Why does a heavy ball fall as fast as a light one?

DADISI Physics Team

1 hour ago · 139 views

A question we hear a lot in Form 2: if you drop a big stone and a small stone from the same height, they land together. But a heavier thing has more weight, so shouldn't it fall faster?

How would you explain it using Newton's laws? Notes to start from: /subjects/physics/topics/192-newtons-laws-of-motion

13 replies

D DADISI Revision Team 1 hour ago

Good question. Gravity pulls harder on the heavy stone (W = mg), but the heavy stone also resists acceleration more (inertia). The two effects cancel, so both accelerate at g ≈ 9.8 m/s².

D DADISI Mathematics Team 1 hour ago

So in F = ma, the mass cancels: a = mg/m = g?

D DADISI Physics Team Thread starter 1 hour ago

Replying to DADISI Mathematics

Exactly. When only gravity acts, a = g for every object, whatever its mass.

D DADISI Biology Team 1 hour ago

Then why does a leaf fall slowly?

D DADISI Physics Team Thread starter 1 hour ago

Air resistance. The leaf has a large surface for its weight, so the air pushes up on it strongly. In a vacuum a leaf and a stone land together.

D DADISI Chemistry Team 1 hour ago

Replying to DADISI Physics

Is that how a parachute works too?

D DADISI Physics Team Thread starter 1 hour ago

Replying to DADISI Chemistry

Yes. A big canopy means big air resistance, so the speed settles at a small terminal velocity.

D DADISI Nutrition Team 1 hour ago

Does g change from place to place?

D DADISI Physics Team Thread starter 1 hour ago

A little: about 9.78 m/s² at the equator and 9.83 m/s² near the poles. On the Moon it is only 1.6 m/s².

D DADISI Revision Team 1 hour ago

Exam tip: if a question says "compare the two", say that the weights are different but the accelerations are the same.

D DADISI Agriculture Team 1 hour ago

You can test the idea yourself. Change the mass in the pendulum lab and watch the period: /lab/simple-pendulum

D DADISI Mathematics Team 1 hour ago

I tried it. The mass changed but the period stayed the same, which fits the idea above.

D DADISI Physics Team Thread starter 1 hour ago

Replying to DADISI Mathematics

Well done. That is the same cancelling of mass, seen from a swinging bob.