Exercise Questions and Answers
Q1: Match items in Column A with the items in Column B.
A1: Here’s the correct matching:
(i) Muscular force → (b) A child lifting a school bag
(ii) Magnetic force → (e) A compass needle pointing North
(iii) Frictional force → (a) A cricket ball stopping on its own just before touching the boundary line
(iv) Gravitational force → (c) A fruit falling from a tree
(v) Electrostatic force → (d) Balloon rubbed on woollen cloth attracting hair strands
Q2: State whether the following statements are True or False.
(i) A force is always required to change the speed of motion of an object.
(ii) Due to friction, the speed of the ball rolling on a flat ground increases.
(iii) There is no force between two charged objects placed at a small distance apart.
A2:
(i) True (A force is needed to change speed, as per the effects of force.)
(ii) False (Friction opposes motion, reducing the speed of a rolling ball.)
(iii) False (Charged objects exert an electrostatic force, attracting or repelling each other.)
Q3: Two balloons rubbed with a woollen cloth are brought near each other. What would happen and why?
A3: The balloons repel each other because they acquire similar charges (e.g., both positive or both negative) when rubbed with the woollen cloth. Like charges repel, as demonstrated in Activity 5.7.
Q4: When you drop a coin in a glass of water, it sinks, but when you place a bigger wooden block in water, it floats. Explain.
A4: The coin sinks because its weight (gravitational force) is greater than the buoyant force exerted by the water. The wooden block floats because its weight is equal to or less than the buoyant force, as the wood is less dense and displaces enough water to balance its weight (Archimedes’ Principle).
Q5: If a ball is thrown upwards, it slows down, stops momentarily, and then falls back to the ground. Name the forces acting on the ball and specify their directions.
(i) During its upward motion
(ii) During its downward motion
(iii) At its topmost position.
A5:
(i) Upward motion: Gravitational force (downward) opposes the ball’s motion, slowing it down. Air friction (downward) also opposes the motion.
(ii) Downward motion: Gravitational force (downward) accelerates the ball toward the Earth. Air friction (upward) slightly opposes the motion.
(iii) Topmost position: Gravitational force (downward) acts on the ball, causing it to begin falling. Air friction is negligible as the ball is momentarily stationary.
Q6: A ball is released from point P and moves along an inclined plane, then along a horizontal surface as shown in Fig. 5.16. It comes to a stop at point A on the horizontal surface. Think of a way so that when the ball is released from the same point P it stops (i) before the point A (ii) after crossing the point A.
A6:
(i) Stop before point A: Increase friction on the horizontal surface by placing a rough material (e.g., sandpaper or cloth) before point A. This increases the frictional force, stopping the ball sooner.
(ii) Stop after crossing point A: Reduce friction by making the horizontal surface smoother (e.g., using a polished or lubricated surface). This decreases the frictional force, allowing the ball to travel farther past point A.
Q7: Why do we sometimes slip on smooth surfaces like ice or polished floors? Explain.
A7: Smooth surfaces like ice or polished floors have fewer surface irregularities, reducing frictional force. This low friction makes it harder to maintain grip, causing slipping when walking or applying force.
Q8: Is any force being applied to an object in non-uniform motion?
A8: Yes, an object in non-uniform motion (changing speed or direction) experiences a net force. This force, such as gravity, friction, or an applied push/pull, causes the acceleration or deceleration.
Q9: The weight of an object on the Moon becomes one-sixth of its weight on the Earth. What causes this change? Does the mass of the object also become one-sixth of its mass on the Earth?
A9: The weight on the Moon is one-sixth that on Earth because the Moon’s gravitational force is one-sixth that of Earth’s. Weight is the gravitational force (W = m × g), and since g is smaller on the Moon, weight decreases. The mass of the object remains the same, as mass is the amount of matter and does not depend on location.
Q10: Three objects 1, 2, and 3 of the same size and shape, but made of different materials are placed in water. They dip to different depths as shown in Fig. 5.17. If the weights of the three objects are w₁, w₂, and w₃, respectively, then
(i) w₁ = w₂ = w₃
(ii) w₁ > w₂ > w₃
(iii) w₁ < w₂ < w₃
(iv) w₁ > w₃ > w₂
A10: (ii) w₁ > w₂ > w₃
Objects dipping to different depths indicate different buoyancies due to their densities. The object that sinks deepest (highest density) has the greatest weight, as it displaces less water and experiences less buoyant force. Assuming object 1 sinks deepest, object 2 is partially submerged, and object 3 floats highest, their weights follow w₁ > w₂ > w₃.