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.
6.1 The Concept of Force
This section defines force as a physical quantity that can cause an object to move from rest, change its speed or direction, or alter its shape. Examples include kicking a ball to start its motion, striking a cricket ball to change its direction, and squeezing a lemon to change its shape. The section emphasizes that force is a vector quantity, meaning it has both magnitude and direction. The SI unit of force is the newton (N). The magnitude expresses the strength of the force, and the direction specifies where the force acts. It is noted that changing either magnitude or direction of a force changes its effect. The section also introduces the spring balance as an instrument to measure force, explaining its working principle. It mentions that forces as small as millinewtons can be felt, while scientists can measure forces as tiny as yoctonewtons in specialized experiments.
📊 Diagram: You may recall learning earlier that a force can make an object move from rest, change the speed and direction of motion of a moving object, and can even change the shape of an object. For example, a ; (a) Kicking a ball; (b) Striking a ball; (c) Squeezing a lemon.
🔗 Connection: Prepares for understanding how forces combine and how to measure them.
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.
- खाद्य संसाधनों में सुधार | Class 9 Science Notes
Clear NCERT-aligned notes on खाद्य संसाधनों में सुधार for Class 9 Science.