Forces and Laws of Motion Test

Result

Forces and Laws of Motion Test - 56

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Question 1
1 / -0

If $$120 N $$ of force is acting on a body with $$6 ms^{-2}$$ acceleration, Its mass is _____________.

A
$$0.05 Kg $$

B
$$20 Kg $$

C
$$200Kg $$

D
$$720 Kg $$

Solution

According to newton second law
$$F(force)=m(mass) \times a(acceleration)$$
$$F=120N$$ $$a=6ms^{-2}$$
Mass of the body is,

$$m = \frac{F}{a}$$

$$ = \frac{{120}}{6}$$

$$= 20\;{\rm{kg}}$$
Question 2
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A force produces an acceleration of $$5.0 \:cm \:s^{-2}$$ when it acts on a body of mass 20 g. Find the force acting on the body.

A
$$2\times 10^{-3}N$$

B
$$4\times 10^{-3}N$$

C
$$1.0\times 10^{-3}N$$

D
$$5\times 10^{-3}N$$

Solution
Question 3
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A bullet of mass $$50$$ gram is fired from a $$5kg$$ gun with a velocity of $$1km/s$$, the speed of recoil of the gun is

A
$$5m/s$$

B
$$1m/s$$

C
$$0.5m/s$$

D
$$-10m/s$$

Solution
Question 4
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In CID serial (on TV ), Abhijeet fires a bullet of mass 100g with a speed of 100 $${ ms }^{ -1 }$$ on a soft plywood of thickness 4 cm. The bullet emerges with 10% of its intial KE. Find the emergent speed of the bullet.

A
31.62 m/s

B
30.52 m/s

C
32.75 m/s

D
33.84 m/s

Solution
Question 5
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A bullet is of mass $$5g$$ is fired at a velocity $$900m{s}^{-1}$$ from a rifle of mass $$2.5kg$$. What is the recoil velocity of the rifle?

A
$$9m{s}^{-1}$$

B
$$180m{s}^{-1}$$

C
$$900m{s}^{-1}$$

D
$$1.8m{s}^{-1}$$

Solution
Question 6
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A bomb of mass $$16kg$$ at rest explodes into two pieces of masses $$4kg$$ and $$12kg$$. The velocity of the $$12kg$$ mass is $$4m{s}^{-1}$$. The kinetic energy of the other mass is

A
$$192J$$

B
$$96J$$

C
$$144J$$

D
$$288J$$
Question 7
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If the action and reaction forces are always equal in magnitude, then these forces

A
will produce accelerations of equal magnitudes.

B
may not produce accelerations of equal magnitudes.

C
produce velocities of equal magnitudes.

D
will not produce accelerations of equal magnitudes

Solution

According to newton's 3rd law of motion, Every action has equal and opposite reaction. Action and reaction always act in pairs, and on different bodies.
Let two masses $$m_1$$ and $$m_2$$ are pairs of masses applying force on each other ie $$F$$.
Acceleration of masses will be $$a_1=\dfrac F{m_1}$$ and $$a_2=\dfrac F{m_2}$$ Hence acceleration of masses also depends on the mass.
So, these forces may not produce accelerations of equal magnitudes.
Question 8
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A ball rests upon a flat piece of paper on a table top. The paper is pulled horizontally but quickly towards right as shown. Relative to its initial position with respect to the table, the ball
I. Remains stationary if there is no friction between the paper and the ball.
II. Moves to the left and starts rolling backwards, i.e. to the left if there is a friction between the paper and the ball.
III. Moves forward, i.e., in the direction in which the paper is pulled.
Here, the correct statements is/ are

A
I and II only

B
III only

C
I only

D
II only

Solution
Question 9
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A target of mass $$M$$ is moving with a velocity $$u$$. Lead shots, each of mass $$m$$, are fired on it in the opposite direction with a velocity $$v$$. Find the number of bullets required to stop the target.

A
$$\dfrac{Mu}{mv}$$

B
$$\dfrac{mv}{Mu}$$

C
$$\dfrac{(M+m)u}{v}$$

D
$$\dfrac{mu(M+m)}{v}$$

Solution

Let $$n$$ be the number of lead shots required to stop the target.
Given:
Mass of the target = $$M$$
Velocity of the target = $$u$$
Mass of each shot = $$m$$
Velocity of each shot = $$v$$

Initial momentum of the target and the shots is $$P_1=Mu-nmv$$ (the minus sign indicates that the shots are moving in the opposite direction)
Final momentum of the target and the shots is $$P_2=0$$
From the conservation of momentum,
$$P_1=P_2$$
$$\Rightarrow Mu-nmv=0$$
$$\Rightarrow n=\dfrac{Mu}{mv}$$
Question 10
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Which of the following cannot be explained using Newton's third law of motion?

A
Working of rocket in the fire crackers

B
Revolution of satellite around a planet

C
Functioning of satellite launch vehicle

D
Walking of a person on ground
Solution
The Third Law of Motion indicates that when one object exerts a force on another object, the second object instantaneously exerts a force back on the first object. These two forces are always equal in magnitude, but opposite in direction.
Rocket in fire crackers when fired releases gases in downward direction so the gases experience a downward force which in reaction exert upward force on a rocket which makes rocket to go up.
Revolution of satellite around a planet is not due to action-reaction effect.
Functioning of satellite launch vehicle is similar to rocket in fire crackers .The exhaust gases going down experience downward force so in reaction the satellite to be launched experience upward force which moves it upwards.
Walking on ground involves pushing of the ground by the person's foot in backward direction and in reaction the ground exert equal force but in forward direction hence the person is able to move.