The Laws of Motion – Maharashtra Board Class 9 Solutions for Science and Technology (English Medium)
- The tendency of a body to resist acceleration is called inertia.
- In the SI system, newton is the unit of force.
- In a collision momentum is always conserved.
- The motion of rocket is based on Newton’s third law of motion.
- An unbalanced force acting on an object brings it in motion.
- Lifting a bucket filled with water; lifting the stone lying on the ground
- Pushing a table; pushing a handcart
- Pulling a drawer; pulling a chair
- Use of magnet to attract iron nails; rubbing a plastic scale and attracting bits of paper with it
- Stretching a rubber band to change its shape; pressing a rubber ball to change its shape
- Stone has more inertia than the ball of the same size.
- The train has more inertia than the bicycle.
- A five-rupee coin has more inertia than a one-rupee coin.
- Every object has inertia that is, it resists a change in its state of rest or in state of motion.
- If the luggage kept on the roof of the bus is not tied to it with a rope, then due to inertia, it may fall off the roof
- in the backward direction if the bus is accelerated or suddenly started.
- in the forward direction if the bus is retarded by applying breaks.
- in the outward direction if the bus takes a sharp turn at high speed.
3. So to avoid this, it is advised to tie any luggage kept on the roof of the bus with a rope.
According to Newton’s third law of motion, action and reaction force act on different bodies. Thus, action and reaction forces do not cancel each other even though they have equal magnitude and opposite direction.
- The momentum of a body is the product of its mass and velocity. Even if the velocity of the tennis ball and cricket ball is the same, the mass of a cricket ball will be greater than that of the tennis ball. Hence, the momentum of the cricket ball will be greater than that of the tennis ball.
- Also a cricket ball is harder than a tennis ball. Therefore, it is easy to stop a tennis ball than a cricket ball moving with the same velocity.
|P = m v
|F = ma
Data: m1 = 16 kg, a1 = 3 m/s2, F = ?, m2 = 24 kg, a2 = ?
(i) F = m1a1 = 16 kg × 3 m/s2 F = 48 N
The force acting on the object is 48 N.
The acceleration of the object is 2 m/s2.
Data: m1 = 10 g =10 × 10-3 kg, m2 = 90 g = 90 × 10-3 kg u1 = 1.5 m/s, u2 = 0 m/s, v1 = v2 = v = ?
By the principle of conservation of momentum,
m1u1 + m2u2 = m1v1 + m2v2
As u2 = 0 m/s and v1 = v2 = v, we have
m1u1 = (m1 + m2) v
The speed with which the plank containing the bullet moves
is 0.15 m/s.
|1||Force is a physical quantity which changes or tends to change the state of rest or of uniform motion of a body in a straight line.||Momentum is the total quantity of motion contained in a body.|
|2||Force = rate of change of momentum, that is,
F m × a.
|Momentum = product of mass and velocity, that is
P = m × v.
|3||The SI unit of force is newton.||The SI unit of momentum is kg m/s.|
|Inertia of motion||Inertia of direction|
|1||The inherent property of a body by virtue of which it cannot change its state of motion is called inertia of motion.||The inherent property of a body by virtue of which it cannot change its direction of motion is called inertia of direction.|
|2||Due to inertia of motion, a rotating fan continues to rotate even after it is switched off.||While sharpening a knife, due to inertia of direction, sparks fly off tangentially from the grinding stone.|
|Inertia of motion||Inertia of rest|
|1||The inherent property of a body by virtue of which it cannot change its state of motion is called inertia of motion.||The inherent property of a body by virtue of which it cannot change its state of position of rest is called inertia of rest.|
|2||Due to inertia of motion, a rotating fan continues to rotate even after it is switched off.||When a bus starts suddenly, the passengers experience a backward jerk due to inertia of rest.|