# ICSE Class 9th Physics 3 – Laws of Motion MCQs

#### If a ball thrown up with a certain velocity attains a height of 40 m and comes back, then displacement of the ball is 80 m.

#### A person standing on a bathroom scale sees a reading on the scale of 148 pounds. This person is acted on by:

Two equal forces act in opposite directions.

#### If the earth is in the shape of a perfect sphere and of uniform density, the acceleration due to gravity at equator is ____ its value at poles.

If a body is at the equator, it is moving along a circular path of radius 'R'. A part of the gravitational attractive force acts as the required centripetal force and the rest helps the body gain 'acceleration'. If the same body is at poles, total gravitational attractive force helps in accelerating the body. Hence acceleration due to gravity is more at poles than at equator even when the earth is a perfect sphere of uniform density.

#### The SI unit of 'G' and 'g' are ____ and ____ respectively.

The SI unit of universal gravitational constant 'G' is N m^{2} kg^{−2} and that of acceleration due to gravity is m s^{−2}.

#### The gravitational force between two masses is independent of ____ the two bodies.

The gravitational force between two masses is independent of state of matter, charge on bodies and the medium between them.

#### If a body covers equal displacements in equal intervals of time then it is said to be moving with ____, however small these time intervals may be.

If a body covers equal displacements in equal intervals of time, then it is said to be moving with uniform velocity however, small these time intervals may be.

#### A body moving along a circular track of radius R completes one rotation in 2 seconds. Its average velocity in 2 second is ____ m/s.

The body completes one rotation in 2 second, so its displacement in 2 seconds is 0 m. Hence, the average velocity of the body in 2 seconds is 0 m/s.

#### A body moves from A to B with a velocity of 20 m/s and comes back from B to A with a velocity of 30 m/s. The average velocity of the body during the whole journey is ____ m/s.

Average velocity is the rate of net displacement during the whole journey. As initial and final points are same, displacement is zero. Hence, average velocity of the body is zero.

#### The force of attraction between two unit masses separated by unit distance is known as ____

The force of attraction between two unit masses separated by unit distance is known as universal gravitational constant.

#### If the rotational speed of the earth increases, then 'G' ____ and ‘g’ ____.

If the rotational speed of the earth increases, then 'G' remains the same but 'g' decreases. As the rotational speed of the earth increases, the required centripetal force increases, and hence the acceleration due to gravity decreases.

#### 'g' on any planet is directly proportional to ____ of the planet and inversely proportional to ____ of the planet.

Acceleration due to gravity on the surface of a planet is directly proportional to the mass of the planet and is inversely proportional to the square of the radius of the planet.

#### Which of the following is incorrect?

As speed has only magnitude but no direction, it is a scalar quantity. Velocity, retardation, acceleration due to gravity and displacement has both direction as well as magnitude. Hence, they are vectors.

#### A particle is acted upon by a force of constant magnitude which is always perpendicular to the velocity of the particle. Then, the velocity of the particle is constant.

When a force is applied perpendicular to the velocity of the body, then the motion of the body is circular due to change in its direction at every point. Hence, the body moves with a constant speed and variable velocity.

#### The velocity-time graph of a body with uniform velocity is a straight line ____.

When we say that body is moving with uniform velocity it means there is no acceleration. The slope of the velocity-time graph has zero degree inclination to the X-axis i.e., tanθ = tan0° = 0. Hence, the acceleration is zero and the velocity-time graph is parallel to the X-axis.

#### When two identical masses are placed at a separation of 1 cm, the gravitational attractive force between them is 0.00667 N. Mass of each body is ____.

F = G. Mm/R^{2 }
0.00667 = 6.67 × 10^{−11} × M^{2}/(0.01)^{2}
Solving for 'M' we get M = 100 kg.

#### A child on a cart with wheels throws a sandbag forward. As a result:

Child moves in the backward direction as a consequence of action reaction law. Sand bag exerts an equal and opposite force in the backward direction.

#### An object can be at rest as well as in motion at the same time.

An object can be at rest relative to one object as well as in motion at the same time relative to another object.

#### The universal law of gravitation was given by

The universal law of gravitation was given by newton

#### If the displacement-time graph of a particle represents a line parallel to the time-axis, then the velocity of the particle is ____.

The inclination of the displacement-time graph with time axis gives the magnitude of velocity. As the graph representing a line parallel to the time-axis, the velocity of the particle is zero.

#### What is the product of force and the time for which force acts known as?

Impulse of force (I) = Force × Time = Change in momentum

#### The initial velocity of a body is 10 m/s. After 5 s, the velocity of the body is 20 m/s. Its velocity after 7 s is ____ m/s.

According to the problem,
initial velocity, u = 10 m/s
final velocity, v = 20 m/s
time duration, t = 5 s
Acceleration of the body is given by, a =v-u/t
a= 20 −10/ 5
a= 2 m/ s^{2}
So, the velocity after 7 s, is given by, v = u + at
v = 10 + 2 × 7
v = 24 m/s
v = 24 m/s

#### The velocity-time graph for a body with non-uniform motion having constant acceleration is a ____.

The velocity-time graph for a body with non-uniform motion having constant acceleration is a straight line inclined to the time axis.

#### In the equation of motion, x = at + bt^{2}, the units of a and b are ____ respectively.

Unit of 'a' is m/s and 'b' is m/s^{2}.

#### Unit of acceleration is m/s^{2}.

Unit of acceleration is m/s^{2}.

#### On the surface of any planet, acceleration due to gravity of a body is independent of ____.

On the surface of a planet acceleration due to gravity of a body is independent of mass of the body.

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