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CBSE NOTES CLASS 11 PHYSICS CHAPTER 5

NEWTONS LAWS OF MOTION

Newton’s 1st law or law of inertia

Inertia

Linear momentum

Newton’s second law of motion

Impulse

Principle of conservation of momentum

Equilibrium of a particle

Second law of motion is the real law of motion

First law of motion from second law of motion

Third law of motion from second law of motion

Free body diagrams

Common forces in mechanics

Non contact forces

Contact forces

Normal reaction forces (N or R)

Tension force in a string (T)

Spring force

Frictional forces

Static frictional force

Kinetic frictional force

Rolling frictional force

Laws of limiting friction

Solving problems related to forces

Force in a circular motion

Motion of a car on a level road

Motion of a car on banked road

CBSE NOTES CLASS 11 PHYSICS CHAPTER 5

NEWTONS LAWS OF MOTION

Newton’s First Law or Law of Inertia

An object continues to be in its state of rest or of uniform motion in a straight line unless compelled by some external force to act otherwise.

Or

If the net external force on a body is zero, its acceleration is zero. Acceleration can be non zero only if there is a net external force on the body.

Inertia

The property by virtue of which a body opposes any change in its state of rest or of uniform motion is known as inertia.

Greater the mass of the body, greater is its inertia. That is mass is the measure of the inertia of the body.

If F = 0; u = constant (In the absence of external applied force velocity of body remains unchanged.)

Examples of Inertia

  1. When a moving vehicle suddenly stops, passenger’s head gets jerked in the forward direction.

  2. When a stationery vehicle suddenly starts moving passenger’s head gets jerked in the backward direction.

  3. On hitting used mattress by a stick, dust particles come out of it.

  4. In order to catch a moving bus safely we must run forward in the direction of motion of bus.

  5. Whenever it is required to jump off a moving bus, we must always run for a short distance after jumping on road to prevent us from falling in the forward direction.

  6. A spaceship out in interstellar space, far from all other objects and with all its rockets turned off, has no net external force acting on it. If it is in motion, it must continue to move with a uniform velocity.

Linear Momentum

Momentum of a body is defined to be the product of its mass m and velocity v, and is denoted by p,

p = m v  Δp = m Δv

Momentum is a vector quantity. SI unit is kg ms-1

Physical Significance of Momentum

Newton’s Second Law of motion

The rate of change of momentum of a body is proportional to the applied force and takes place in the direction in which force acts.

F ΔpΔt    F = kΔpΔt

As Δt → 0, we have,

F =limΔt  0ΔpΔt= dpdt 

For a body of fixed mass,

F = k mdvdt   F = k m a

Here k is a constant of proportionality.

The unit of force has been defined in such a way that k = 1.

 F = m a

The SI unit of force, Newton (N), is one that causes an acceleration of 1 m s-2 to a mass of 1 kg,

i.e. 1 N = 1 kg m s-2.

Examples

(i) Body kept on horizontal plane is at rest, Fx = 0 and Fy = mg.

(ii) Body kept on horizontal plane is accelerating horizontally under single horizontal force, Fx = max and Fy = mg.

(iii) Body kept on horizontal plane is accelerating horizontally towards right under two horizontal forces. (F1 > F2), Fx = F1 - F2 = max and Fy = mg.

Impulse

The change in momentum of an object, when a force acts on it for a short duration, is called impulse.

F =mv  muΔt

J= t1t2F dt

A large force, acting for a short time to produce a finite change in momentum is called an impulsive force.

Examples of Impulsive Forces

  1. Force applied by foot on hitting a football.

  2. Force applied by boxer on a punching bag.

  3. Force applied by bat on a ball in hitting it to the boundary.

  4. While catching a ball a player lowers his hand to save himself from getting hurt.

  5. A person falling on a cemented floor receives more jerk as compared to one falling on a sandy floor.

Newton’s Third Law of Motion

To every action, there is always an equal and opposite reaction.

Here action refers to the force applied by first body on the second body and reaction refers to the force applied by second body on the first one.

Or

Forces always occur in pairs. Force on a body A by B is equal and opposite to the force on the body B by A.

