CLASS 9 – SCIENCE
Chapter: Force and Laws of Motion
Topic: How Force Affects Motion
1. Force
Definition: A force is a push or pull acting on an object that can change its state of rest or motion, direction, speed, or shape.
SI Unit: Newton (N)
Examples: Pushing a table, kicking a football, and pulling a cart.
2. Effects of Force
Force can produce the following effects:
1. It can move a stationary object.
2. It can stop a moving object.
3. It can increase or decrease the speed of an object.
4. It can change the direction of motion.
5. It can change the shape or size of an object.
3. Balanced Forces
Definition: When two or more forces acting on an object cancel each other, the net force is zero. These forces are called balanced forces.
Example: Two people push a box with equal forces in opposite directions.
Net Force = 0 N
Balanced forces do not change the state of motion of an object, although they may change its shape.
4. Unbalanced Forces
Definition: When the net force acting on an object is not zero, the forces are called unbalanced forces.
Example: Kicking a stationary football makes it move.
Unbalanced forces can change the speed, direction, or state of motion of an object.
5. Newton's First Law of Motion
Statement: An object remains at rest or continues to move with uniform velocity in a straight line unless acted upon by an unbalanced external force.
This law is also called the Law of Inertia.
Example: Passengers tend to fall forward when a moving bus stops suddenly because of inertia of motion.
6. Inertia
Definition: The tendency of an object to resist any change in its state of rest or uniform motion is called inertia.
Inertia depends on the mass of an object. Greater mass means greater inertia.
Types of Inertia:
1. Inertia of rest
2. Inertia of motion
3. Inertia of direction
Example: When a stationary bus starts suddenly, passengers tend to fall backward due to inertia of rest.
7. Momentum
Definition: The momentum of an object is the product of its mass and velocity.
Formula:
Momentum = Mass × Velocity
[
p=mv
]
Where:
- p = Momentum
- m = Mass
- v = Velocity
SI Unit: kg m/s
Momentum depends on the mass and velocity of an object.
8. Newton's Second Law of Motion
Statement: The rate of change of momentum of an object is directly proportional to the applied unbalanced force and takes place in the direction of that force.
For an object of constant mass:
[
\boxed{F=ma}
]
Where:
- F = Force
- m = Mass
- a = Acceleration
SI Unit of Force: Newton (N)
One Newton: One newton is the force required to produce an acceleration of 1 m/s² in an object of mass 1 kg.
9. Newton's Third Law of Motion
Statement: For every action, there is an equal and opposite reaction.
Examples:
1. We walk by pushing the ground backward.
2. A swimmer pushes water backward to move forward.
3. A gun recoils when a bullet is fired.
4. A rocket moves forward by pushing gases backward.
Action and reaction forces act on two different objects.
10. Law of Conservation of Momentum
Statement: When no external net force acts on a system, its total momentum remains constant.
For two interacting objects:
[
m_1u_1+m_2u_2=m_1v_1+m_2v_2
]
This law is based on the principle that the total momentum before an interaction equals the total momentum after the interaction, provided the system is isolated from external forces.
11. Important Formulae
1. Momentum: p=mv
2. Force: F=ma
3. Acceleration: a=\frac{v-u}{t}
4. Force due to change in momentum: F=\frac{m(v-u)}{t}
5. Conservation of momentum:
[
m_1u_1+m_2u_2=m_1v_1+m_2v_2
]
Here, u is initial velocity, v is final velocity, and t is time.
12. Important Questions
Q1. What is force?
Ans. Force is a push or pull that can change the state of motion or shape of an object.
Q2. What is inertia?
Ans. Inertia is the tendency of an object to resist a change in its state of rest or uniform motion.
Q3. What is momentum?
Ans. Momentum is the product of the mass and velocity of an object.
Q4. State Newton's First Law of Motion.
Ans. An object remains at rest or in uniform motion in a straight line unless acted upon by an unbalanced external force.
Q5. State Newton's Second Law of Motion.
Ans. The rate of change of momentum is directly proportional to the applied unbalanced force and occurs in the direction of that force.
Q6. State Newton's Third Law of Motion.
Ans. For every action, there is an equal and opposite reaction.
Q7. What is the SI unit of force?
Ans. The SI unit of force is the newton (N).
Q8. On what factor does inertia depend?
Ans. Inertia depends on the mass of an object.
Q9. Why do passengers fall forward when a moving bus stops suddenly?
Ans. Passengers tend to continue moving forward due to inertia of motion.
Q10. Why does a gun recoil when a bullet is fired?
Ans. The bullet moves forward and the gun experiences an equal and opposite force backward, according to Newton's Third Law of Motion.
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