Grade 11 · ⚛️ Physics · Mechanics · PHY-G11-M2.1
The coin and card — see inertia for yourself
Flick a card out from under a coin and watch Newton's first law drop it straight into a cup.
Why this matters
When a Bajaj brakes hard and your body lurches forward, that's Newton's first law — your body wants to keep moving. The same three laws let Ethiopian Airlines pilots calculate the thrust needed to lift a 787 off the runway at Bole.
Watch the demo
Video by Make Science Fun
The science
First law (inertia)
An object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted on by a net external force. The tendency to resist changes is called inertia, and it scales with mass.
Second law (F = ma)
The net force on an object equals its mass times its acceleration: F_net = m · a. A force acting for only a very short time produces only a small change in velocity.
Third law (action–reaction)
If A pushes B with force F, then B pushes A with force −F. The two forces act on different objects.
You'll need
- A small coin (1 Birr or any coin)
- A stiff card (playing card, ID card, or thick paper)
- A small empty cup or glass
Safety first
- Use a plastic cup if you have one, in case it tips over.
Step by step
0/4 doneWhat you should see
With a sharp flick, the coin drops straight into the cup. When you pull the card slowly, the coin moves with the card and falls off to the side.
Why it happens
The card moves so quickly that friction acts on the coin for only a tiny moment, so the coin barely gains any sideways velocity — its inertia keeps it almost still. Once the card is gone, gravity pulls the coin straight down. Pulling slowly lets friction act for longer, dragging the coin along.
Didn't work? Try this
The coin flies off with the card.
Flick faster and keep your finger moving parallel to the card. A smoother card (like a playing card) helps.
The cup tips over.
Hold the cup steady with your other hand, or use a heavier glass.
Still stuck? Ask the lab assistant
Check your understanding
1. Why does the coin fall into the cup instead of flying away with the card?
2. If you double the net force on an object, what happens to its acceleration (mass unchanged)?
Your lab notebook
Loading…