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 Second Law of Motion
Newton's second law of motion establishes the quantitative relationship between force, mass, and acceleration. It states that when a net force acts on an object, the object accelerates in the direction of the net force. The magnitude of the acceleration is directly proportional to the magnitude of the net force and inversely proportional to the mass of the object. Mathematically, this is expressed as a = F/m or equivalently F = ma, where F is the net force, m is the mass, and a is the acceleration. This law explains everyday observations such as a gentle push causing slow acceleration and a strong push causing rapid acceleration. It also explains why heavier objects require more force to achieve the same acceleration as lighter objects. Classroom activities using carts, pulleys, and weights demonstrate this relationship by varying force and mass and measuring the resulting acceleration. For example, increasing the hanging mass in a pulley system increases the force pulling the cart, resulting in higher acceleration. Conversely, increasing the cart's mass while keeping force constant decreases acceleration. The unit of force, the newton (N), is defined as the force required to accelerate a 1 kg mass by 1 m/s². The second law also applies to gravitational force, where the force acting on an object due to Earth's gravity is F = mg, with g ≈ 9.8 m/s². Real-world applications include safety features like airbags, which increase the time over which a force acts, reducing acceleration and injury. Newton's second law is fundamental in physics and engineering for predicting motion under various forces.
📊 Diagram: Figures 6.17a and 6.17b show a cart with ball bearing wheels and a pulley system used to demonstrate the relationship between force, mass, and acceleration. Figure 6.18 shows a cricket fielder catching a ball, illustrating force and acceleration concepts. Figure 6.19 depicts an inflated airbag in a vehicle, demonstrating safety applications.
🧪 Activity: Activity 6.3 involves measuring acceleration of a cart pulled by a hanging mass using a pulley system, varying force and mass to observe changes in acceleration. Activity 6.4 repeats the experiment with increased cart mass to observe decreased acceleration.
🔗 Connection: This section builds on Newton's first law by quantifying how forces cause acceleration, setting the stage for Newton's third law about forces in pairs.
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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- खाद्य संसाधनों में सुधार | Class 9 Science Notes
Clear NCERT-aligned notes on खाद्य संसाधनों में सुधार for Class 9 Science.