Force and Motion Questions

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Very Short Answers Questions [1 Mark Questions]

Ques. If a bus is approaching a bus stop, what will be its acceleration?

1. Positive acceleration
2. Uniform acceleration
3. Negative acceleration
4. Non-uniform acceleration

Ans. The correct answer is c. Negative acceleration

Explanation: While approaching a bus stop, the velocity of the bus needs to be decreased. Decreasing velocity is referred to as negative velocity. The acceleration of the bus is the rate of change of its velocity. Therefore, the acceleration of the bus will be negative.

Ques. Force is a polar vector.

1. True
2. False

Ans. The correct answer is a. True

Explanation: The force is a push or a pull that has the potential to modify a body's state of rest or uniform motion, implying that force is a polar vector with a point of application.

Ques. What will happen if a wolf chases a rabbit and the rabbit runs in a zigzag path?

1. The wolf catches the rabbit easily
2. The wolf gets confused
3. It becomes tough for the wolf to catch the rabbit
4. The rabbit dies

Ans. The correct answer is c. It becomes tough for the wolf to catch the rabbit

Explanation: When the rabbit runs in a zigzag manner, it is more difficult for the wolf to catch it because the wolf has greater mass and hence more inertia of direction than the rabbit.

Ques. Which among the following can be observed as an example of motion in three dimensions?

1. Train moving on a straight track
2. Planets revolving around the sun
3. A kite flying in the sky on a windy day
4. A free-falling body

Ans. The correct answer is c. A kite flying in the sky on a windy day

Explanation: When all three coordinates describing an object's location change with respect to time, the motion of the object is said to be three-dimensional. As a result, a kite flying in the sky on a windy day is one of the best examples of three-dimensional motion.

Ques. A person sitting inside a moving bus_____ when it suddenly stops.

1. Falls down
2. Falls backward
3. Remains unaffected
4. Falls forward

Ans. The correct answer is d. Falls forward

Explanation: As the bus comes to a stop, the bottom half of the person's body comes to rest with it, but the upper part of his body remains in motion because of inertia, and the person falls forward.

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Short Answers Questions [2 Marks Questions]

Ques. Define Motion.

Ans. An object is said to be in motion with respect to an observer, if its position changes with time, with respect to that observer.

Ques. What is Force?

Ans. A push or pull acting on a body is referred to as force acting on that body. It is defined as a physical quantity that causes motion or which opposes the motion or which changes the direction of motion of a body.

Ques. Define uniform motion.

Ans. Uniform motion is the motion of an object along a straight path with a constant speed. This represents that an object travels the same distance in the same period of time. For example, a car is moving at a constant speed of 40 km/h on a straight road.

Ques. Define frame of reference.

Ans. A frame with respect to which an observer observes the position and motion of an object is known as the frame of reference or reference frame. For example, if an observer observes the motion of an object while standing on Earth then his frame of reference is Earth.

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Long Answers Questions [3 Marks Questions]

Ques. A mass of 5 kg is acted upon by a force of 1 N. Starting from rest, how much distance is covered by the mass in 10 s?

Ans. Given

• Mass, m = 5 kg
• Force, F = 1 N
• Time, t = 10 s
• Initial velocity, u = 0

Acceleration, a = F/m = 1/5 = 0.2 m/s²

Distance covered by the mass is given by

s = ut + 1/2 at²

⇒ s = (0 x 10) + (1/2 x 0.2 x 10²) = 10 m

Ques. A particle is moving with a uniform acceleration. Its position-time relationship is given by x = 3 + 4t + 5t² in SI units. Calculate the magnitude of its

(i) Initial velocity

(ii) Velocity at t = 3s

(iii) Uniform acceleration

(iv) Position at t = 5s

Ans. Given

x = 3 + 4t + 5t²

⇒ velocity, v = dx/dt = d/dt (3 + 4t + 5t²)

⇒ v = 4 + 10t

(i) For initial velocity, t = 0

⇒ vinitial = 4 + (10 x 0) = 4 m/s

(ii) Velocity at t = 3s

⇒ v = 4 + (10 x 3) = 34 m/s

(iii) Acceleration at any time t is given by

a = dv/dt = d/dt (4 + 10t)

⇒ a = 10 m/s²

(iv) At t = 5 s

x = 3 + (4 x 5) + (5 x 5²) = 148 m

Ques. Find the maximum speed with which a car can turn on a bend without skidding. If the radius of the bend is 20 m and the coefficient of friction between the road and tyres is 0.4.

