Force And Laws Of Motion Class 9 Science Chapter 8 Notes are available here. These notes are prepared by the subject experts of our team.
Force And Laws Of Motion Class 9 Science Chapter 8 Notes
- The previous chapter covered the motion of objects along a straight line in terms of position, velocity, and acceleration, noting that motion can be uniform or non-uniform.
- The causes of motion were not yet discussed, leading to questions about why speed changes over time and if all motions require a cause.
- This chapter aims to address these curiosities about motion and its causes.
- For centuries, the causes of motion puzzled scientists and philosophers, with the belief that rest is the natural state of an object until challenged by Galileo Galilei and Isaac Newton.
- Everyday observations show that effort is needed to move a stationary object or stop a moving one, experienced as muscular effort through pushing, hitting, or pulling.
- The concept of force is based on these actions (push, hit, pull).
- Force is not directly seen, tasted, or felt but its effects are observed when applied to objects.
- Pushing, hitting, and pulling are ways to bring objects into motion by making a force act on them.
- Forces can change the magnitude of an object’s velocity (speed it up or slow it down), change its direction, and alter its shape and size.
Balanced and Unbalanced Forces
Wooden Block on Table:
- Two strings (X and Y) are tied to opposite faces of a wooden block on a horizontal table.
- Pulling string X moves the block to the right; pulling string Y moves it to the left.
- Pulling both strings with equal force results in no movement (balanced forces).
Balanced and Unbalanced Forces:
- Balanced forces do not change the state of rest or motion of an object.
- Pulling with two opposite forces of different magnitudes causes the block to move in the direction of the greater force (unbalanced forces).
- An unbalanced force acting on an object brings it into motion.
Friction and Motion:
- Pushing a box on a rough floor with a small force doesn’t move it due to friction balancing the pushing force.
- Pushing harder until the pushing force exceeds the friction force causes the box to move (unbalanced force).
- Friction arises between surfaces in contact, opposing motion.
Bicycle Motion:
- Stopping pedaling on a bicycle causes it to slow down due to friction forces acting opposite to the direction of motion.
- Continuous pedaling is needed to keep the bicycle moving.
- An object moves with uniform velocity when forces acting on it are balanced and there’s no net external force.
- An unbalanced force causes a change in speed or direction of motion.
- To accelerate an object’s motion, an unbalanced force is required.
- If the unbalanced force is removed, the object continues to move with the acquired velocity.
First Law of Motion
Galileo’s Observations:
- Objects move with a constant speed when no force acts on them.
- A marble rolling down an inclined plane increases in velocity due to the unbalanced force of gravity.
- When a marble rolls up an inclined plane, its velocity decreases.
Frictionless Plane Experiment:
- On an ideal frictionless plane inclined on both sides, a marble released from one side would roll down and climb up the opposite side to the same height.
- If the inclination on the opposite side is gradually reduced, the marble travels further but reaches the original height.
- On a horizontal plane, the marble would travel forever to reach the original height, indicating zero unbalanced forces.
- An unbalanced (external) force is required to change motion, but no net force is needed to sustain uniform motion.
Practical Situations:
- Zero unbalanced force is difficult to achieve due to friction.
- Frictional force opposes motion, causing the marble to stop after some distance.
- Friction can be minimized using a smooth marble, smooth plane, and lubricant.
Newton’s Laws of Motion:
- Newton built on Galileo’s ideas and formulated three fundamental laws of motion.
- First Law of Motion (Law of Inertia): An object remains in a state of rest or uniform motion unless acted upon by an external force.
- Objects resist changes in their state of motion due to inertia.
Law of Inertia in Everyday Experiences:
- Braking in a Car: When brakes are applied, the car slows down but the body continues moving forward due to inertia, which can cause injury.
- Safety Belts: Worn to prevent injury by exerting a force to slow the body’s forward motion.
- Bus Movement: Sudden starts cause passengers to fall backward as their feet move with the bus, but their upper body resists due to inertia.
- Sharp Turns in a Car: Passengers are thrown to one side due to their inertia while the car changes direction.
Inertia and Mass
Inertia:
- Inertia is the property of an object to resist a change in its state of motion.
- An object at rest tends to remain at rest; an object in motion tends to keep moving.
Examples Illustrating Inertia:
- Pushing an empty box is easier than pushing a box full of books.
- Kicking a football causes it to fly away; kicking a stone of the same size hardly moves it and may cause injury.
- Using a one-rupee coin in an activity requires less force than using a five-rupees coin.
- A small cart can pick up a large velocity with a certain force, but the same force produces negligible change in the motion of a train.
Inertia and Mass:
- Heavier or more massive objects have larger inertia.
