FORCE AND LAWS OF MOTION

DONE BY CHRIS VINOD

FORCEForce is the effort needed to push or pull or to change the shape of a body. It is any interaction that, when unopposed, will change the motion of an object. In other words, a force can cause an object with mass to change its velocity i.e. to accelerate. A force has both magnitude and direction, making it a vector quantity. It is measured in the SI unit of Newtons and represented by the letter F.

EFFECTS OF FORCE Force can cause motion in a stationary body. Force can stop a moving body. Force can change the direction of motion of a

body. Force can change the speed of a moving body. force can change the shape and size of a body.

BALANCED AND UNBALANCED FORCE

Balanced ForceWhen a number of forces acting simultaneously on a body do not bring about any change in its state of rest or uniform motion in a straight line, then the forces acting on the body are said to be balanced forces.The net force will be zero in this case.Eg: a boy holding school bag in his hand.

• Unbalanced forceWhen a number of forces acting on a body bring about a change in its state of rest or uniform motion in a straight line, then the forces acting on the body are said to be unbalanced forces.Thus to move an object, an unbalanced force is required.eg:

NEWTON’S FIRST LAW OF MOTION

Every body continuous to be in its state of rest or of uniform motion in a straight line unless it is compelled by some external applied force to change that state.

INERTIA

The inherent property of a body by virtue of which it resists any change in its state of rest or of uniform motion in a straight line on its own is called inertia.

INERTIA OF REST

eg: A passenger in a bus fall backward when the bus starts suddenly.When the bus starts suddenly the lower part of the passenger’s body starts moving forward with the bus. But the upper part of the body tends to remain in the state of rest due to the inertia of rest.

INERTIA OF MOTION

A passenger in a moving bus tends to lean forward when the bus stops suddenly.When the bus stops suddenly the lower part of the passengers body comes to rest along with the bus. The upper part of the body tends to remain in the state of motion

LINEAR MOMENTUM

The linear momentum of a body of mass m, travelling with a velocity v is defined as the product of its mass and velocity.P = mass * velocityP =mvSI unit of momentum = kg m/s

NUMERICAL

Calculate the momentum of a toy car weighing 200g moving with the velocity of 5m/s.Mass = 200g200g = 0.2 kgVelocity = 5m/sp = mv0.2 * 5 = 1 kg m/s

NEWTON’S SECOND LAW OF MOTION

The rate of change of momentum of a body ids directly proportional to the applied unbalanced force in the direction of force.

MATHEMATICAL FORMULATION OF SECOND LAW OF MOTION.Consider a body of mass ‘m’, having an initial velocity ‘u’. A force ‘F’ act on this body for time ‘t’. Let the final velocity be ‘v’.Initial momentum of the body = muFinal momentum of the body = mvChange in momentum of the body = m(v – u) According to the second law of motion,F = m(v – u)/ tF = Km(v – u)/t [ { v – u }/t = a ]F = Kma [ K = 1 ]F = ma

NUMERICAL

What is the acceleration produced by a force of 12N exerted on an object of mass 3kg.Force of the object = 12NMass = 3 kgAcceleration = ?F = ma A = f/m = 4 m/s

NEWTON’S 3RD LAW OF MOTION

For every action there is an equal or opposite reaction.