Short Answer Questions — Chapter 1: Laws of Motion
Inertia is the property of a body by which it resists any change in its state of rest or uniform motion in a straight line.
Example: When a bus suddenly starts moving, passengers tend to fall backward. This is because their bodies resist the change from rest to motion — inertia of rest.
More examples:
- A coin placed on a card over a glass falls into the glass when the card is flicked — the coin's inertia of rest keeps it in place while the card moves.
- When a moving bus stops suddenly, passengers lean forward — their bodies want to continue in motion (inertia of motion).
| Type | Definition | Example |
|---|---|---|
| Inertia of rest | Resistance to starting motion from rest | Passengers fall back when bus starts |
| Inertia of motion | Resistance to stopping or changing speed | Passengers fall forward when bus brakes |
| Inertia of direction | Resistance to changing direction | Mud flies off a rotating wheel tangentially |
Statement: The rate of change of momentum of a body is directly proportional to the net external force acting on it, and the change takes place in the direction of the net force.
Mathematical form:
F = ma
Where:
- F = Net force (in Newtons, N)
- m = Mass of the body (in kg)
- a = Acceleration produced (in m/s²)
Derivation:
- Initial momentum = mu; Final momentum = mv
- Change in momentum = mv − mu = m(v − u)
- Rate of change = m(v − u)/t = ma
Impulse is defined as the product of the force and the time for which the force acts on a body.
Since F = m(v − u)/t:
Impulse = F × t = m(v − u) = change in momentum
SI unit: Newton-second (N·s) or kilogram metre per second (kg·m/s)
Physical significance: A large force acting for a short time (like a bat hitting a ball) or a small force acting for a long time can produce the same impulse.
Statement: For every action, there is an equal and opposite reaction. The action and reaction forces act on different bodies.
Mathematically: If body A exerts force F on body B (action), then body B exerts force −F on body A (reaction).
Examples:
| Situation | Action | Reaction |
|---|---|---|
| Rocket propulsion | Hot gases pushed downward | Rocket pushed upward |
| Swimming | Swimmer pushes water backward | Water pushes swimmer forward |
| Walking | Foot pushes ground backward | Ground pushes foot forward |
| Recoil of gun | Gun exerts force on bullet (forward) | Bullet exerts force on gun (backward) |
Important: Action and reaction are equal in magnitude, opposite in direction, and act on different objects — so they do NOT cancel each other.
Momentum is defined as the product of the mass and velocity of a body.
Where:
- p = momentum
- m = mass of the body (kg)
- v = velocity of the body (m/s)
SI unit: kilogram metre per second (kg·m/s)
Key points:
- Momentum is a vector quantity (has both magnitude and direction)
- Direction of momentum = direction of velocity
- A heavier and faster body has more momentum
Statement: When no external force acts on a system of objects, the total momentum of the system remains constant (is conserved).
Mathematically: If two objects collide with no external force:
m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂
Where:
- m₁, m₂ = masses of the two objects
- u₁, u₂ = initial velocities
- v₁, v₂ = final velocities after collision
Examples:
- Recoil of a gun: gun + bullet system momentum = 0 before firing; after firing, bullet goes forward and gun recoils backward with equal magnitude momentum.
- Rocket propulsion: exhaust gases pushed backward, rocket pushed forward.
| Property | Mass | Weight |
|---|---|---|
| Definition | Quantity of matter in a body | Force of gravity on the body |
| Formula | — | W = mg |
| Unit | kilogram (kg) | Newton (N) |
| Type | Scalar | Vector |
| Varies with location? | No — constant everywhere | Yes — different on Moon, Earth, space |
| Value in space | Remains same | Zero (weightlessness) |
Couple: A couple consists of two equal, parallel, and opposite forces acting on a body at different points.
Characteristics of a couple:
- The two forces are equal in magnitude
- They act in opposite directions
- They are separated by a perpendicular distance (called the arm of the couple)
- The resultant force = 0 (no translational motion), but they produce rotation
Moment of a couple = Torque = Force × Perpendicular distance between the forces
The turning effect produced by a couple is called Torque (also called the moment of the couple).
Unit: Newton-metre (N·m)
Examples: Turning a steering wheel, opening a bottle cap, using a screwdriver.
Centripetal force is the force that acts on a body moving in a circular path, always directed toward the centre of the circular path.
Where:
- F = centripetal force (N)
- m = mass of the body (kg)
- v = velocity of the body (m/s)
- r = radius of the circular path (m)
Key points:
- "Centripetal" means "centre-seeking"
- Without centripetal force, a body would move in a straight line (Newton's 1st Law)
- It does no work (always perpendicular to motion)
Examples:
- Earth revolving around the Sun — gravity provides centripetal force
- Stone tied to a string swung in a circle — tension provides centripetal force
- Car turning on a road — friction provides centripetal force
Principle of Moments: When a body is in equilibrium under the action of several forces, the sum of the clockwise moments about any point is equal to the sum of the anticlockwise moments about the same point.
Mathematically: Sum of clockwise moments = Sum of anticlockwise moments
F₁ × d₁ = F₂ × d₂
Application: This principle is used to find the unknown force or distance in problems involving levers, seesaws, and beams.
Example: A seesaw is balanced when a heavier person sits closer to the pivot and a lighter person sits farther away.
📚 Samacheer Kalvi 10th Science Guide — Buy Online