Simple harmonic motion is a type of periodic motion where the restoring force is directly proportional to the displacement and acts in the direction opposite to that of displacement.

Simple harmonic motion can serve as a mathematical model for a variety of motions, such as the oscillation of a spring. In addition, other phenomena can be approximated by simple harmonic motion, including the motion of a simple pendulum as well as molecular vibration. Simple harmonic motion is typified by the motion of a mass on a spring when it is subject to the linear elastic restoring force given by Hooke’s Law. The motion is sinusoidal in time and demonstrates a single resonant frequency. For simple harmonic motion to be an accurate model for a pendulum, the net force on the object at the end of the pendulum must be proportional to the displacement. This will be a good approximation when the angle od swing is small.

Topical Notes, Topical Notes, Problems, Presentations, Quiz, Test, Investigations and Videos              
Simple Harmonic Motion
Mass on a spring
Pendulum and other oscillations                   
Newton’s law of gravity
Orbits of planets and satellites                                  
a) Circular
b) General
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Gravity is a force of attraction between masses. It provides the centripetal force needed to keep a satellite in orbit around a planet, or a planet in orbit around a star such as the Sun. Geostationary satellites are used for communications and stay in a fixed position above the Earth’s surface. Low polar orbit satellites are closer to the ground and are used for weather forecasting and imaging the Earth.

Planets and Earth

A moving object will continue to move in a straight line at the same speed unless a force acts on it. For an object to move in a circle, a force has to act on it all the time.  This force is called the centripetal force. It acts towards the centre of the circle. Gravity is the centripetal force that keeps planets moving around the Sun, and satellites moving around planets. Gravity is the universal force of attraction between masses. The greater the mass involved, the greater the force.

The force of gravity between two objects with small masses like a table and a chair is small because the objects are. However, large objects like planets have a much greater mass and so have a much greater gravitational force.