Choose the type at the top: 🧲 elastic (two balls bounce apart, no kinetic energy is lost) ·
🩹 inelastic (they stick together, kinetic energy is lost) · 💥 explosion (a spring pushes them apart from rest).
Changing the type keeps the masses and velocities, so you can compare the types.
Above the scene, choose how B moves before the collision: ● at rest · →→ same direction (A catches up with B) ·
→← opposite direction (towards A, head-on). The masses and A's speed stay; you can still type any velocity in the boxes.
Pick an example (class demo, slide problems, skaters, cannon) or set the masses and velocities yourself. Velocities have a sign:
+ is to the right. Set a value with ▼ ▲ or type it in the box.
Set each value with ▼ and ▲ (hold to go faster; the ↑ ↓ keys or the mouse wheel in the box also work, Shift = ×10), or type a value in the box. ↺ puts it back to its start value. Outside the classroom range the box turns amber ⚠: fine to explore, but the lab can't do this.
Move the pointer over any graph (or tap it): a dashed line shows the values where it cuts each curve, or the values of the nearest data point.
Press ▶ Start. The contact lasts only 0.01 s — almost instant, like hard balls. ◂ / ▸ (or the ← → keys) step through it in 1 ms steps and pause.
The middle column shows the free-body diagrams of A and B during the contact — the two contact forces are a Newton's third law
pair: equal in size, opposite in direction, for the same time — and a bar chart of the momentum before and after.
After the contact the largest pair stays on the diagram, faded.
Graphs: p–t shows that the total momentum stays the same. On F–t the Area tool gives the impulse on
each body (equal and opposite). KE–t shows what happens to the kinetic energy A.3 link.
● Record saves a trial; the Data tab plots momentum after against momentum before (and KE after against KE before).
Work through the Tasks tab. Write explanations on your worksheet.
The physics
p = mv · Σpbefore = Σpafter when no external resultant force acts (isolated system)
The panel under the scene works out Σp before and Σp after with the numbers of the run (inelastic: they move on
together, so Σp after = (mA + mB)v).
Why: during the contact FA on B = −FB on A for the same Δt, so the impulses are equal
and opposite: ΔpA = −ΔpB.
Inelastic (they stick together): mAuA + mBuB = (mA + mB)v.
Explosion from rest: 0 = mAvA + mBvB (recoil).
A.3 linkEk = ½mv² = p²/2m. Elastic: total KE the same before and after.
Inelastic (they stick together): some KE becomes internal energy, sound….
Explosion: KE increases (from the spring's elastic energy, or chemical energy). Momentum is conserved in all of them.
Model: frictionless surface (no track: the bench is smooth enough over a short time). The bodies act like a spring that
gives back all of the relative speed (elastic) or none of it (inelastic: they stick); the contact lasts 0.01 s (almost instant, for every body).
Explosion: a spring plunger releases over 0.1 s. Bodies are drawn the same size whatever their mass; the scale bar gives distances.
Newton's laws
The badge above the scene says which law applies at this moment: 1st law — at rest or moving at a constant velocity, the resultant force is zero (ΣF = 0); 2nd law — accelerating, ΣF = ma. The table under the free-body diagram adds up the forces (ΣF) and compares them with m a (▾ folds it). The two purple contact forces in the free-body diagrams are a 3rd-law pair: the same size and type, in the opposite direction, acting on the other body. During the 0.01 s hit they are far bigger than W and FN; when W and FN would be too short to see they are drawn dashed (not to scale — read their values).