Assertion: In the reference frame of centre of mass, net force acting on system is always zero. Reason: A pseudo force given by P → S = - m a → cm (where m is mass of system and a → cm is acceleration of centre of mass) acts on system which balances all the external forces.

If both Assertion and Reason are true and the Reason is correct explanation of the Assertion.

If both Assertion and Reason are true, but Reason is not correct explanation of the Assertion.

If Assertion is true, but the Reason is false.

If Assertion is false, but Reason is true.

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Assertion: A body whatever its motion is always at rest in reference frame which is fixed to the body itself.

Reason: The relative velocity of a body with respect to itself is always zero.

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Important Questions on Laws of Motion

EASY

Physics>Mechanics>Laws of Motion>Inertial and Non-inertial Frames of Reference

A block

A

takes

2 s

to slide down a frictionless incline of

30 °

and length

l

, kept inside a lift going up with uniform velocity

v

. If the incline is changed to

45 °

, the time taken by the block, to slide down the incline, will be approximately:

HARD

Physics>Mechanics>Laws of Motion>Inertial and Non-inertial Frames of Reference

Two wooden blocks are moving on a smooth horizontal surface such that the block having mass $m$ remains stationary with respect to block of mass $M$ as shown in the figure below. The magnitude of the applied force $P$ is,

Question Image

EASY

Physics>Mechanics>Laws of Motion>Inertial and Non-inertial Frames of Reference

A block of mass

M

placed inside a box descends vertically with acceleration

a

. The block exerts a force equal to one-fourth of its weight on the floor of the box. The value of

a

will be

EASY

Physics>Mechanics>Laws of Motion>Inertial and Non-inertial Frames of Reference

A person is standing in an elevator. In which situation, he experiences weight loss ?

MEDIUM

Physics>Mechanics>Laws of Motion>Inertial and Non-inertial Frames of Reference

A boy is standing on a weighing machine inside a lift. When the lift goes upwards with acceleration

\frac{g}{4},

the machine shows the reading

50 kg .

wt. When the lift goes downward with acceleration

\frac{g}{4},

the reading of the machine in

kg

wt

. would be

EASY

Physics>Mechanics>Laws of Motion>Inertial and Non-inertial Frames of Reference

A man weighing

60 kg

is in a lift moving down with an acceleration of

1.8 {ms}^{- 2}

. The force exerted by the floor on him is?

MEDIUM

Physics>Mechanics>Laws of Motion>Inertial and Non-inertial Frames of Reference

When a lift of mass

800 kg

is ascending with an acceleration of

5 m s^{- 2}

, the tension in its cable will be

______

take

(g = 10 m s^{- 2})

HARD

Physics>Mechanics>Laws of Motion>Inertial and Non-inertial Frames of Reference

A particle is moving along the

x

-axis with its coordinate with time

t

given by

x (t) = 10 + 8 t - 3 t^{2} .

Another particle is moving along the

y

-axis with its coordinate as a function of time given by

y (t) = 5 - 8 t^{3} .

t = 1 s,

the speed of the second particle as measured in the frame of the first particle is given as

\sqrt{v} .

Then

v (i n m s^{- 1})

is ___________.

HARD

Physics>Mechanics>Laws of Motion>Inertial and Non-inertial Frames of Reference

A girl drops an apple from the window of a train which is moving on a straight track with speed increasing with a constant rate. The trajectory of the falling apple as seen by the girl is

EASY

Physics>Mechanics>Laws of Motion>Inertial and Non-inertial Frames of Reference

A plumb line is suspended from a ceiling of a car moving with horizontal acceleration

a

. What will be the angle of inclination with vertical?

MEDIUM

Physics>Mechanics>Laws of Motion>Inertial and Non-inertial Frames of Reference

Two blocks

A

and

B

of masses

m_{A} = 1 k g

and

m_{B} = 3 k g

are kept on the table as shown in figure. The coefficients of friction between

A

and

B

0.2

and between

B

and the surface of the table is also

0.2 .

