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CBSE | Class 9 | science
Read the chapter in a clean notebook-style flow before moving into deeper practice.
Summary Note
Key concept recap
This chapter explores the causes behind the motion of objects, emphasizing the concept of force as a push or pull that changes an object's state of motion or shape.
Everyday experiences such as pushing a cart or hitting a ball involve the application of force.
Force can change the speed, direction, or shape of an object, and its presence is inferred from these effects.
Summary Note
Key concept recap
Balanced forces do not change the state of motion or rest of an object. When equal and opposite forces act on an object, it remains in its current state.
Unbalanced forces cause a change in motion—either by setting a stationary object in motion or by altering the motion of a moving object.
For example, if children push a box hard enough to overcome friction, the box starts moving due to an unbalanced force.
Summary Note
Key concept recap
The first law states: An object remains at rest or in uniform motion unless acted upon by an unbalanced force.
Galileo's experiments showed that objects in motion would continue moving unless a force (like friction) stopped them.
This tendency of an object to resist change in its state is called inertia. The mass of an object is a measure of its inertia.
Summary Note
Key concept recap
Inertia is the property by which an object resists change in motion or rest.
Heavier objects have more inertia than lighter ones; hence a train has more inertia than a cart.
Mass and inertia are directly proportional—greater the mass, greater the inertia.
Summary Note
Key concept recap
This law relates force, mass, and acceleration. It states that the rate of change of momentum is proportional to the applied unbalanced force.
Mathematically, F = ma where F is force, m is mass, and a is acceleration.
It explains that a greater force causes a greater change in momentum, and is useful for measuring force.
Summary Note
Key concept recap
Momentum is the product of mass and velocity, represented as p = mv.
It is a vector quantity and has the same direction as velocity.
SI unit of momentum is kg m/s. A change in momentum is caused by force over time.
Summary Note
Key concept recap
Examples include catching a fast ball, jumping on a sandbed, and applying brakes in a car.
Increasing the time of impact reduces the force exerted, which is why hands are pulled back while catching a ball.
Force can also be calculated from change in momentum using F = (mv - mu)/t.
Summary Note
Key concept recap
The third law states: To every action, there is an equal and opposite reaction.
The forces act on different objects but are equal in magnitude and opposite in direction.
Examples include recoil of a gun, jumping off a boat, and walking, where backward push results in forward motion.
Summary Note
Key concept recap
Force – A push or pull acting on an object.
Inertia – Resistance of an object to change its motion.
Momentum – Product of mass and velocity (p = mv).
Balanced Force – Equal and opposite forces, no motion change.
Unbalanced Force – Causes acceleration or deceleration.
Newton's First Law – Object remains at rest or in motion unless disturbed.
Second Law – F = ma; rate of change of momentum is force.
Third Law – Action and reaction are equal and opposite.
SI Unit of Force – Newton (N).
