Laws Of Motion: Class 11 Physics NCERT Chapter 5

Key Features Of NCERT Material for Class 11 Science Chapter 5  –  Laws Of Motion

In the last chapter 4, you learned about Motion in Plane. In this chapter, you all will get to know about Laws Of Motion.

Quick revision notes

Dynamics is the part of material science where we study the movement of a body by mulling over the reason i.e., power which delivers the movement. 

  • Force 

Power is an outer reason as push or pull, which creates or attempts to deliver movement in a body very still, or stops/attempts to stop a moving body or changes/attempts to alter the course of movement of the body. The natural property, with which a body opposes any adjustment in its condition of movement is called dormancy. Heavier the body, the dormancy is more and lighter the body, lesser the idleness. 

Law of dormancy expresses that a body has the powerlessness to change its condition of rest or uniform movement (i.e., a movement with consistent speed) or course of movement without anyone else. 

  • Newton’s Laws of Motion 

Law 1. A body will stay very still or keep on moving with uniform speed except if an outer power is applied to it. 

First law of movement is likewise alluded to as the ‘Law of inactivity’. It characterizes dormancy, power and inertial edge of reference. I here is consistently a need of ‘edge of reference’ to portray and comprehend the movement of molecule, lhc easiest ‘edge of reference’ utilized are known as the inertial edges. A casing of referent, e is known as an inertial edge it, inside, everything increasing velocities of any molecule are brought about by the activity of ‘genuine powers’ on that molecule. At the point when we talk about increasing speeds created by ‘invented’ or ‘pseudo’ powers, the casing of reference is a non-inertial one. 

Law 2. At the point when an outside power is applied to an assortment of steady mass the power creates an increasing speed, which is legitimately relative to the power and conversely corresponding to the mass of the body.

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Law 3. “To each activity there is equivalent and inverse response power”. At the point when a body An applies a power on another body B, B applies an equivalent and inverse power on A. 

  • Linear Momentum 

The direct force of a body is characterized as the result of the mass of the body and its speed. 

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  • Impulse 

Powers representing brief term are called incautious powers. Drive is characterized as the result of power and the little league stretch for which it acts. It is given by 

4-35

Drive of a power is a vector amount and its SI unit is 1 Nm. 

— If power of a motivation is changing with time, at that point the drive is estimated by finding the region limited forcibly time diagram for that power. 

— Impulse of a power for a given time is equivalent to the absolute change in force of the body during the given time. In this manner, we have 

5-28

  • Law of Conservation of Momentum

The absolute force of a segregated arrangement of particles is moderated. 

At the end of the day, when no outside power is applied to the framework, its absolute energy stays consistent. 

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  • Recoiling of a firearm, trip of rockets and stream planes are some basic uses of the law of protection of straight energy. 

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  • Concurrent Forces and Equilibrium

“A gathering of powers which are acting at one point are called simultaneous powers.” 

Simultaneous powers are supposed to be in harmony if there is no adjustment in the situation of rest or the condition of uniform movement of the body on which these simultaneous powers are acting. For simultaneous powers to be in balance, their resultant power must be zero. If there should arise an occurrence of three simultaneous powers acting in a plane, the body will be in balance if these three powers might be totally spoken to by three sides of a triangle taken all together. On the off chance that number of simultaneous powers is more than three, at that point these powers must be spoken to by sides of a shut polygon all together for harmony. 

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  • Commonly Used Forces 

(I) Weight of a body. It is the power with which earth draws in a body towards its middle. On the off chance that M is mass of body and g is quickening because of gravity, weight of the body is Mg vertically descending way. 

(ii) Normal Force. In the event that two bodies are in contact a contact power emerges, if the surface is smooth the heading of power is ordinary to the plane of contact. We call this power as Normal power.

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Example. Let us consider a book resting on the table. It is acted upon by its weight in vertically downward direction and is at rest. It means there is another force acting on the block in opposite direction, which balances its weight. This force is provided by the table and we call it as normal force.

(iii) Tension in string. Suppose a block is hanging from a string. Weight of the block is acting vertically downward but it is not moving, hence its weight is balanced by a force due to string. This force is called ‘Tension in string’. Tension is a force in a stretched string. Its direction is taken along the string and away from the body under consideration.