The maximum force

F

that can be applied on

B

horizontally, so that the block

A

does not slide over the block

B

is : [Take

g = 10 m / s^{2}

]

MEDIUM

Physics>Mechanics>Laws of Motion>Inertial and Non-inertial Frames of Reference

A rocket is fired vertically from the earth with an acceleration of 2g, where g is the gravitational acceleration. On an inclined plane inside the rocket, making an angle

θ

with the horizontal, a point object of mass m is kept. The minimum coefficient of friction

μ_{m i n}

between the mass and the inclined surface such that the mass does not move is:

EASY

Physics>Mechanics>Laws of Motion>Inertial and Non-inertial Frames of Reference

A body of mass $50 kg$ is suspended using a spring balance inside a lift at rest. If the lift starts falling freely, the reading of the spring balance is

HARD

Physics>Mechanics>Laws of Motion>Inertial and Non-inertial Frames of Reference

A circular disc with a groove along its diameter is placed horizontally on a rough surface. A block of mass

1 kg

is placed as shown. The co-efficient of friction between the block and all surfaces of groove and horizontal surface in contact is

μ = \frac{2}{5} .

The disc has an acceleration of

25 {ms}^{- 2}

towards left. Find the acceleration of the block with respect to disc. Given,

\cos θ = \frac{4}{5}, \sin θ = \frac{3}{5}

Question Image

MEDIUM

Physics>Mechanics>Laws of Motion>Inertial and Non-inertial Frames of Reference

Assertion: In the reference frame of centre of mass, net force acting on system is always zero.

Reason: A pseudo force given by

{\vec{P}}_{S} = - m {\vec{a}}_{cm}

(where

m

is mass of system and

{\vec{a}}_{cm}

is acceleration of centre of mass) acts on system which balances all the external forces.

MEDIUM

Physics>Mechanics>Laws of Motion>Inertial and Non-inertial Frames of Reference

Assertion: An observer moving with constant speed must be an inertial reference frame.

Reason: An observer is called inertial if its change in velocity is zero.

MEDIUM

Physics>Mechanics>Laws of Motion>Inertial and Non-inertial Frames of Reference

A piece of wire is bent in the shape of a parabola

y = k x^{2}

(

y -

axis vertical) with a bead of mass

m

on it. The bead can slide on the wire without friction. It stays at the lowest point of the parabola when the wire is at rest. The wire is now accelerated parallel to the

x -

axis with a constant acceleration

a

. The distance of the new equilibrium position of the bead, where the bead can stay at rest with respect to the wire, from the

y -

axis is -

HARD

Physics>Mechanics>Laws of Motion>Inertial and Non-inertial Frames of Reference

Two blocks of mass

m_{1} = 10 kg

and

m_{2} = 5 kg

, connected to each other by a massless inextensible string of length

0.3 m

are placed along a diameter of a turn table. The coefficient of friction between the table and

m_{1}

0.5

while there is no friction between

m_{2}

and the table. The table is rotating with an angular velocity of

10 {rads}^{- 1}

about a vertical axis passing through its centre

O

. The masses are placed along the diameter of the table on either side of the centre

O

such that the mass

m_{1}

is at a distance of

0.124 m

from

O

. The masses are observed to be at rest with respect to an observer on the turn table. Calculate the frictional force on

m_{1}

EASY

Physics>Mechanics>Laws of Motion>Inertial and Non-inertial Frames of Reference

Assertion: The apparent weight of a body in an elevator moving with some downward acceleration is less than the actual weight of body.

Reason: The part of the weight is spent in producing downward acceleration, when body is in elevator.

EASY

Physics>Mechanics>Laws of Motion>Inertial and Non-inertial Frames of Reference

Assertion: In a free fall, weight of a body becomes effectively zero.

Reason: Acceleration due to gravity acting on a body having free fall is zero.

Assertion: A body whatever its motion is always at rest in reference frame which is fixed to the body itself. Reason: The relative velocity of a body with respect to itself is always zero.

Important Questions on Laws of Motion

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Assertion: A body whatever its motion is always at rest in reference frame which is fixed to the body itself.

Reason: The relative velocity of a body with respect to itself is always zero.