Physics Essentials

Why things move, heat up, shine and spark

Physics Essentials is a Bertopia flashcard pack of 62 cards: why things move, heat up, shine and spark. It covers forces and motion; energy, work and power; waves, sound and light; electricity and magnetism; heat and matter; atoms, nuclei and space.

Sample cards

A hockey puck slides across smooth ice and keeps going. Why?

A forward force from the stick keeps pushing it along the ice
Moving objects naturally store up a force that slowly runs out
The ice pulls the puck forward with a small magnetic force
No net force acts on it, so its velocity stays constant

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Answer

No net force acts on it, so its velocity stays constant

Newton's first law: an object keeps its velocity unless a net force acts. Friction on ice is tiny, so the puck barely slows. No force is needed to keep it moving.

You hold a heavy box still for a minute. How much work do you do on it?

A lot, because holding a heavy weight is tiring
Work equal to its weight times one minute of holding
Work equal to its weight times its height above the floor
None, because it does not move

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Answer

None, because it does not move

Work is force times displacement in the direction of the force. No displacement means no work on the box, even though your muscles use energy.

How do sound waves differ from light waves?

Sound is transverse and light is longitudinal
Sound travels faster than light in air
Light needs air or another medium to travel through, but sound can cross a vacuum
Sound is longitudinal and needs a medium; light does not

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Answer

Sound is longitudinal and needs a medium; light does not

Sound is a pressure wave of particles vibrating along its direction of travel, so it cannot cross a vacuum. Light is an electromagnetic transverse wave.

What is electric current?

The energy stored in a battery
The rate of flow of charge
The force that pushes electrons
The resistance of a wire

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Answer

The rate of flow of charge

Current is charge per second, measured in amperes. Potential difference is energy per charge; resistance opposes current.

Why does a metal spoon in hot soup heat up faster than a wooden one?

Metal has a lower melting point than wood
Wood reflects all heat away from itself
Metal attracts heat by magnetism
Free electrons in metal carry energy quickly

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Answer

Free electrons in metal carry energy quickly

Metals have delocalised electrons that move and transfer energy rapidly, as well as lattice vibrations. Wood relies on slower vibrations alone.

Rutherford fired alpha particles at gold foil and most went straight through. What did that show?

Atoms are mostly empty space with a small dense nucleus
Atoms are solid spheres of positive charge with electrons dotted through them
Gold atoms have no electrons at all
Alpha particles cannot be deflected by anything

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Answer

Atoms are mostly empty space with a small dense nucleus

Most passed straight through, but a few bounced back sharply, implying a tiny, massive, positive nucleus. This replaced the plum pudding model.

Sources

2 sources

OpenStax College Physics 2e, OpenStax, Rice University
NIST Reference on Constants, Units and Uncertainty, National Institute of Standards and Technology

Physics Essentials

Sample cards

Sources

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