Slide vs Spin

Why it matters

Push a heavy box dead centre and it slides. Push the SAME box with the SAME force at a corner — and it spins on the spot. The force didn't change. What did?

The idea in plain words

The two motions a rigid body can have. Tap each.

translation · rotation

Why do you push a door at the handle, not near the hinge?

Same push, biggest turn — that's why handles live far from hinges

• sliding: translation — every point of the body moves the same way — force through the CG
• spinning: rotation — the body turns about an axis — force off the CG creates a moment
• the key idea: line of action — the line along which the force acts — its distance from the axis decides the turning
• The hinge is the axis of rotation
• Handle = maximum distance from the axis = maximum moment for the same force

Predict first

A force acts on a box along a line passing exactly through its centre of gravity. The box will…

Through the CG there is no moment arm — no turning. Off the CG, the same force gets a moment and the box rotates. Line of action is everything.

• slide without turning (pure translation) — correct
• spin without moving
• both slide and spin

What you do

Push the box both ways and watch the two motions. Same force, different line of action.

Check yourself

A wheel spinning on a fixed axle shows…

Its points move in circles about the axle: rotation. A sliding box is translation; a rolling wheel is both.

To rotate a body, the applied force must…

A tiny force with a big moment arm out-turns a huge force through the axis (zero arm, zero moment).

A rolling football has…

Rolling is the combination: the centre travels while the ball turns about it.

• pure rotational motion — correct
• pure translational motion
• no motion — it stays in place
• act along a line NOT passing through the axis / CG — correct
• be very large
• act for a long time
• translation of its centre AND rotation about it — correct
• only rotation
• only translation