NCERT Book Class 11 Physics Chapter 4 Laws of Motion

CHAPTER FOUR

LAWS OF MOTION

4.1 INTRODUCTION
In the preceding Chapter, our concern was to describe the
motion of a particle in space quantitatively. We saw that
4.1 Introduction uniform motion needs the concept of velocity alone whereas
4.2 Aristotle’s fallacy non-uniform motion requires the concept of acceleration in
4.3 The law of inertia addition. So far, we have not asked the question as to what
4.4 Newton’s first law of motion governs the motion of bodies. In this chapter, we turn to this
4.5 Newton’s second law of
basic question.
motion Let us first guess the answer based on our common
4.6 Newton’s third law of motion experience. To move a football at rest, someone must kick it.
To throw a stone upwards, one has to give it an upward
4.7 Conservation of momentum
push. A breeze causes the branches of a tree to swing; a
4.8 Equilibrium of a particle strong wind can even move heavy objects. A boat moves in a
4.9 Common forces in mechanics flowing river without anyone rowing it. Clearly, some external
4.10 Circular motion agency is needed to provide force to move a body from rest.
4.11 Solving problems in Likewise, an external force is needed also to retard or stop
mechanics motion. You can stop a ball rolling down an inclined plane by
Summary applying a force against the direction of its motion.
Points to ponder In these examples, the external agency of force (hands,
Exercises wind, stream, etc) is in contact with the object. This is not
always necessary. A stone released from the top of a building
accelerates downward due to the gravitational pull of the
earth. A bar magnet can attract an iron nail from a distance.
This shows that external agencies (e.g. gravitational and
magnetic forces ) can exert force on a body even from a
distance.
In short, a force is required to put a stationary body in
motion or stop a moving body, and some external agency is
needed to provide this force. The external agency may or may
not be in contact with the body.
So far so good. But what if a body is moving uniformly (e.g.
a skater moving straight with constant speed on a horizontal
ice slab)? Is an external force required to keep a body in
uniform motion?