How Forces Affect Motion | Class 9 Science Notes
By ConceptScroll Team · Published on 17 July 2026 · 4 min read
How Forces Affect Motion – this guide gives you a concise, exam-ready overview of How Forces Affect Motion from Class 9 Science, written by ConceptScroll editors and reviewed against the latest NCERT textbook.
Newton's First Law of Motion
Newton's first law of motion, also known as the law of inertia, states that an object at rest remains at rest, and an object in motion continues to move with a constant velocity unless acted upon by a net external force. This means that if the total force acting on an object is zero, there will be no change in its state of motion. The object will either remain stationary or keep moving in a straight line at a constant speed. This law formalizes the concept of inertia, which is the tendency of objects to resist changes in their state of motion. For example, a book lying on a table remains at rest because the forces acting on it (gravity and the normal force) are balanced. Similarly, a moving car will continue moving at the same speed and direction if no net force acts on it, such as friction or braking forces. The first law highlights that motion does not require a continuous force to be maintained; rather, force is needed to change the velocity (speed or direction) of an object. This was a revolutionary idea that contradicted earlier beliefs that a force was necessary to keep an object moving. The law also implies that acceleration occurs only when a net force acts on the object. Graphical representations include position-time graphs showing constant position for an object at rest and straight-line graphs for objects moving with constant velocity. Velocity-time graphs show zero velocity for stationary objects and constant velocity (horizontal line) for moving objects without acceleration. Understanding this law is crucial for analyzing motion and forms the foundation for Newton's second law, which quantifies the relationship between force and acceleration.
📊 Diagram: Figures 6.15a and 6.15b show position-time and velocity-time graphs for an object at rest (constant position, zero velocity). Figures 6.16a and 6.16b show position-time and velocity-time graphs for an object moving with constant velocity (linear position increase, constant velocity).
🔗 Connection: This section introduces the concept of inertia and zero net force, leading naturally to Newton's second law, which relates force and acceleration quantitatively.
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 forward force on the shore. According to Newton's third law, the shore exerts an equal and opposite force backward on the sailor. To conserve momentum, the boat moves backward as 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 placed to increase the time over which the athlete comes to rest, thereby reducing the impact force on the athlete's body. This cushioning prevents injuries by absorbing the energy of the fall.
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.
the loaded cart and the empty cart, both exert an equal magnitude of force on each other.
According to Newton's third law of motion, the forces two bodies exert 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 Newton's second law, Force F = mass m × acceleration a.
Given the acceleration-mass graph, for each mass, acceleration is known. Multiplying mass and acceleration gives the force.
Since the force is constant (as the acceleration decreases with increasing mass), the force-mass graph will be a horizontal straight line indicating constant force irrespective of mass.
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Clear NCERT-aligned notes on खाद्य संसाधनों में सुधार for Class 9 Science.