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  • Simple Pulley

For instance take two  masses m1 and m2 tied at the ends of an in extensible string, which passes over a light and friction less pulley. Let m1 > m2. The heavier body will move downwards and the lighter will move upwards. Let a be the common acceleration of the system of two bodies. Acceleration is given by

12-10

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  • Apparent Weight and Actual Weight

— ‘Evident weight’ of a body is equivalent to its ‘genuine weight’ if the body is either in a condition of rest or in a condition of uniform movement. 

— Apparent load of a body for vertically upward quickened movement is given as 

Obvious weight =Actual weight + Ma = M (g + a) 

— Apparent load of a body for vertically descending quickened movement is given as 

Obvious weight = Actual weight Ma = M (g – a). 

  • Friction

The resistance to any relative movement between two surfaces in contact is alluded to as grinding. It emerges due to the ‘bury coinciding’ of the surface anomalies of the two surfaces in contact. 

  • Static and Dynamic (Kinetic) Friction

The frictional powers between two surfaces in contact (I) previously and (ii) after a relative movement between them has begun, are alluded to as static and dynamic erosion separately. Static grinding is consistently somewhat more than dynamic contact. 

The extent of dynamic frictional power is additionally corresponding to typical power. 

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  • Limiting Frictional Force

This frictional power acts when body is going to move. This is the greatest frictional power that can exist at the contact surface. We compute its worth utilizing laws of erosion. 

Laws of Friction:

(I) The greatness of restricting frictional power is relative to the ordinary power at the contact surface.

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(ii) The magnitude of limiting frictional force is independent of area of contact between the surfaces.

  • Coefficient of Friction

The coefficient of grating (μ) between two surfaces is the proportion of their restricting frictional power to the ordinary power between them, i.e., 

16-7

  • Angle of Friction

It is the point which the resultant of the power of restricting grinding F and the ordinary response R makes with the heading of the typical response. On the off chance that θ is the point of grating, we have 

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  • Angle of Repose

Edge of rest (α) is the point of a slanted plane with the level at which a body put over it just starts to slide down with no speeding up. Point of rest is given by α = tan-1 (μ) 

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  • Motion on a Rough Inclined Plane

Assume a movement up the plane happens under the activity of pull P acting corresponding to the plane. 

  • Centripetal Force

Centripetal power is the power needed to move a body consistently all around. This power demonstrations along the range and towards the focal point of the circle. It is given by

19-8

where, v is the straight speed, r is the range of roundabout way and ω is the precise speed of the body. 

  • Centrifugal Force

Radial or centrifufal is a force that emerges when a body is moving really along a roundabout way, by ideals of propensity of the body to recapture its common straight line way.

The magnitude of centrifugal force is same as that of centripetal force.

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  • Motion in a Vertical Circle

The movement of a molecule in an even circle is not the same as the movement in vertical circle. In flat circle, the movement isn’t affected by the increasing speed because of gravity (g) though in the movement of vertical circle, the movement isn’t affected by the quickening because of gravity (g) while in the movement of vertical circle, the estimation of ‘g’ assumes a significant job, the movement for this situation doesn’t stay uniform. At the point when the molecule climb from its least position P, its speed constantly diminishes till it arrives at the most noteworthy purpose of its roundabout way. This is because of the work done against the power of gravity. At the point when the molecule descends the circle, its speed would continue expanding. 

23-4

Let us consider a molecule moving in a roundabout vertical way of span V and focus o tide with a string. L be the quick situation of the molecule with the end goal that 24-3

Here the accompanying powers follow up on the molecule of mass ‘m’. 

(I) Its weight = mg (verticaly downwards). 

(ii) The pressure in the string T along LO. 

25-2

We can take the flat bearing at the absolute bottom ‘p’ as the situation of zero gravitational expected vitality. Presently according to the standard of protection of vitality, 

26-2

From this connection, we can figure the pressure in the string at the absolute bottom P, mid-way point and at the most noteworthy situation of the moving molecule. 

Case (I) : At the absolute bottom P, θ = 0° 

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At the point when the molecule finishes its movement along the vertical circle, it is alluded to as “Circling the Loop” for this the base speed at the most reduced position must be √5gr

  • IMPORTANT TABLES

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