JAC Board Class 9th Science Chapter 9 Notes Force and Laws of Motion
→ Force: It is a push or pull on an object that produces acceleration in the body on which it acts. The SI unit of force is ‘newton’. Forces are used in our daily life actions like pushing, lifting, pulling, stretching, twisting and pressing.
→ A force cannot be seen. It can be judged only by the effects which it can produce in several bodies (or objects) around us.
→ Force has both magnitude and direction, making it a vector quantity. It is measured in the SI unit of newton and represented by the symbol ‘F’
→ Effects of Force:
a. Force can set a stationary body in motion. For example, a football can be set in motion by kicking it, i.e. by applying a force.
b. Force can stop a moving body. For example, by applying brakes, a running cycle or a running vehicle can be stopped.
c. Force can change the direction of a moving object. For example, by applying force, i.e., by moving handle, the direction of a running bicycle can be changed. Similarly by moving steering, the direction of a running vehicle is changed.
d. Force can change the speed of a moving body. For example, by accelerating, the speed of a running vehicle can be increased or by applying breaks, the speed of a running vehicle can be decreased.
e. Force can change the shape and size of an object. For example, by hammering, a block of metal can be turned into a thin sheet. Also by hammering, a stone can be broken down into pieces.
→ Balanced forces: If the resultant force of several forces acting on a body is zero, the forces are said to be ‘balanced forces’.
→ Unbalanced forces: If the resultant force of the several forces acting on a body is not zero, the forces are said to be ‘unbalanced forces’. An object in rest can be moved by applying unbalanced forces. Only an unbalanced force can produce the effects of force.
→ Newton’s First Law of Motion: An object remains in its 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 its inertia.
- Inertia of rest: It is the tendency of a body to remain in its state of rest.
- Inertia of motion: It is the tendency of a body to remain in its state of uniform motion in a straight line.
- Inertia of direction: It is the inability of a body to change, by itself, its direction of motion.
→ Newton’s Second Law of Motion: The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of force.
Mathematically,
Force = Mass × Acceleration or,
F ∝ \(\frac{P_{1}-P_{2}}{t}\) or \(\frac{\mathrm{m}(\mathrm{v}-\mathrm{u})}{\mathrm{t}}\) or ma
This law defines the unit of force. One unit force is that force which produces unit acceleration in a body of unit mass.
→ Newton (N): It is the SI unit of force. One Newton is that force which produces an acceleration of lm/s2 in a body of mass 1 kg.
→ 1 newton = 1 kg × 1 m/s2 or 1N = 1 kg m/s2
→ 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 SI unit is kg m/s.
Momentum = Mass × Velocity or p = mv
→ Newton’s Third Law of Motion: To every action, there is an equal and opposite reaction and they act on two different bodies.
→ Law of Conservation of Momentum:
It states that the total momentum of any system of objects remains constant in the absence of any external force. According to the law of conservation of momentum, for collision between two bodies:
Total momentum before collision = Total momentum after collision
m1u1 + m2u2 = m1v1 + m2v2
→ Frictional force: Whenever a body slides or rolls over the surface of another body, a force comes into action which acts in the opposite direction as that of the motion of a body. This opposing force is called ‘friction’.