical advantage
Load ÷ effort — how much a machine multiplies force.
Questions for your notebook
Write these down, then discuss
1
When a ball thrown upwards reaches its highest point, which of these are correct? (i) The force acting on the ball is zero. (ii) The acceleration of the ball is zero. (iii) Its kinetic energy is zero. (iv) Its potential energy is maximum.
2
Identify the energy transformation in each situation.
3
A crane lifts a mass m to the 10th floor in a certain time t. It then raises the same mass to the 20th floor in double the time (2t). How much more energy and power are required?
Project as-is — students copy the questions, then the class discusses answers together.