ScientistGrades 10–12⏱ 20 min

Conservation of energy

Send a skater down a ramp and watch energy change between motion, height and heat, while the total never changes.

Standards: HS-PS3-1HS-PS3-2

1Wonder

A roller coaster has no engine after the first hill, yet it races through loops and climbs more hills. Where does it get the energy?

Energy skate park

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Turn your phone sideways, or open this lesson on a tablet or computer. The explanation and quiz below work here.

Speed 0 m/s Height 2.96 m KE + PE + Heat = 1.45 kJ

Track

Skater mass m 50 kg Friction

Try this

Watch the bars as the skater rolls. When is the motion energy biggest? The height energy?
Without friction, does the skater ever go higher than the dashed start line?
Turn on friction. Where does the lost energy go? Does the total change?
Change the mass. Does the skater go faster or slower? What about the energy?
On the hill track, drag the skater low on the left. Can they get over the hill?

Step 1 of 5

As the skater rolls, energy keeps changing between height energy (potential, PE) and motion energy (kinetic, KE). The total stays the same.

Conservation of energy

\htmlClass{c-red}{\tfrac{1}{2}mv^2} + \htmlClass{c-blue}{mgh} + \htmlClass{c-amber}{E_{\text{heat}}} = \text{constant}

Energy cannot be created or destroyed; it only changes form.

Question 1 of 5

A 50 kg skater stands 2 m above the ground (g = 9.8 m/s²). How much potential energy is that?

Energy is conserved: it changes form but the total stays the same.
Potential energy $PE = mgh$ depends on height; kinetic energy $KE = \tfrac12 mv^2$ depends on speed.
Without friction, a skater can rise back to exactly the starting height.
Speed at the bottom: $v = \sqrt{2gh}$ , independent of mass.
Friction turns mechanical energy into heat.