01 · Explore
The Concept of Force
While motion is described by position and velocity, force is the underlying cause that changes these states. It is an interaction that can alter an object's rest, speed, direction, or shape.
A force is a push or a pull. In everyday life, we see its effects when kicking a ball (starting motion), striking a cricket ball with a bat (changing direction), or squeezing a lemon (changing shape). It is not just the presence of force that matters, but also its strength and where it is directed.
Force is a vector quantity, meaning it requires both magnitude and direction to be fully described. For instance, gravity pulls objects downwards, while buoyancy in water acts upwards. If you change either the magnitude or the direction of the applied force, the resulting effect on the object changes.
The SI unit of force is the newton, symbolized as N. The magnitude of a force represents its strength. We can measure this magnitude using a spring balance, which uses the stretching of an internal spring to indicate the force applied. In scientific terms, 1 N is roughly the upward force you apply to hold a 100 g mass steady in your palm.
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What are the three primary effects a force can have on an object?
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Why is force considered a vector quantity?
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Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 1, 2, 11.
02 · Explore
Balanced and Unbalanced Forces
In most real-world scenarios, multiple forces act on an object simultaneously. The resulting motion depends on the net force, which is the vector sum of all individual forces.
When two forces of equal magnitude act on an object in exactly opposite directions, they are called balanced forces. In this state, the net force is zero. A stationary object remains at rest, and a moving object continues at a constant velocity without changing speed or direction.
If the forces acting on an object are unequal, they are unbalanced forces. This results in a non-zero net force. The object will accelerate in the direction of the larger force. For example, in a tug-of-war, the rope moves toward the team that pulls with a larger magnitude of force.
To calculate the net force when forces act along the same line: if they are in the same direction, add their magnitudes; if they are in opposite directions, subtract the smaller magnitude from the larger one. The direction of the net force will be the direction of the larger individual force.
In the absence of friction, once an object is moving, no further force is required to keep it moving with a constant velocity. However, to change the velocity or stop the object, an unbalanced force must be applied.
Calculating Net Force on a Block
F_net = F1 - F2
If a force of 10 N acts to the right and 6 N acts to the left, the net force is 10 N - 6 N = 4 N towards the right. If both forces (10 N and 6 N) acted to the right, the net force would be 10 N + 6 N = 16 N to the right.
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If a rope in a tug-of-war does not move, what can you conclude about the forces?
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Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 2, 3, 8.
03 · Explore
The Force of Friction
Friction is a contact force that arises between two surfaces and always acts in a direction opposite to the relative motion of the objects.
When you push a heavy box on the floor, it may not move at first. This is because the force of friction between the box's bottom and the floor opposes your push. The box only begins to move when your applied force exceeds the maximum frictional resistance.
Friction depends on the nature of the surfaces in contact. Smooth surfaces like polished marble offer less friction than rough surfaces like a cemented floor. This is why a stack of coins travels further on a smooth table than on a wooden one when given the same initial push.
To keep an object moving at a constant velocity in the presence of friction, a continuous external force must be applied to balance the frictional force. If the applied force is removed, friction will eventually bring the moving object to rest. Friction can also be helpful; for example, it provides the necessary grip for us to walk without slipping.
Friction Opposing Motion
- 1
Applied Force
A person pushes a box in the forward direction.
- 2
Frictional Resistance
Friction acts at the contact surface in the backward direction.
- 3
Net Force Action
If Applied Force > Friction, the box accelerates forward.
- 4
Deceleration
If Applied Force is removed, only friction acts, slowing the box to a stop.
This sequence illustrates how friction opposes an applied force and eventually stops motion once the external force is no longer present.
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Why is it difficult to walk on a wet polished floor?
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Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 4, 5, 6, 7.
04 · Explore
Newton's First Law of Motion
Often called the Law of Inertia, this law describes the behavior of objects when the net external force acting on them is zero.
Newton's first law states that an object at rest remains at rest, and an object in motion continues to move with a constant velocity (same speed and straight-line direction), unless a net force acts upon it. This means that in the absence of a net force, an object's acceleration is zero.
The inherent tendency of an object to resist any change in its state of rest or motion is called inertia. Galileo Galilei first argued that if all impediments like friction were removed, a body moving on a horizontal plane would continue to move indefinitely. Newton later formalized this as his first law.
Constant velocity implies no change in either speed or direction. Therefore, if an object is moving in a straight line at a steady speed, we can conclude that the net force acting on it is zero. To change the velocity or stop the object, an unbalanced force must be applied.
| Initial State | Net Force | Resulting Motion |
|---|---|---|
| At Rest | Zero | Stays at Rest |
| Moving | Zero | Moves with Constant Velocity |
| At Rest | Non-Zero | Starts Moving (Accelerates) |
| Moving | Non-Zero | Changes Speed or Direction |
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If an object is moving with a constant velocity, is there a net force acting on it?
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Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 7, 8.