- The train has more inertia than the cart because it has a much lesser tendency to change its state of motion.
- Inertia is quantitatively measured by the mass of an object.
- The mass of an object is a measure of its inertia.
Relationship Between Inertia and Mass:
- Inertia is the natural tendency of an object to resist a change in its state of motion or rest.
- The mass of an object quantifies its inertia.
Second Law of Motion
First Law of Motion:
- When an unbalanced external force acts on an object, its velocity changes, causing acceleration.
Force and Acceleration:
- The acceleration of an object depends on the force applied to it.
- Everyday observations:
- A table tennis ball does not hurt a player on impact, but a fast-moving cricket ball can hurt a spectator.
- A parked truck requires no attention, but a moving truck can be dangerous even at low speeds.
- A small mass like a bullet can be lethal when fired from a gun.
- These examples indicate that the impact depends on the object’s mass and velocity.
Momentum:
- Momentum (p) is defined as the product of an object’s mass (m) and velocity (v): ( p = mv ).
- Momentum has both direction and magnitude, matching the direction of velocity.
- The SI unit of momentum is kilogram-meter per second (kg·m/s).
- An unbalanced force changes the velocity of an object, thereby changing its momentum.
Example with a Car:
- Pushing a car with a dead battery requires a continuous push over time to gradually accelerate it to the needed speed.
- The change in momentum is determined by both the magnitude of the force and the duration of its application.
- The force needed to change an object’s momentum depends on the rate at which the momentum changes.
Second Law of Motion:
- The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force.
Mathematical Formulation of the Second Law of Motion
Initial Conditions:
- An object of mass ( m ) is moving with an initial velocity ( u ).
- The object is uniformly accelerated to a velocity ( v ) in time ( t ) by a constant force ( F ).
Momentum:
- Initial momentum ( p1 = mu )
- Final momentum ( p2 = mv )
- Change in momentum ∝ p2−p1 ∝ mv−mu ∝ m×(v−u)
Rate of Change of Momentum:
- Rate of change of momentum ∝ m(v−u)/t
- Applied force F ∝ m(v−u)/t
- F = k×m(v−u)/t = kma
- Here, a=(v−u)/t is the acceleration.
- ( k ) is a constant of proportionality.
SI Units and Constant ( k ):
- SI units of mass: (kg)
- SI units of acceleration: (m s)^(-2)
- Unit of force chosen such that ( k = 1 )
- 1 unit of force = k×(1kg)×(1m s^−2)
- Value of ( k ) becomes 1.
Second Law of Motion:
- F = ma
- Unit of force: (kg m s)^(-2) or newton (N).
Applications and Examples:
- A fielder in cricket pulls hands back while catching a fast-moving ball to increase the time of impact, reducing the acceleration and force.
- In high jump, athletes fall on cushioned or sand beds to increase the time of impact, reducing the rate of change of momentum and force.
- Karate players break ice slabs with a single blow by applying a large force in a short time.
First Law of Motion Derived from Second Law:
- From ( F = ma )
- F = m(v – u)/t
- Ft = mv – mu
- When ( F = 0 ), ( v = u ) for any time ( t ), indicating uniform motion.
- If ( u = 0 ), then ( v = 0 ), meaning the object remains at rest.
Third Law of Motion
Basic Principle:
- When one object exerts a force on another, the second object exerts an equal and opposite force back on the first.
- These forces are equal in magnitude but opposite in direction.
- They act on different objects, not on the same object.
Examples and Illustrations:
- Football Collision: Players feel hurt because each applies an equal and opposite force to the other (action and reaction forces).
- Spring Balances Experiment:
- Two spring balances connected together show equal readings.
- Force exerted by balance A on B is equal and opposite to the force exerted by balance B on A.
- Any of these forces can be called action and the other reaction.
- Action-Reaction Forces:
- Always act on two different objects simultaneously.
- For walking, you push the road backwards, and the road pushes you forward with an equal and opposite force.
- Forces may not produce equal accelerations because they act on objects with different masses.
Practical Applications:
- Gun Firing:
- Gun exerts forward force on the bullet.
- Bullet exerts an equal and opposite force on the gun, causing recoil.
- Gun’s greater mass results in less acceleration compared to the bullet.
- Sailor Jumping from Boat:
- Sailor jumps forward, and the boat moves backward due to the equal and opposite force.
Experiments and Activities:
- Children on Carts:
- Different accelerations observed for the same force due to different masses of the carts.
- Recommended construction: 12 mm or 18 mm thick plywood board (50 cm x 100 cm) with ball-bearing wheels.
Third Law Statement: To every action, there is an equal and opposite reaction.
Importance of Different Masses: The same force can result in different accelerations depending on the mass of the objects involved.