Forces and Laws of Motion Test

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Forces and Laws of Motion Test - 55

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Question 1
1 / -0

Four alternatives are given to the following incomplete statement/question. Choose the right answer.
If the forces acting on an object are balanced then

A
the object will be accelerated

B
the object will be in motion

C
the object will have zero acceleration,

D
the object loses its shape

Solution

From Newton's second law of motion - $$F=ma$$, where $$F$$ is the net force acting on a body, $$m$$ is the mass of the body, and $$a$$ is the acceleration of the body.
If the forces acting on the object are balanced then $$F=0$$.
$$\therefore a=0$$, as $$m$$ can not be zero.
Hence, the body will not have any acceleration.
Question 2
1 / -0

What is the momentum of an object of mass m, moving with a velocity $$V$$ ?

A
$$(mV)^{2}$$

B
$$mV^{2}$$

C
$$\frac{1}{2}mV^{2}$$

D
$$mV$$

Solution

Correct Answer: D
Explanation:
Momentum defined as the quantity of motion of a body. It is a vector quantity and measured by the product of mass and velocity of the body. So,
momentum = mass ×velocity
$$=m×V=mV$$
Hence Option (D) is the right answer.
Question 3
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Newton’s second law of motion says that the mass of object times its acceleration is equal to the net force on the object. Which of the following gives the correct units for force?

A
$$kg.m/s^{2}$$

B
$$kg.m^{2}/s^{2}$$

C
$$kg/m.s^{2}$$

D
$$kg.m^{2}/s$$

E
None of those answer

Solution

The SI unit of mass is $$kg$$.
The SI unit of acceleration is $$m/s^2$$.
We know,
$$\text{Force}=\text{mass}\times \text{acceleration}$$
The unit of force is $$kg.m/s^2$$.
Question 4
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A body of mass $$10\ kg$$ undergoes a change in velocity of $$10\ m/s$$ in $$5\ s$$. What is the force acting on it?

A
$$100\ N $$

B
$$25\ N $$

C
$$50\ N $$

D
$$20\ N $$

Solution

Acceleration $$ a = \dfrac{\text{velocity}}{\text{time}} $$
Given ,
Velocity $$ = 10\ m/s $$
Time $$ = 5s $$
Mass $$ = 10\ kg $$

$$a= \dfrac{10}{5}\ m/s^{2} $$

$$ = 2\ m/s^{2} $$

Force = mass $$ \times $$ acceleration

$$ = 10 \times 2 = 20 N $$
Question 5
1 / -0

In the above figure, a locomotive has broken through the wall of a train station. During the collision, what can be said about the force exerted by the locomotive on the wall?

A
The force exerted by the locomotive on the wall was larger than the force the wall could exert on the locomotive.

B
The force exerted by the locomotive on the wall was the same in magnitude as the force exerted by the wall on the locomotive.

C
The force exerted by the locomotive on the wall was less than the force exerted by the wall on the locomotive.

D
The wall cannot be said to “exert” a force; after all, it broke.

Solution

Answer (B). Newton’s 3rd law describes all objects, breaking or whole. The force that the locomotive exerted on the wall is the same as that exerted by the wall on the locomotive. The framing around the wall could not exert so strong a force on the section of the wall that broke out.
Question 6
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An athlete jumps straight up in the air to catch a ball. If he took $$0.4$$ seconds to jump up and $$0.4$$ seconds to come back down, how high did he jump? Take $$g = 9.8\ m/s^{2}$$.

A
$$0.784\ m $$

B
$$1.568\ m $$

C
$$2.352\ m $$

D
$$3.136\ m $$

Solution

Given,
Time taken by the athlete to jump up $$=$$ Time taken by the athlete to come down
$$t=0.4\ s$$
Distance travelled by the athlete while moving up $$=$$ distance travelled by the athlete while coming down
Let the distance travelled while moving up or down be $$h$$

While coming down,
Initial velocity, $$u=0$$
Acceleration due to gravity, $$g=9.8\ m/s^2$$

Using Newton's second equation of motion :
$$s = ut + \dfrac{1}{2} at^{2} $$

$$h = ut + \dfrac{1}{2} gt^{2} $$

$$ \therefore h = (0\times 0.4) +\dfrac{1}{2} \times 9.8 (0.4)^{2} $$

$$ h =\dfrac{1}{2} \times 9.8\times 0.16 $$

$$h = 0.784\ m $$
Question 7
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Which of these examples can be explained by Newtons first law of motion ?

