Class 9 – Science Extra Questions – Ch. 8 – Force and Laws of Motion

Ans: B

Explanation: Fluids are substances that can flow and take the shape of their container. This property is characteristic of liquids and gases. Solids, on the other hand, have a fixed shape and volume and do not flow. Therefore, fluids encompass both liquids and gases.
Correct Option: B

Ans: D

Explanation: Archimedes’ principle states that the buoyant force on an object submerged in a fluid is equal to the weight of the fluid displaced by the object. Therefore, the magnitude of the buoyant force is the weight of the liquid displaced.
Correct Option: D

Ans: D

Explanation: A velocity-time graph where the line is straight and passes through the origin with a positive slope indicates that the velocity is increasing linearly with time. A straight line on a velocity-time graph represents constant acceleration. Passing through the origin means the initial velocity is zero. A positive slope means the acceleration is positive, so the velocity is increasing. Therefore, the car is accelerating uniformly from rest. Option A is incorrect because a constant velocity would be represented by a horizontal line. Option B is incorrect because the car is moving and its velocity is changing. Option C is incorrect because deceleration implies a negative acceleration, which would be a negative slope.
Correct Option: D

Ans: D

Explanation: The car accelerates uniformly from rest, meaning its initial velocity (\(u\)) is 0 m/s. The time taken (\(t\)) is 10 seconds, and the distance covered (\(s\)) is 50 meters. We can use the kinematic equation that relates displacement, initial velocity, time, and acceleration: \(s = ut + \frac{1}{2}at^2\). Plugging in the given values, we have: \(50 = (0)(10) + \frac{1}{2}a(10)^2\). This simplifies to \(50 = 0 + \frac{1}{2}a(100)\), so \(50 = 50a\). Dividing both sides by 50, we get \(a = \frac{50}{50} = 1\) m/s².
Correct Option: D

Ans: D

Explanation: When a tree branch is shaken vigorously, the branch experiences rapid acceleration. The leaves, being less rigidly attached to the branch (via the petiole), tend to resist this change in motion due to their inertia. The petiole, which is a relatively weaker point of connection, is subjected to forces that exceed its tensile strength as it tries to keep the leaves moving with the branch. This inertia causes the leaves to lag behind the motion of the branch, leading to their detachment. Gravity plays a role in the subsequent fall of the leaf, but inertia is the primary reason for the initial detachment during shaking. Capillary action is related to water transport and photosynthesis is a metabolic process, neither of which directly explains the detachment from shaking.
Correct Option: D

Ans: C

Explanation: When ice floats in water, it displaces a volume of water whose weight is equal to the weight of the ice. According to Archimedes’ principle, a floating object displaces a volume of fluid whose weight is equal to the weight of the object. Ice is less dense than water, which is why it floats. When ice melts, it turns into water. The key is that the volume of water produced by melting ice has the same mass (and therefore the same weight) as the original ice. Since the original ice displaced a volume of water equal to its weight, and the melted ice now becomes water with that same weight, the volume of water it occupies is exactly the same as the volume it displaced when it was ice. Therefore, the water level remains the same.
Correct Option: C

Ans: A

Explanation: When a ball rests on the surface of water, it is experiencing stable equilibrium. If the ball is slightly pushed down into the water, the buoyant force acting on it increases, pushing it back up to its original position. If the ball is lifted slightly out of the water, the buoyant force decreases, and gravity pulls it back down to its original position. This tendency to return to its original position after a disturbance indicates stable equilibrium.
Correct Option: A

Ans: A

Explanation: We can use Newton’s second law of motion, F = ma, to find the acceleration.
Given:
Force (F) = 5000 N
Mass (m) = 2000 kg
Acceleration (a) = F / m = 5000 N / 2000 kg = 2.5 m/s²

Since the object starts from rest, its initial velocity (u) is 0 m/s. We can use the kinematic equation v = u + at to find the velocity after 5 seconds.
Given:
Initial velocity (u) = 0 m/s
Acceleration (a) = 2.5 m/s²
Time (t) = 5 s
Velocity (v) = 0 m/s + (2.5 m/s² * 5 s) = 12.5 m/s

Therefore, the acceleration of the object is 2.5 m/s² and its velocity after 5 seconds is 12.5 m/s.
Correct Option: A

Ans: A

Explanation: Force is measured in Newtons (N). A Newton is defined as the force required to accelerate a mass of one kilogram by one meter per second squared, which is kg m s⁻². Therefore, the unit kg m s⁻¹ as given in option A is incorrect for force. Pressure is force per unit area, so its unit is N/m² or Pa. Buoyancy is a force, so it is measured in Newtons (N). Density is mass per unit volume, so its unit is kg/m³.
Correct Option: A

Ans: C

Explanation: When the bus is stationary and the ball is thrown upwards, the ball’s horizontal motion is zero. When the bus starts moving with uniform velocity, it means the bus and everything inside it, including the ball and the thrower, are moving horizontally with the same velocity. When the ball is thrown upwards, it inherits the horizontal velocity of the bus. Since the bus, the thrower, and the ball all have the same horizontal velocity, the ball will continue to move horizontally with the bus and will land back in the thrower’s hand. This is because, in the absence of air resistance, the horizontal and vertical motions are independent. The ball’s vertical motion is affected by gravity, and its horizontal motion is unaffected by the upward throw.
Correct Option: C