What went wrong when the king rode under the arch?

It was too low and knocked off his crown

How does the chain of blame unfold in the poem?

Each accused person blames another, ending with the king's own changes

State Newton's first law of motion.

A body continues in its state of rest or of uniform motion in a straight line unless compelled by a net external force to change that state.

State Newton's second law of motion.

The rate of change of linear momentum of a body is proportional to the applied force and is in its direction: \(F=\mathrm{d}p/\mathrm{d}t=ma\).

Define impulse and relate it to momentum.

Impulse is force multiplied by the time of action; it equals the change in momentum, \(J=F\,\Delta t=\Delta p\).

State Newton's third law of motion.

To every action there is always an equal and opposite reaction; the two forces act on two different bodies.

State the law of conservation of linear momentum.

When no net external force acts, the total linear momentum of a system is conserved, as seen in the recoil of a gun.

What was Aristotle's fallacy?

The mistaken belief that a constant external force is needed to keep a body moving at constant velocity, ignoring friction.

State the laws of static and kinetic friction.

Static friction is self-adjusting up to a maximum \(f_s\le\mu_s N\); kinetic friction is constant, \(f_k=\mu_k N\), and \(\mu_k<\mu_s\).

What provides the centripetal force for a car on a level circular road?

The force of static friction between the tyres and the road provides the required centripetal force.

Why are roads banked at curves?

Banking lets the horizontal component of the normal reaction supply centripetal force, reducing dependence on friction and allowing higher safe speeds.

What is a free-body diagram?

A diagram showing a chosen system with all the external forces acting on it, used to apply the laws of motion systematically.

Laws of Motion

Q: What provides the centripetal force for a car on a level circular road?

The force of static friction between the tyres and the road provides the required centripetal force.

Q: Why are roads banked at curves?

Banking lets the horizontal component of the normal reaction supply centripetal force, reducing dependence on friction and allowing higher safe speeds.

Q: What is a free-body diagram?

A diagram showing a chosen system with all the external forces acting on it, used to apply the laws of motion systematically.

Class 11 Physics · Physics Part I · Chapter 4

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Summary

This chapter explains what governs motion through Newton's three laws. Aristotle wrongly believed a constant force is needed to keep a body moving; Galileo's study of inclined planes led instead to the law of inertia. Newton's first law states that a body continues in rest or uniform motion unless acted upon by a net external force, defining inertia and force. The second law states that the rate of change of momentum equals the applied force, \(F=\mathrm{d}p/\mathrm{d}t=ma\), giving the newton as the SI unit of force; impulse equals the change in momentum. The third law states that to every action there is an equal and opposite reaction, the two forces acting on different bodies. From the second and third laws follows the law of conservation of linear momentum for an isolated system, illustrated by recoil of a gun. Equilibrium of a particle requires the net force to be zero. Common forces in mechanics include gravity, normal reaction, tension and friction; static, kinetic and rolling friction are governed by \(f_s\le\mu_s N\) and \(f_k=\mu_k N\). For circular motion the centripetal force is provided by friction or by banking of roads, and free-body diagrams are used to solve mechanics problems systematically.

Aristotle's fallacy and the law of inertiaNewton's first law of motionNewton's second law of motionNewton's third law of motionConservation of momentumEquilibrium of a particleCommon forces in mechanics and frictionCircular motion and banking of roads

Key terms

Inertia

The tendency of a body to remain in its state of rest or uniform motion unless acted upon by an external force.

Momentum

The product of mass and velocity of a body, \(p=mv\); a vector quantity whose rate of change equals the applied force.

Impulse

The product of force and the time for which it acts, equal to the change in momentum it produces.

Newton's third law

To every action there is an equal and opposite reaction, the two forces acting on different bodies.

Conservation of momentum

In the absence of an external force, the total linear momentum of a system remains constant.

Friction

The force that opposes relative motion or tendency of motion between surfaces in contact.

Extra questions & answers

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Newton's first law of motion is also called the law of:

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