Speeding Up, Slowing Down

Why it matters

Throw a ball straight up at 30 m/s and gravity starts a countdown: 30, 20, 10, 0 — one number smaller every second. It's the most reliable metronome on Earth.

The idea in plain words

The rate of the rate. Tap the terms.

a = (v − u) ÷ t

A car goes from 10 m/s to 25 m/s in 5 s. Acceleration?

3 m/s²

• m/s²: a — acceleration — change in VELOCITY per second
• m/s²: retardation — negative acceleration — slowing down (brakes, upward throws)
• the constant: g — 9.8 ≈ 10 m/s² downward for EVERYTHING, feather to hammer (in vacuum)
• the setup: free fall — motion under gravity alone — u = 0 when dropped
• a = (25 − 10) ÷ 5

Predict first

A ball thrown up at 30 m/s (g = 10 m/s²) reaches its top after…

Gravity strips 10 m/s every second: 30 → 20 → 10 → 0. Time to stop = u/g = 3 s. Then the countdown runs in reverse on the way down.

• 3 seconds — correct
• 30 seconds
• 10 seconds

What you do

Read the second-by-second speed strip, then throw a heavier ball and catch gravity not caring.

Check yourself

At the very TOP of the throw, the ball's velocity and acceleration are…

Gravity never pauses — that's exactly why the ball comes back down.

Dropped from rest, a stone's speed after 4 s (g = 10) is…

v = u + gt = 0 + 10×4 = 40 m/s — ten more every second.

A hammer and feather dropped together on the Moon…

Apollo 15 filmed exactly this. On Earth only AIR separates them, never gravity.

• v = 0 but a = g still (10 m/s² down) — correct
• both zero
• v = 0, a = 0 for an instant
• 40 m/s — correct
• 10 m/s
• 80 m/s
• land together — same g, no air — correct
• hammer lands first
• feather lands first