FAB (force on A by B) = – FBA (force on B by A)

Examples

Principle of Conservation of Momentum

The total momentum of an isolated system of interacting particles is conserved.

Or

When two bodies collide, the total momentum of the system before the collision is equal to the total momentum of system after the collision,

Proof

Consider two bodies A and B, with initial momenta pA and pB. The bodies collide for a duration of time Δt and get apart, with final momenta p'A and p'Brespectively.

By the second Law

and

By third law, FAB = − FBA, we have

This could also be written as,

m1u1 + m2u2 = m1v1 + m2v2

Equilibrium of a Particle

Equilibrium (translational) of a particle refers to the situation when the net external force on the particle is zero.

[Actually equilibrium of a body requires not only translational equilibrium (zero net external force) but also rotational equilibrium (zero net external torque)].

According to the first law, this means that, the particle is either at rest or in uniform motion.

If only two forces are acting on a particle then the two forces must be equal and opposite.

If n forces F1, F2, …. Fn, act on a particle, then

This implies that

where F1x, F1y and F1z are the components of F1 along x, y and z directions respectively and so on.

Examples of Conservation of Momentum

(i) Recoil of gun – when bullet is fired in the forward direction gun recoils in the backward direction.

(ii) When a person jumps on the boat from the shore of river, boat along with the person on it moves in the forward direction.

When a person on the boat jumps forward on the shore of river, boat starts moving in the backward direction.

(iii) In rocket propulsion fuel is ejected out in the downward direction due to which rocket is propelled up in vertically upward direction.

Second Law of Motion is the Real Law of Motion

Newton's second law is the real law of motion in the sense that the first and third law of motion can be derived from the 2nd law of motion.

First Law from Second Law

From Newton’s 2nd law, F = m a

If F = 0, then a = 0

This means that if no force is applied on the body, its acceleration will be zero. If the body is at rest, it will remain in the state of rest and if it is moving, it will remain moving in the same direction with the same speed. Thus, a body not acted upon by an external force, does not change its state of rest or motion. This is the statement of Newton’s first law of motion.

Third Law of Motion from Second Law of Motion

Consider an isolated system of two bodies A and B. An isolated system is such that no external force acts on the system.

Let us consider that there is an interaction between object A and B.

Let FAB bet the force applied by B on A and FBA be the force applied by A on B.

Now,

FBA =dpBdt

And

FAB =dpAdt

Adding the two, we get,

FBA+FAB =dpBdt+dpAdt

= d(pB+pA)dt

But total momentum of the system is constant, i.e.,

d(pB+pA)dt=0  

 FBA+FAB=0

 FBA=-FAB

Free Body Diagrams

Free-body diagrams are diagrams used to show the relative magnitudes and directions of all forces acting upon an object in a given situation.

Common Forces in Mechanics

Non Contact Forces

Gravitational force, electrical force and magnetic force.

Contact Forces

Normal Reaction Forces (N or R)

When bodies are in contact (e.g. a book resting on a table, a system of rigid bodies connected by rods, hinges etc.), there are mutual contact forces, for each pair of bodies, satisfying the third law. The component of contact force normal to the surfaces in contact is called normal reaction.

Tension Force in String (T)

Force applied by string, rope, chain, rod etc. on an object is known as tension. Tension of the string always acts away from the body to which it is attached irrespective of the direction.

Examples of Tension

Fixed Pulley

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It is a simple machine in the form of a circular disc or rim supported by spokes having groove at its periphery. It is free to rotate about an axis passing through its center and perpendicular to its plane.

In case of light pulley, tension in the rope on both the sides of the pulley is same.

Spring Force

Restoring force F = - k x, where k is the spring constant and x is the extension or compression (displacement) in the spring.

Frictional Forces

Friction is property by virtue of which relative motion between two objects is resisted.

Frictional Forces are tangential forces developed between the two surfaces in contact, so as to oppose their relative motion.

Types of Frictional Forces

  1. Static frictional force

  2. Kinetic frictional force

  3. Rolling frictional force

Static Frictional Force

Frictional force acting between the two surfaces in contact, which are relatively at rest, so as to oppose their impending relative motion under the effect of an external force is known as static frictional force.