Ans. Given

• Coefficient of static friction, µ_s = 0.4
• The radius of the bend, R = 20 m

The maximum speed with which a car can turn on a bend without skidding is given by

v_max = √(µ_sgR)

⇒ v_max = √(0.4 x 9.8 x 20)

⇒ v_max = 8.85 m/s

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Very Long Answers Questions [5 Marks Questions]

Ques. A block of mass 5 kg is moving with a speed of 8 m/s. A force of 10 N is applied to it for 3 s. Find the final speed of the block if force is applied

(i) Along the motion

(ii) Opposite to the motion

(iii) Normal to the initial direction of motion of the block

Ans. Given

• Mass of the block, m = 5 kg
• The initial velocity of the block, u = 8 m/s
• Force applied to the block, F = 10 N
• Time, t = 3 s

Acceleration produced by force, a = F/m

⇒ a = 10/5 = 2 m/s²

(i) When the force is applied in the direction of motion, the final speed of the block is given by

v = u + at

⇒ v = 8 + (2 x 3) = 14 m/s

(ii) When the force is applied opposite to the direction of motion, then acceleration is taken as negative. The velocity is given by

v = u + at

⇒ v = 8 + (-2 x 3) = 2 m/s

(iii) When the force acts perpendicular to the direction of motion.

Let the block moving along the x-axis acceleration be along the y-axis, then

• Initial velocity, u = 8\(\hat{i}\) m/s
• Acceleration, a = 2\(\hat{j}\) m/s²

Final velocity is given by

v = u + at

⇒ v = 8\(\hat{i}\) + (2\(\hat{j}\) x 3)

⇒ v = 8\(\hat{i}\) + 6\(\hat{j}\)

⇒ |v| = 10 m/s

Ques. A bullet of mass 200 g fired from a gun moving with a velocity of 20 m/s hits a wooden log. The bullet stops after traveling a distance of 40 cm in the wooden log. Calculate the retrading force exerted by the log on the bullet.

Ans. Given

• Initial velocity of the bullet, u = 20 m/s
• Mass of the bullet, m = 200 g = 0.2 kg
• Distance traveled by the bullet, s = 40 cm = 0.4 m

Since the bullet stops after traveling 40 cm distance in the wooden log, then the final velocity of the bullet, v = 0.

The retardation of the bullet can be calculated using the equation of motion

v² = u² + 2as

⇒ a = (v² – u²) / 2s

⇒ a = (0² - 20²) / (2 x 0.4)

⇒ a = – 500 m/s²

Hence, the retarding force is given by

F = ma

⇒ F = 0.2 x (-500)

⇒ F = - 100 N

Ques. A stone of mass 1 kg is thrown with a velocity of 20 m/s across the frozen surface of a lake and it comes to rest after traveling a distance of 50 m. What is the magnitude of the force opposing the motion of the stone?

Ans. Given

• The initial velocity of the stone, u = 20 m/s
• Mass of the stone, m = 1 kg
• Distance traveled by the stone, s = 50 m

Since the stone stops after traveling a 50 m distance across the frozen lake, then the final velocity of the bullet, v = 0.

The retardation of the stone can be calculated using the equation of motion

v² = u² + 2as

⇒ a = (v² – u²) / 2s

⇒ a = (0² – 20²) / (2 x 50)

⇒ a = – 4 m/s²

Hence, the retarding force is given by

F = ma

⇒ F = 1 x (- 4)

⇒ F = – 4 N

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