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# Tag Archives: motion

## Size of body neglected in Circular motion

Why in  circular  motion the  size  of  the moving body is  neglected?

Not only in circular motion, but in entire kinematics we discuss the case of point objects only. In kinematics we study motion only and not the objects undergoing the motion.

A point object is an object having negligible dimensions compared to the distance travelled by it. For example, a car moving from Delhi to Mumbai can be considered a point object.

Further, when we discuss the concepts in Kinematics, even a large object is dealt considering the entire body to be a single point. This point will be the centre of mass of the body.

If a person is revolving a stone tied to a string, the centre of mass of the stone is taken for determining the length starting from the point about which it revolves.

In dynamics, we have to consider the shape and size of the body also.

## Problem from Kinematics

the distance between Ahmadabad and vadodara is 100 km.two trains set off simultaneously from Ahmadabad towards vadodara each other.the speed of these trains are 45kmh-1 And 30kmh-1.when will they cross each other?

## Sequence of Learning Physics

In what sequence should these chapters be prepared for my exams:-
units and dimensions,kinematics,laws of motion,work power and energy,motion of system of particles and rigid body dynamics,gravitation.

It is better to follow the same pattern as given in NCERT text book

## A Problem from Motion

“Q. Velocity of a moving particle is given by v = t2 – 4. Find out:-
(i) Time at which velocity is zero.
(ii) Acceleration at which t= 1s
(iii) What type of motion uniform or non-uniform.”

Ans:

In the equation, put v=0 to find the time at which velocity equals zero

Differentiate the equation and then put t=1 to find acceleration at t=1s since a = dv/dt

The motion is non uniform as its velocity varies with time.

## Question from Beats (Waves)

"the displacement of a particle in periodic motion is given by "y=(cos2t-2cost+2sin^2t+1). What is the no. of beats that can be heard in 10 seconds?"

## Angle between velocity and acceleration

Discuss a circumstance in which angle between velocity and acceleration is less than 90.

Discuss a circumstance in which angle between velocity and acceleration lies batween 180 and 90.

(Hifsa Zafar asked this)

In projectile motion when the object is moving up, the angle between velocity and acceleration (due to gravity g) is between 90 degree and 180 degree.

during the downward motion of a projectile, the angle between velocity and acceleration is between 0 and 90 degree.

## Motion under gravity

Two stones of mass 5 kg and 10 kg are dropped from the same height above the ground level. Which one will hit the ground first? Why?

Answer: Both the stones will reach the ground simultaneously (provided we neglect the air resistance (the viscosity) and buoyant force.

This is because acceleration due to gravity is independent of mass of the freely falling body.

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