If there is no external force, there is no static friction.

where fs is the static friction, N is the normal reaction and μs is called coefficient of static friction. This is constant for two given surfaces.

The limiting value of static friction (fs)max, when the object is just about to move, is independent of the area of contact and is given by

Kinetic Frictional Force

Frictional force that opposes relative motion between surfaces in contact and moving relative to each other, is called kinetic or sliding friction and is denoted by fk

Hence (fs)max is also known as limiting frictional force.

Rolling Frictional Force

Frictional force which opposes the rolling of bodies (like cylinder, sphere, ring etc.) over any surface is called rolling frictional force and is given by µrN.

Laws of limiting friction

  1. The direction of limiting frictional force is always opposite the direction of motion.

  2. Limiting friction acts tangential to the two surfaces in contact.

  3. The magnitude of limiting friction is directly proportional to the normal reaction between the two surfaces.

  4. The limiting friction depends upon the material, the nature of the surfaces in contact and their smoothness.

  5. For any two given surfaces, the magnitude of limiting friction is independent of the shape or the area of the surfaces in contact so long as the normal reaction remains the same.

Solving Problems Related to Forces

  1. Draw a diagram showing schematically the various parts of the assembly of bodies, the links, supports, etc.

  2. Choose a convenient part of the assembly as one system.

  3. Draw a separate diagram which shows this system and all the forces on the system by the remaining part of the assembly. Include also the forces on the system by other agencies. Do not include the forces on the environment by the system.

  4. In a free-body diagram, include information about forces (their magnitudes and directions. The rest should be treated as unknowns to be determined using laws of motion.

  5. If necessary, follow the same procedure for another choice of the system. In doing so, employ Newton’s third law. That is, if in the free-body diagram of A, the force on A due to B is shown as F, then, in the free-body diagram of B, the force on B due to A should be shown as –F.

  6. While drawing the diagram choose the coordinate system in such a way so that one or more of the axes are along directions in which no motion of the object is taking place.

Force in Circular Motion

Acceleration of a body moving in a circle of radius R with uniform speed v is v2/R directed towards the centre.

According to the second law, the force f, providing this acceleration is

f=mv2R

This force directed forwards the centre is called the centripetal force.

For a stone rotated in a circle by a string, the centripetal force is provided by the tension in the string.

Motion of a car on a level road

Three forces act on the car.

(i) The weight of the car, mg

(ii) Normal reaction, N

(iii) Frictional force, f

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As there is no acceleration in the vertical direction

The centripetal force required for circular motion is along the surface of the road, and is provided by the component of the contact force between road and the car tyres along the surface. The static friction (since the car is not going out of the circular path) provides this force.

f   μsN =mv2R 

 μs m g =m vmax2R

  vmax=μsR g

Motion of a Car on Banked Road

In case of horizontal road necessary centripetal force mv2/r is provided by static frictional force. When heavy vehicles move with high speed on a sharp turn (small radius) then all the factors contribute to huge centripetal force which if provided by the static frictional force may result in the fatal accident. To prevent this roads are banked by lifting their outer edge.

Due to this, normal reaction of road on the vehicle gets tilted inwards such that it’s vertical component balances the weight of the body and the horizontal component provides the necessary centripetal force.

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Since there is no acceleration along the vertical direction, the net force along this direction must be zero. Hence,

The centripetal force is provided by the horizontal components of N and f.

But fs ≤ μs N

Thus to obtain vmax we put fs = μs N, in (1) and (2)

So we get,

N=mgcos- μssin

Substituting value of N in (4)

mgcos- μssin×sin+ μscos= mv2R

 vmax=R gsin+ μscoscos- μssin

 vmax=R gμs+ tan1- μstan

Take examples

What is the acceleration of the block and trolley system, if the coefficient of kinetic friction between the trolley and the surface is 0.04? What is the tension in the string? (Take g = 10 m s-2). Neglect the mass of the string.

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[NCERT]

Motion of a Block Sliding on an Inclined Plane

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Choose x-axis along OB and y-axis perpendicular to it.

Now balance the forces along both axes.

Angle of Repose

The maximum angle, at which an object can rest on an inclined plane, without sliding down, is called angle of repose.

For a = 0,

sin- μs  cos =0