Spark & Anvil Privacy

Teach & case

Help a near-peer reason through the idea — when you explain it, you understand it best — then read the real decision it comes from.

A case · you decide

Torin and the probe that coasts to Mars

Torin — a student on a mission-design team arguing about whether a coasting spacecraft needs its engine on

The idea in play: Newton's laws — a net force CHANGES velocity; steady motion needs no forward push.

Torin's team is planning a probe that cruises to Mars. Someone insists the engine has to burn the whole trip "to keep it moving." Fuel for a months-long burn is impossible — so is that insistence even right?

In the near-vacuum between planets there's almost no friction and no air. Once the engine has brought the probe up to cruise speed, the only real forces are tiny gravitational tugs — the net force is essentially zero.

Newton's first law: with zero net force, velocity doesn't change. The probe keeps its speed and direction on its own. The engine is for CHANGING velocity — a burn to speed up, slow down, or turn — not for "holding" a speed.

Mission control shuts the engine off at cruise speed, mid-flight. Does the probe gradually slow down and stop?

A teammate wants a small continuous thrust "just to maintain cruise speed." Newton's-laws-wise, is that needed?

Predict the forces yourself in the Prediction Lab →

When two ideas meet: 2 cases with two people

A case · you decide

Colt and Suki: what a collision keeps

Colt & Suki — a student tracking momentum through a crash, and a friend tracking energy

The idea in play: momentum is conserved in every collision; kinetic energy is conserved only in elastic ones, and a sticky collision turns some of it into heat and sound.

A 2 kg cart moving at 3 m/s hits a 1 kg cart at rest. They stick together.

Colt adds up momentum: 2 × 3 = 6 kg·m/s before, shared by 3 kg afterwards.

Suki adds up kinetic energy: ½ × 2 × 3² = 9 J before.

How fast do the joined carts move?

Suki finds only ½ × 3 × 2² = 6 J of kinetic energy afterwards. Where did the other 3 J go?

Open the Prediction Lab →

A case · you decide

Torin and Bexley: nothing is pushing it

Torin & Bexley — a student who reads forces, and a friend who reads the motion graph

The idea in play: a flat velocity-time graph means zero acceleration, and zero acceleration means zero net force.

A hockey puck slides across smooth ice. Bexley's velocity-time graph is a flat line.

Most people's instinct says something must be pushing the puck forward to keep it moving.

What is the net force on the puck while the graph is flat?

Open the Prediction Lab →