Motion Energy: ½mv²

Lesson 3 of 10 · 7 min · Free, no account needed

What you will learn

Motion energy (kinetic energy) is KE = ½ × m × v²: mass in kg, speed in m/s, energy in joules.
Speed is squared: twice the speed means 4 times the energy, three times the speed means 9 times.
Brakes have to remove that energy, so braking distance also grows with speed squared.

Driving at 60 instead of 30 km/h. Is the braking distance twice as long?
Send the car into the boxes at 10, 20 and 30 km/h.
Each box takes the same energy to knock over, so boxes count the car's motion energy.
Put your results on the graph: drag each dot to its number of boxes.
Motion energy has a formula too.
Kinetic energy, or motion energy: KE = ½ × m × v². m is the mass in kg, v the speed in m/s, KE in joules.
The mass counts once, the speed is squared. A car at 20 m/s carries 4 times the energy of the same car at 10 m/s.
To stop, the brakes must turn all of it into heat. With a steady braking force the stopping distance grows like v² too. Simplified model: no reaction time.
Game: Braking test. Put the cone where each car will stop.
At 30 km/h this car stops in 5 m. Three cars, as few misses as you can.
A slow heavy truck or a fast light car. Which carries more motion energy?
Tap your pick, then Check.
Math: KE = ½mv² is a parabola, y = ax². Tap the point with 4 times the energy of the green one.
At 30 km/h this car stops in 5 m. Where does it stop from 90 km/h?
Drag the marker, then Check.
A crossing 10 m ahead. One driver at 30 km/h, one at 50. Press Play.

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