IGCSE Physics / Work and power
Linking GPE, KE and work
Edexcel International GCSE Physics (4PH1), chapter 16: Work and power, specification point 4.15, 4.3. Free lesson.
Read the revision notes · IGCSE Physics past papers
Gravitational potential energy (GPE) can be transferred to kinetic energy (KE) as an object falls. If friction is present, some energy is transferred to the thermal store of the surroundings, so less becomes KE.
How energy moves on a roller coaster
At the top of a hill, a roller coaster car is almost still, so most of its energy is in its gravitational store. As it rolls down, it loses height and speeds up. This transfers energy to its kinetic store. Climbing the next hill, it slows down as energy transfers back to the gravitational store. At the bottom of a dip, the car is lowest and fastest, so its kinetic store is biggest. With friction and air resistance, some energy transfers to the thermal store of the track and air. This leaves less energy for the next hill, so it is lower.
Finding speed after a fall
If there is no air resistance, all the GPE lost becomes KE gained. Using GPE = m × g × h and KE = 1/2 × m × v² gives m × g × h = 1/2 × m × v². The mass cancels, so the speed depends on the drop height, not the object's mass. Rearranging gives v = √(2 × g × h). For a 20 m drop with g = 10 N/kg: v = √(2 × 10 × 20) = 20 m/s. Without air resistance, heavy and light objects land at the same speed.
Calculating speed from GPE lost
Set out the energy transfer one store at a time. First find the GPE lost. With no air resistance, this equals the KE gained. Then substitute into the kinetic energy equation and solve for the speed. For a dropped object, GPE lost = m × g × h and KE gained = 1/2 × m × v².
Finding work done against friction
When friction acts, the GPE lost is shared between the KE gained and the work done against friction. The work is the energy that did not become kinetic energy, and it ends up in the thermal store of the surroundings. Work done against friction = GPE lost − KE gained.
Finding the greatest height after a throw
At the greatest height, the ball is momentarily still. With no air resistance, all its initial KE has become GPE. The mass cancels, so use the initial speed and g to find the height. For an initial speed of 6 m/s, half of 6² is 18. Then 18 ÷ 10 = 1.8 m.