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Physics of Diving

When a high diver in a swimming event springs from the board and “tucks in”, a rapid spin result. Why is this?

Answer: This is the consequence of conservation of angular momentum.

The angular momentum of a body is the product of Moment of inertia (A measure of rotational inertia and it depends on the mass as well as distribution of mass about the axis of rotation. Farther the masses, greater will be the rotational inertia) and the angular velocity (The speed of rotation)

The angular momentum of a body remains unchanged in the absence of any external torque.

When the diver dives, he is giving his body a turning and takes off with his limbs stretched. In the stretched position, the moment of inertia is more. When he “tucks in”, the moment of inertia decreases. But since this happens without any external torque, it would result in an increase in angular velocity so as to keep the angular momentum constant.

Why gravitational force of attraction is necessary for a satellite to revolve around earth?

The question was posted by Raj Kumar.

The satellite has a velocity. To revolve around, the direction of velocity must change. In the absence of a force it can move only in a straight line. It is the gravitational force which helps the satellite top change direction and revolve around the earth.

In other words, for a satellite to revolve in a circular path, a centripetal force is required. This centripetal force is provided by the force of gravitation only.

A question from Magnetism

Rishi asked:

“It is said that an iron piece behaves as a magnet when an external magnetic field is applied. Then otherwise why doesn’t it behave as a magnet even though the earth’s magnetic field is acting on it.”

Ans:

Every molecule of iron possess magnetic behavior. But in the absence of an external strong magnetic field, the individual molecular magnets form closed loops or they are arranged in a random fashion in such a way that the net magnetic effect is zero.

Since the magnetic dipole moments of all the molecular magnets inside the iron add up to zero, the iron rod does not respond to earth’s magnetic field like a magnet.

Further, the strength of earth’s magnetic field is not so large to magnetize a piece of iron. The coercivity of iron is also very less (it can easily lose any acquired magnetism)

However, steel buried in North South direction is found to acquire some magnetism.

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