Class 9 Science Notes: Force and Laws of motion

 Force and Laws of Motion

• Force is defined as a push or pull on the body.

• Force is an external effort that can change an object's state of motion or rest, or change its shape .

• Force is a vector quantity.

• Its SI Unit is Newton (N).

• Its CGS unit is dyne. (1 N = 100000 dyne)

Effects of Forces:

→ Force can make a stationary body move.

e.g.: a football can be set to move by kicking it i.e. by applying a force.

→ Force can stop a moving body.

e.g., by applying brakes, a running cycle or a running vehicle can be stopped.

→ Force can change the direction of a moving object.

e.g: by applying force i.e. by moving steering, the direction of a running vehicle is changed.

→ Force can change the speed of a moving body.

E.g: When you push a swing, you apply force to make it go higher and faster.

→ Force can change the shape and size of an object.

e.g.: by hammering, a block of metal can be turned into a thin sheet. By hammering, a stone can be broken into pieces.

Balanced Force	Unbalanced Force
1. When two or more forces act on a body and produce a net force equal to zero, the forces are called balanced forces.	1. When two or more forces act on a body and produce a net force not equal to zero, the forces are called unbalanced forces.
2. There is no change in speed of an object.	2. There is a change in speed of an object.
3. The object has zero acceleration.	3. The object has some acceleration.

Resultant force: The resultant force is the total force acting on an object when multiple forces are combined. It's also known as the net force. The resultant force has the same effect on an object as the original system of forces.

Aristotle’s Belief: Natural state of body is rest

Galileo's Observation:

• Speed increases when ball rolls down an inclined plane

• Speed decreases when ball rolls down up inclined plane

Experiment on Horizontal Plane:

• On a smooth surface, the ball continues to move.

• Speed remains constant with no external force or friction.

Conclusion: Bodies naturally oppose changes in their state of rest or motion.

Inertia:

• Inertia is defined as a property of matter by which it tries to maintain its state of rest or of uniform motion along a straight line.

• Inertia of an object is measured by its mass. Inertia is directly proportional to the mass. This means inertia increases with increase in mass and decreases with decrease in mass.

Inertia of Rest: An object stays at rest, and it remains at rest until an external force affects it. Example: When a car accelerates, passengers may feel as though their bodies are moving backwards. In reality, inertia is making their bodies stay in place as the car moves forward.

Inertia of Motion: An object will continue to be in motion until a force acts on it. Example: A hockey puck will continue to slide across the ice until acted upon by an outside force.

Newton’s First Law of Motion:

A body remains in the state of rest or uniform motion in a straight line unless

and until an external force acts on it.

There are two conditions on which the 1st law of motion is dependent:

• Objects at rest: When an object is at rest, velocity (v = 0) and acceleration (a = 0) are zero. Therefore, the object continues to be at rest.

• Objects in motion: When an object is in motion, velocity is not equal to zero (v ≠ 0), while acceleration (a = 0) is equal to zero. Therefore, the object will continue to be in motion with constant velocity and in the same direction.

Examples:

• A person standing in a bus falls backward when the bus starts moving suddenly.

• A person standing in a moving bus falls forward if the driver applies brakes suddenly.

• Before hanging the wet clothes over the laundry line, usually many jerks are given to the clothes to get them dried quickly.

Momentum:

Momentum is the quantity of motion of an object. The product of velocity and mass is called the momentum. Momentum is denoted by ‘p’.

p = m x v

where, p = momentum, m = mass of the object and v = velocity of the object.

Examples:

• Even a small bullet is able to kill a person when it is fired from a gun because of its momentum due to great velocity. 

• A person gets injured severely when hit by a moving vehicle because of the momentum of the vehicle due to mass and velocity.

Newton’s Second Law of Motion:

The rate of change of momentum of an object is directly proportional to the applied unbalanced force in the direction of the force.

Where k is constant and the value of k is 1. 

So, F = m a

S.I. Unit of Force is kg m/2 or Newton.

Define 1 Newton?

Ans: When an acceleration of 1 m/s2 is seen in a body of mass 1 kg, then the force applied on the body is said to be 1 Newton.

Force	Momentum
Force = mass * acceleration (F = ma)	Momentum = mass * velocity (p = mv)
Forces can exist even when the object is stationary	Momentum for stationary objects is always zero.
The direction of the force depends on    acceleration.	The direction of momentum depends on the direction of velocity.
Force is inversely proportional to time  (F  ∝ 1/t).	Momentum is directly proportional to time (p ∝ t).
Force remains constant for constant acceleration.	Momentum changes with constant               acceleration, but remains constant if velocity is constant.

Newton’s 3rd law of motion:

• It states that every action has an equal and opposite reaction. 

• The mathematical representation of Newton’s third law of motion is:

Let A be the body exerting force F1 on the body B, then body B too exerts a force F2 on body A, which is given as: F1 = - F2