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Force and Friction

Force:
Force is defined as to change the magnitude of velocity of an object or to change its direction of motion. Force can change the shape and size of objects.

Force that does not change the state of an object is called balanced force. Pushing force and frictional force acting on the object are balanced and there is no net external force on it, hence object moves with uniform velocity.

Friction force is the force arises between two surfaces in contact and is always in a direction opposite to the push.

Force that acts on the object and brings it in motion is called unbalanced force. Unbalanced force causes change in speed of the object or change in the direction of the object. If unbalanced force is removed completely, the object would continue to move with the velocity it has acquired till then.

Motion of an object was first described by Galileo with the example or rolling marbles on an inclined plane. He concluded that objects move with a constant speed when no force acts on them, velocity of an object increases when it rolls down on an inclined plane and velocity decreases when it climbs up.

Newton studied Galileo’s ideas on force and motion and presented three fundamental laws of motion. These three laws are known as Newton’s laws of motion.


Newton’s law of motion:
The first law of motion is stated as: An object remains in a state of rest or of uniform motion in a straight line unless compelled to change that state by an applied force.

First law of motion:
All objects resist a change in their state of motion. They have tendency to stay at rest or to keep moving with the same velocity. That is called inertial and the first law of motion is also known as the law of inertia.

Inertia is defined as the tendency of an object to remain at rest or of moving until an external force acts on it. Object having heavier mass has larger inertia. The mass of an object is a measure of its inertia.

The first law of motion indicates that when an unbalanced external force acts on an object, its velocity changes and is accelerated.
Newton has introduced a property that combines the object’s mass and its velocity and has given the name as momentum.

The momentum (p) of an object is defined as the product of its mass (m) and velocity (v).

  p = mv

SI unit of momentum – Kilogram-meter/second or kg-m/s
Momentum has both direction and magnitude. Change of momentum is determined by magnitude of the force and the time rate at which the momentum is changed.


Second law of motion:
The second law of motion states that the rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of force.

The second law of motion measures the force action on an object as a product of its mass and acceleration.

Consider an object with mass (m) is moving along a straight line with an initial velocity (u). It is uniformly accelerated to velocity (v) in time (t) by the application of a constant force (F) throughout the time (t). The final and initial momentum of the object will be
 
p1 = mu
  p
2 = mv

Change in momemntum p ∞ p
2-p1 ∞ mv-mu ∞ m (v-u)
The rate of change of momentum ∞ m (v-u)/t
Or the applied force, F ∞ m (v-u)/t
  F = K m (v-u)/t
  F = K m a   (a = v-u/t is the acceleration)
  K = constant
  F = ma
SI unit of force = kg m s
2 or Newton

The first law of motion can be mathematically stated from the mathematical expression for the second law of motion.
  F = ma
  F = m [(v-u)/t]
  Ft = mv-mu
Now, If F= 0, v = u for whatever time, then the object will continue moving with uniform velocity (u) throughout the time (t).
If u is zero then v will also be zero. That is, the object will remain at rest.


Third law of motion:
Third law of motion states that when one object exerts a force on another object, the second object immediately exerts a force back on the first. These two forces are always equal in degree but opposite in direction. These two opposite forces are also known as action and reaction forces and the law is known as Newton’s third law of motion or laws of action and reaction.
The action and reaction forces are always equal in magnitude but may not produce acceleration of equal magnitude because each force acts on different mass and different object.


Calculations:
Ex-1: Friction force is always opposite to the ………..
a)  action force
b)  Balanced force

c)  Pushing force
d)  None

Answer: Friction force is the force arises between two surfaces in contact and is always in a direction opposite to the push.

Ex-2: Which of the following has more inertial?
a)  A rubber ball and a spoon of the same size
b)  A bicycle and a truck
c)  A five rupee coin or a one rupee coin

Answer: Inertia depends on the mass of the object.
a)  A rubber ball and a spoon of the same size – Spoon
d)  A bicycle and a truck - Truck
e)  A five rupee coin or a one rupee coin – Five rupee coin

Answer: When an object covers equal distance in equal time interval is called uniform motion.

Ex-3: Which of the following scientist has first given the idea of force and motion?

a)  Galileo
b)  Robert Hooke
c)  Robert Brown
d)  Newton

Ex-4: 15 N Forces is requiring accelerating the object around 10 m/s
2, and then what will the mass of that object?
  a)
1.5 kg  b) 1.0 kg  c) 0.05 kg  d) 0.1 kg
Answer:  F = ma
  15 = m x 10
  m = 1.5 kg


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