PHYSICS / FIRST STEPS

Physics

From bending light to inertia, discover the rules behind everyday events.

Start with everyday questions · 2 articles

HANDS-ON PHYSICS / NEW

After braking, what keeps moving?

Make a prediction, then test it on the inertia bench.

Enter the lab ↗

01 / Light and refraction

Why does a chopstick look bent in water?

The chopstick has not bent. The light has changed direction.

Start reading ↓02 / Forces and motion

Why do you lean forward when a vehicle brakes?

Objects tend to keep their existing state of motion. This property is called inertia.

Start reading ↓

Why does a chopstick look bent in water?

The chopstick has not bent. The light has changed direction.

When light travels obliquely from water into air, its speed changes and its direction usually changes too. This is refraction. We see the underwater part of a chopstick because light from it crosses the water surface and reaches our eyes.

Our brain traces incoming light back along a straight line to estimate where an object is. Refraction makes the submerged part appear shallower than it really is, so the parts above and below the surface seem misaligned.

Light travelling perpendicular to the water surface does not change direction there. The viewing angle matters: crossing between two media does not always make light bend.

Think about it: viewed obliquely from the bank, does the bottom of a pool appear deeper or shallower?

Usually shallower. This is one reason not to judge water depth by appearance alone.

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Why do you lean forward when a vehicle brakes?

Objects tend to keep their existing state of motion. This property is called inertia.

Open the inertia lab ↗

When a car moves forward, its passengers move forward too. Braking slows the car, while the body tends to keep its previous velocity. Relative to the car, the passenger therefore leans forward.

A seat belt exerts a force that slows the body. Inertia is not a force pushing someone forward; it is a property of all objects. Greater mass means greater resistance to a change in velocity.

Newton’s first law states that an object stays at rest or moves in a straight line at constant speed when the net force on it is zero. Continued motion does not require a continued push. Changing speed or direction requires a nonzero net force.

Think about it: when a stationary bus suddenly accelerates forward, which way does a standing passenger lean relative to the bus?

Backward. The feet move with the bus while the upper body tends to remain at rest. Sit securely or hold on; do not perform experiments in a moving vehicle.

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