A
It is easier to push an empty shopping cart than a full shopping cart

B
It takes a smaller force to roll a marble than a bowling ball

C
When a person steps forward, he exerts a backward force on ground

D
A runner does not come to a stop immediately upon crossing the finish line

Solution

The first law of motion was Inertia and when a runner reaches a finish line he does not comes to stop immediately due to inertia.
Newton's first law of motion states that a body at rest remains at rest, or if in motion, remains in motion at a constant velocity unless acted on by a net external force. This is also known as the law of inertia. Inertia is the tendency of an object to remain at rest or remain in motion.
Question 8
1 / -0

A bullet of mass A and velocity B is fired into a block of wood of mass C. If loss of any mass and friction be neglected, then velocity of the system must be

A
$$ \dfrac{AB}{A+C}$$

B
$$ \dfrac{A + C}{B+C}$$

C
$$ \dfrac{AC}{B+C}$$

D
$$ \dfrac{A+B}{AC}$$

Solution

Given:
The bullet strikes the block and after collision they become a system and
$$(Mass)_{bullet} = A$$
$$(Velocity)_{bullet} = B$$
$$(Mass)_{block} = C$$
$$(Velocity)_{block} = 0$$
Momentum before impact = Momentum after impact
$$AB + 0 =(A+C)(v)_{system
}$$
$$(v)_{system}=\dfrac{AB}{A+C}$$
The velocity of the system $$(v)_{system}=\dfrac{AB}{A+C}$$
Question 9
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A shell of mass 100 g moving with the speed of $$50 m s^{-1}$$ along a straight line bursts into two pieces. The piece of mass 60 g starts moving horizontally with a speed of $$50 m s^{-1}$$. The other piece moves with a speed of

A
$$50 m s^{-1}$$ opposite to the first piece.

B
$$100 m s^{-1}$$ in vertically downward direction.

C
$$50 m s^{-1}$$ in the same direction as the first piece

D
zero (remains at rest)

Solution
Question 10
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When a bus starts suddenly, the passengers are pushed back. This is an example of which of the following?

A
Newton's first law

B
Newton's second law

C
Newton's third law

D
None of Newton's laws

Solution

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Forces and Laws of Motion Test - 55

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Question 1

the object will be accelerated

the object will be in motion

the object will have zero acceleration,

the object loses its shape

Solution

Question 2

$$(mV)^{2}$$

$$mV^{2}$$

$$\frac{1}{2}mV^{2}$$

$$mV$$

Solution

Question 3

$$kg.m/s^{2}$$

$$kg.m^{2}/s^{2}$$

$$kg/m.s^{2}$$

$$kg.m^{2}/s$$

None of those answer

Solution

Question 4

$$100\ N $$

$$25\ N $$

$$50\ N $$

$$20\ N $$

Solution

Question 5

The force exerted by the locomotive on the wall was larger than the force the wall could exert on the locomotive.

The force exerted by the locomotive on the wall was the same in magnitude as the force exerted by the wall on the locomotive.

The force exerted by the locomotive on the wall was less than the force exerted by the wall on the locomotive.

The wall cannot be said to “exert” a force; after all, it broke.

Solution

Question 6

$$0.784\ m $$

$$1.568\ m $$

$$2.352\ m $$

$$3.136\ m $$

Solution

Question 7

It is easier to push an empty shopping cart than a full shopping cart

It takes a smaller force to roll a marble than a bowling ball

When a person steps forward, he exerts a backward force on ground

A runner does not come to a stop immediately upon crossing the finish line

Solution

Question 8

$$ \dfrac{AB}{A+C}$$

$$ \dfrac{A + C}{B+C}$$

$$ \dfrac{AC}{B+C}$$

$$ \dfrac{A+B}{AC}$$

Solution

Question 9

$$50 m s^{-1}$$ opposite to the first piece.

$$100 m s^{-1}$$ in vertically downward direction.

$$50 m s^{-1}$$ in the same direction as the first piece

zero (remains at rest)

Solution

Question 10

Newton's first law

Newton's second law

Newton's third law

None of Newton's laws

Solution

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