What is Force?
A force is a push or pull acting on an object that can change its state of rest, state of motion, speed, direction of motion, or shape.
Force is a Vector Quantity:
A force has both:
- Magnitude – How large the force is (measured in newtons).
- Direction – The direction in which the force acts.
Example
If Rahul pushes a box with \(20\ N\) towards the east, then
- Magnitude \(=\) \(20\ N\)
- Direction \(=\) East
Without direction, the force is not completely described.
Balanced and Unbalanced Forces:
| Characteristics | Balanced Force | Unbalanced Force |
| Definition | Balanced forces are equal in magnitude and opposite in direction. | When forces acting on an object are not equal, the net force is not zero. |
| Effects |
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| Example |
Two students pull a rope with \(100\ N\) each in opposite directions.
Net force
\(100−100=0\ N\)
The rope does not move.
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One student pulls with \(80\ N\) and another with \(50\ N\).
Net force
\(80−50=30\ N\)
The object moves towards the larger force.
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Net Force and Constant Velocity:
According to Newton's First Law, if an object is moving with constant velocity, then
- \(Acceleration\ =\ 0\)
- \(Net\ force\ =\ 0\ N\)
This does not mean no forces are acting.
It means all forces are balanced.
Example
A car moving at constant speed on a straight road has
- \(Engine\ force\ =\ Friction\ +\ Air\ resistance\)
Therefore,
\(Net\ force\ =\ 0\)
Friction:
Friction is the force that opposes the relative motion between two surfaces in contact.
Important Points:
- Always acts opposite to the direction of motion
- Exists only when two surfaces are in contact
- Depends on the nature of the surfaces.
| Advantages of Friction | Disadvantages of Friction | Reducing Friction |
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Force Produces Acceleration:
Whenever the velocity of an object changes, there must be an unbalanced force acting on it.
A change in velocity may mean
- Increase in speed
- Decrease in speed
- Change in direction
All these are forms of acceleration.
Newton's First Law of Motion:
An object continues to remain at rest or continues to move with constant velocity in a straight line unless acted upon by an unbalanced external force.Examples
Examples:
- Passengers fall backward when a bus starts suddenly.
- Passengers fall forward when a moving bus stops suddenly.
- Dust comes out when a carpet is beaten.
Inertia:
Inertia is the property of an object by which it resists any change in its state of rest or motion.
Greater mass \(\Rightarrow\) Greater inertia.
Types of Inertia:
| Inertia of Rest | Inertia of Motion | Inertia of Direction |
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An object at rest tends to remain at rest.
Example
Passengers fall backward when a bus starts.
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A moving object tends to continue moving.
Example
Passengers fall forward when a moving bus stops suddenly.
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An object resists any change in its direction.
Example
Passengers are pushed sideways when a car takes a sharp turn.
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Newton's Second Law of Motion:
The acceleration produced in an object is directly proportional to the net force acting on it and inversely proportional to its mass.
\(F=ma\)
Where
- \(F\ =\ Force\ (N)\)
- \(m\ =\ Mass\ (kg)\)
- \(a\ =\ Acceleration\ (m/s^{2})\)
SI Unit of Force:
From
\(F=ma\)
\(SI\ unit\ of\ mass = kilogram (kg) \)
\(SI\ unit\ of\ acceleration = metre\ per\ second\ squared (m/s^{2}) \)
Therefore,
\(1N = 1kg\times 1m/s^{2}\)
One newton is the force required to produce an acceleration of \(1\ m/s^{2}\) in a body of mass \(1\ kg\).
Newton's Third Law of Motion:
For every action, there is an equal and opposite reaction.
Characteristics
- Equal in magnitude
- Opposite in direction
- Act simultaneously
- Act on different objects
Therefore, they never cancel each other.
Example:
Walking
Swimming
Rocket launch
Gun recoil
Rowing a boat
Applying Newton's Laws in Real-Life:
| Situation | Law / Concept applied | Explanation |
|---|---|---|
| Wearing a seat belt | Newton's first law | Seat belt prevents passengers from moving forward due to inertia when the car stops suddenly. |
| Pushing an empty shopping trolley | Newton's Second Law | A lighter trolley accelerates more than a loaded trolley for the same force. |
| Rocket launch | Newton's Third Law | Exhaust gases are pushed downward, producing an equal and opposite thrust that moves the rocket upward. |
| Tug of war | Balanced/Unbalanced Forces | Equal pulls keep the rope stationary; unequal pulls make it move toward the larger force. |
| Braking a bicycle | Friction | Friction between the brake pads and wheel reduces speed and brings bicycle to rest. |