Newton's Laws of Motion Newtons Laws are the basic of Mechanics. First Law
Every body continues in its state of rest or of uniform motion in a straight line unless it is compelled by an external force to change that state. Second Law
The rate of change of momentum of a body is directly proportional to the eternal force acting upon it and takes place in the direction of the force. Third Law
To every action, there is always an equal and opposite reaction. Importance of First law : 1. It explains the property of Inertia. 2. It defines the Force. Inertia The property of a body by which it cannot by itself change its state of rest or of its uniform motion in a straight line is called Inertia. Units : gram (and) kilogram Force Force is that which changes or tends to change the state of rest of a body o of its uniform motion in a straight line. Absolute Units C.G.S : Dyne
1 Dyne = 1 gm * 1 cm/sec2M.K.S: Newton
1 Newton = 1 kg * 1 m/sec2Gravitational units :C.G.S : gm . wt1 gm.wt : A force with which a body of mass 1 gm is attracted by the earth.
1 gm.wt = g dynes.
M.K.S : kg . wt
1 kg.wt : A force with which a body of mass 1kg is attracted by earth.
1 kg.wt = g newtons.
Weight of the body W = mg
Importance of second law : It gives a measure of the force by F = maMomentum :
It is defined as the product of mass and velocity. (OR) P = mv and it is a vector quantity.
Units of Momentum :
C.G.S : gm – cm/s
M.K.S : kg – m/sImpulse :
It is defined as the product of the force and time. An impulsive force is a large forceacting for a short interval of time. (OR) I = Ft = mv- mu and it is a vector quantity.
Units :
C.G.S : dyne – sec (OR) gm – cm/s
M.K.S : newton –sec (OR) kg – m/sImportance of Third law :
It explains that a force always accompanied by an opposite force.
It leads to the law of conservation of momentum.
Law of Conservation of Momentum :
When no external force acts on a system, the algebraic sum of the momenta of the bodies in the system remains constant in any direction after any number of collisions.
m1v1 + m2v2 = m1u1 + m2u2
i.e., total momentum after collision = total momentum before collision.
Apparent and real weights :
Let a man of real weight W = mg be in a stationary lift.Then the normal reaction(R) of the floor is equal W. This normal reaction we call it as the apparent weight and R = W = mg.
2. If the lift moves up or don with uniform velocity, R = W = mg
3. Let the lift move upwards acceleration a.
In this case W1 (apparent weight) > W (real weight)
R – W = ma.
R = W1 = W + ma
= mg + ma
= m (g + a)
4. If the lift moves down with an acceleration a , the real weight W is greater than
the normal reaction or apparent weight,
(R = W2)
and W – R = ma.
Or, R = W – ma = mg – ma
Or, W2 = m (g – a)
5. If the lift moves down with a = g, or in the case of a freely falling lift,
R = mg – ma = 0
Hence the apparent weight of a person is zero.Work Work is said to be done by a force when a body moves in the direction of force.
W = F * s
Units :
C.G.S : erg
M.K.S : joule(J)
1 erg = 1 dyne * 1 cm
1 Joule = 1 newton * 1 metre
1 Joule = 107 ergs.
Kilo Watt hour (K.W.H) :
1 K.W.H = 36 * 105 J Power Power is defined as the time rate at which work is done is called power.
P = work done / time (OR) W/t
Units :
C.G.S : erg S-1
M.K.S : J s-1 (OR) watt (W)
The power is said to be one watt of ENERGY FICTION
Every body continues in its state of rest or of uniform motion in a straight line unless it is compelled by an external force to change that state. Second Law
The rate of change of momentum of a body is directly proportional to the eternal force acting upon it and takes place in the direction of the force. Third Law
To every action, there is always an equal and opposite reaction. Importance of First law : 1. It explains the property of Inertia. 2. It defines the Force. Inertia The property of a body by which it cannot by itself change its state of rest or of its uniform motion in a straight line is called Inertia. Units : gram (and) kilogram Force Force is that which changes or tends to change the state of rest of a body o of its uniform motion in a straight line. Absolute Units C.G.S : Dyne
1 Dyne = 1 gm * 1 cm/sec2M.K.S: Newton
1 Newton = 1 kg * 1 m/sec2Gravitational units :C.G.S : gm . wt1 gm.wt : A force with which a body of mass 1 gm is attracted by the earth.
1 gm.wt = g dynes.
M.K.S : kg . wt
1 kg.wt : A force with which a body of mass 1kg is attracted by earth.
1 kg.wt = g newtons.
Weight of the body W = mg
Importance of second law : It gives a measure of the force by F = maMomentum :
It is defined as the product of mass and velocity. (OR) P = mv and it is a vector quantity.
Units of Momentum :
C.G.S : gm – cm/s
M.K.S : kg – m/sImpulse :
It is defined as the product of the force and time. An impulsive force is a large forceacting for a short interval of time. (OR) I = Ft = mv- mu and it is a vector quantity.
Units :
C.G.S : dyne – sec (OR) gm – cm/s
M.K.S : newton –sec (OR) kg – m/sImportance of Third law :
It explains that a force always accompanied by an opposite force.
It leads to the law of conservation of momentum.
Law of Conservation of Momentum :
When no external force acts on a system, the algebraic sum of the momenta of the bodies in the system remains constant in any direction after any number of collisions.
m1v1 + m2v2 = m1u1 + m2u2
i.e., total momentum after collision = total momentum before collision.
Apparent and real weights :
Let a man of real weight W = mg be in a stationary lift.Then the normal reaction(R) of the floor is equal W. This normal reaction we call it as the apparent weight and R = W = mg.
2. If the lift moves up or don with uniform velocity, R = W = mg
3. Let the lift move upwards acceleration a.
In this case W1 (apparent weight) > W (real weight)
R – W = ma.
R = W1 = W + ma
= mg + ma
= m (g + a)
4. If the lift moves down with an acceleration a , the real weight W is greater than
the normal reaction or apparent weight,
(R = W2)
and W – R = ma.
Or, R = W – ma = mg – ma
Or, W2 = m (g – a)
5. If the lift moves down with a = g, or in the case of a freely falling lift,
R = mg – ma = 0
Hence the apparent weight of a person is zero.Work Work is said to be done by a force when a body moves in the direction of force.
W = F * s
Units :
C.G.S : erg
M.K.S : joule(J)
1 erg = 1 dyne * 1 cm
1 Joule = 1 newton * 1 metre
1 Joule = 107 ergs.
Kilo Watt hour (K.W.H) :
1 K.W.H = 36 * 105 J Power Power is defined as the time rate at which work is done is called power.
P = work done / time (OR) W/t
Units :
C.G.S : erg S-1
M.K.S : J s-1 (OR) watt (W)
The power is said to be one watt of ENERGY FICTION
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