Laws of MotionIntroduction
Saturday evening in Aman's gully. Riya kicks the football and it shoots off down the lane. Aman throws a stone up to knock down a mango. A gust of wind bends the neem branches, and down at the river a boat drifts along with the current although nobody rows it.
In the last two chapters we described motion: positions, velocities, accelerations. We never asked what causes it. That is the question of this chapter. Every example above has some agent that acts on the body: a foot, a hand, the wind, the water. We say the agent exerts a force. A force is needed to set a body moving, and also to slow it down or stop it.
The agent does not always have to touch the body. A coconut that breaks off speeds up towards the ground although nothing touches it, and a magnet pulls an iron nail from a few centimetres away. Sort these forces: does each one need contact?
So a force needs an agent, and the agent may or may not touch the body. A force starts a resting body moving and stops a moving one. But what about a body that is already moving steadily, like a skater gliding in a straight line on smooth ice? Does it need a force to keep going?
The Big Question
Riya glides across a frozen pond in a straight line at a steady speed. Before you read on, make a guess. To keep her gliding at the same speed, what does she need?
If that surprised you, you are in good company: getting it right took from Aristotle to Galileo. In this chapter we follow that story to Newton's three laws of motion, then use them on momentum and its conservation, on a body held still by several forces, on friction and the other everyday forces, and on motion in a circle. We finish with a method that cracks almost any mechanics problem: the free-body diagram.