How Forces Affect | Class 9 Science Notes
By ConceptScroll Team · Published on 17 July 2026 · 3 min read
How Forces Affect – this guide gives you a concise, exam-ready overview of How Forces Affect from Class 9 Science, written by ConceptScroll editors and reviewed against the latest NCERT textbook.
6.2 Balanced and Unbalanced Forces
This section discusses the interaction of multiple forces acting simultaneously on an object. It explains that forces can be balanced or unbalanced. Balanced forces are equal in magnitude but opposite in direction, resulting in no change in motion. For example, in a tug of war where both teams pull equally, the rope does not move. Unbalanced forces occur when forces are unequal, causing motion in the direction of the larger force. The net force is the vector sum of all forces acting on the object. The section illustrates forces acting in opposite directions with equal or unequal magnitudes, and forces acting in the same direction, explaining how to calculate net force in each case. It also introduces the concept of internal forces causing rotation when equal and opposite forces act on different points of an extended object, such as turning a tap handle.
📊 Diagram: Fig. 6.3: (a) Pushing a box kept on table or floor; Fig. 6.3: (b) A ball floating on water; (a) Two forces applied in opposite direction of equal magnitudes; (b) Two forces applied in opposite direction of unequal magnitudes; Fig. 6.5: Two forces applied in the same direction; (a), (b), (c) Two forces acting on a block in different manners; Fig. 6.7: Equal and opposite forces on an extended object.
🔗 Connection: Prepares for understanding friction as a force opposing motion.
Frequently asked questions
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.
Yes, the boat will move backward. When the sailor jumps forward, he exerts a force on the boat in the backward direction. According to Newton's third law, the boat exerts an equal and opposite force on the sailor. As a result, the boat moves backward while the sailor moves forward.
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.
The landing mat or sand bed is used to increase the time over which the athlete comes to rest after the jump. This reduces the force experienced by the athlete on landing, preventing injury. According to the impulse-momentum principle, increasing the time of impact decreases the force.
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.
Option (iv) is correct. According to Newton's third law of motion, the forces exerted by the two carts on each other are equal in magnitude and opposite in direction, regardless of their masses or velocities.
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.
From the graph of acceleration (a) versus mass (m), since force F = m × a, the force is constant if the product of m and a is constant. The force-mass graph will be a horizontal line indicating constant force irrespective of mass.
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Clear NCERT-aligned notes on खाद्य संसाधनों में सुधार for Class 9 Science.