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Short Notes - Force and Laws of Motion - Science Class 9
- Force: It is a push or pull on an object that produces acceleration in the body on which it acts. S.I. unit of force is Newton.
- A force can do three things on a body.
(a)It can change the speed of a body.
(b)It can change the direction of motion of a body.
(c)It can change the shape and size of a body. - Balanced forces: Forces are said to be balanced forces if they nullify one another and their resultant force is zero.
- Unbalanced forces: When two opposite forces acting on a body, move a body in the direction of the greater force or forces which brings motion in a body are called as unbalanced forces.
- First law of motion: An object remains in a state of rest or of uniform motion in a straight line unless acted upon by an external unbalanced force.
- Inertia: The natural tendency of an object to resist a change in its state of rest or of uniform motion is called inertia.
The mass of an object is a measure of its inertia.
Its S.I. unit is kg.
A body with greater mass has greater inertia. - Frictional force: The force that always opposes the motion of objects is called force of friction.
- 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.
Mathematically,
- Momentum: The momentum of an object is the product of its mass and velocity and has the same direction as that of the velocity. Its S.I. unit is kg m/s.
(p = mv )
1 Newton: A force of one Newton produces an acceleration of 1 m/s2 on an object of mass 1kg.
1N = 1 kg m/s2
(F = ma) - Third law of motion: To every action, there is an equal and opposite reaction and they act on two different bodies.
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FAQs on Short Notes - Force and Laws of Motion - Science Class 9
| 1. What are the three laws of motion? |  |
Ans. The three laws of motion are:
1. Newton's First Law of Motion: Also known as the law of inertia, it states that an object at rest will remain at rest, and an object in motion will continue in motion with a constant velocity unless acted upon by an external force.
2. Newton's Second Law of Motion: It states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. The formula for this law is F = ma, where F is the force, m is the mass, and a is the acceleration.
3. Newton's Third Law of Motion: It states that for every action, there is an equal and opposite reaction. This means that whenever an object exerts a force on another object, the second object exerts an equal and opposite force on the first object.
| 2. What is the SI unit of force? | |
Ans. The SI unit of force is the Newton (N). It is named after Sir Isaac Newton, who made significant contributions to the field of physics, including the laws of motion. One Newton is defined as the force required to accelerate a 1 kilogram mass by 1 meter per second squared.
| 3. What is the difference between mass and weight? | |
Ans. Mass and weight are two different quantities:
- Mass: Mass is the amount of matter in an object. It is a scalar quantity and is measured in kilograms (kg). Mass remains the same regardless of the location of the object.
- Weight: Weight is the force exerted on an object due to gravity. It is a vector quantity and is measured in Newtons (N). Weight can vary depending on the location of the object because it depends on the gravitational force acting on it.
| 4. What is the effect of an unbalanced force on an object? | |
Ans. An unbalanced force acting on an object causes it to accelerate. According to Newton's second law of motion, the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. Therefore, if an unbalanced force is applied to an object, it will either speed up, slow down, or change direction depending on the direction and magnitude of the force.
| 5. How does friction affect motion? | |
Ans. Friction is a force that opposes the motion of objects in contact with each other. It acts in the opposite direction to the motion and can either slow down or completely stop an object. Friction also plays a crucial role in allowing us to walk, drive vehicles, and perform various daily activities. However, excessive friction can waste energy and cause wear and tear on objects.
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