Chapter 6 Solutions — How Forces Affect Motion

Revise, Reflect, Refine

1. 1.
   Using a horizontal force F, a table is moved across the floor at a constant velocity. How much is the frictional force exerted by the floor on the table?
2. 2.
   For a ball moving on a smooth (frictionless) surface, choose the appropriate option that will make the following statements physically correct:

   • (i) If no net force is applied on the ball, the velocity of the ball will remain the same/increase/decrease.
   • (ii) If a net force is applied on the ball in the direction of its motion, the magnitude of the velocity of the ball will remain the…
3. 3.
   Two blocks P and Q on a smooth horizontal surface are shown in Fig. 6.36a and Fig. 6.36b. Two forces of magnitudes 4 N and 5 N are acting in opposite directions on block P, while block Q is moving with a constant velocity. Which of the following statement is correct?

   • (i) P experiences a net force and Q does not experience a net force.
   • (ii) P does not experience a net force and Q experiences a…
4. 4.
   While practising for the snake boat race (Vallum kalli in Kerala), 100 oarsmen are rowing a boat together. Out of these, 95 row backwards to propel the boat forward. But by mistake, 5 oarsmen row in the opposite direction. If each oarsman applies a horizontal force of 200 N, what is the net force on the snake boat? (Ignore drag forces, air friction, etc.)
5. 5.
   When a net force acts on an object, we observe that the object accelerates:

   • (i) opposite to the direction of force, with acceleration proportional to the force acting on the object.
   • (ii) opposite to the direction of force, with acceleration proportional to the mass of the object.
   • (iii) in the direction of force, with acceleration inversely proportional to the force acting on the object.
   • (iv) in…
6. 6.
   The position-time graph for four objects A, B, C and D moving along a straight line are given in Fig. 6.37. A net force acts on:

   • (i) Object A
   • (ii) Object B
   • (iii) Object C
   • (iv) Object D
7. 7.
   A sailor jumps out from a small boat to the shore (Fig. 6.38). As the sailor jumps forward, will the boat move? If yes, in which direction and why.
8. 8.
   During a high jump event, a landing mat or sand bed is placed for the athlete to fall upon (Fig. 6.39). Explain the reason behind it.
9. 9.
   A hand cart loaded with vegetables collides with an identical but empty hand cart. During the collision:

   • (i) the loaded cart exerts a force of larger magnitude on the empty cart.
   • (ii) the empty cart exerts a force of larger magnitude on the loaded cart.
   • (iii) neither cart exerts a force on the other.
   • (iv) the loaded cart and the empty cart, both exert an equal magnitude of force on each other.
10. 10.
    The acceleration-mass graph for the acceleration produced by a force on objects of different masses is plotted in Fig. 6.40. Plot the force-mass graph for this case.
11. 11.
    The velocity-time graph of an object of mass 10 kg moving along a straight line is shown in Fig. 6.41. Calculate the force acting on the object by using the graph.
12. 12.
    A bullet of mass 50 g moving with a speed of 100 m s⁻¹ enters a heavy stationary wooden block and stops after penetrating a distance of 50 cm. Estimate the stopping force acting on the bullet (assume that the bullet undergoes constant acceleration within the block).
13. 13.
    An ace footballer converted a penalty shot by kicking the football with a speed of 108 km h⁻¹. The estimated force they imparted was 800 N. The mass of the football was 0.4 kg. Calculate the time of contact between their foot and the ball.
14. 14.
    An object of mass 2 kg moving with a constant velocity of 10 m s⁻¹ encounters a rough patch where the force of friction on the object is 7 N. At the same time, an additional constant force of 3 N opposing the motion is applied on the object. After entering the rough patch, how much distance does the object travel before coming to rest?
15. 15.
    A tractor pulls a harrow (a ploughing tool) of mass m₁ with a net force F resulting in an acceleration of a₁. The same tractor pulls a trolley of mass m₂ with a force F producing an acceleration of a₂. If the tractor now pulls the trolley with the harrow placed on it (with the same force F), then obtain an expression for the resulting acceleration in terms of a₁ and a₂. Ignore friction.
16. 16.
    When the pole of a bar magnet is brought close to a magnetic compass, the bar magnet and the compass needle (which is also a magnet) exert a magnetic force on each other. As per Newton's third law of motion, both the forces are equal in magnitude and opposite in direction. However, the compass needle moves, whereas the bar magnet does not move (Fig. 6.42). Explain why.