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# Category Archives: KINEMATICS

## Understanding Projectile Motion

IN A PROJECTILE MOTION WE TAKE MOTION IN A HORIZONTAL DIRECTION AS g=0 while in vertical motion as g=-9.8 meter / second square why?

Projectile Motion is an example for motion in two dimension. It can be studied easily by resolving into two components – The vertical motion is affected by gravity and the horizontal motion is not affected by gravity.

Therefore we take g=0 when we consider the horizontal component of motion as the projectile is not accelerated in the horizontal direction. The horizontal component of velocity remains constant through out its motion.

A PowerPoint Presentation on projectile_motion

## Numerical (MCQ) from Kinematics

A person standing on an escalator takes time t1 to reach the top of a tower when the escalator is moving.he takes time t2 to reach the top of the tower when the escalator is standing.how long will he take if he walks up a moving escalator?
a)t2-t1    b)t1+t2    c)t1t2/(t1-t2)  d)t1t2/(t1+t2)

OPEN TO VISITORS TO MAKE A TRY

## 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.

## Kinematics – Signs of displacement and Velocity

When will displacement and velocity be negative ? Can you explain me about angular acceleration and angular velocity.

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The signs are conventional. We take one direction as positive and the direction opposite to it as negative.

When we discuss one-dimensional motion, it is easy to take the help of the number-line, whatever towards right is positive and towards left is negative. If the initial direction of motion is taken as positive and the further displacements does not make the body to go beyond the initial position in opposite direction, the displacement will be positive.

## A thought provoking (HOTS) problem from rotation

Consider a disk rotating in a horizontal plane with a constant angular speed “omega” about its center O.

The disk has a shaded region on one side of the diameter and an unshaded region on the other  side.

When disk is in horizontal plane x-y with left half shaded and right half unshaded, two pebbles P and Q are simultaneously projected at an angle towards R.

The velocity of projection is in the y-z plane and is same for both pebbles with respect to the disk.

Assume that

(i) they land back on the disk before the disk has completed one-eighth rotation,

(ii) their range is less than half the disk  radius,and

(iii)”omega” remains constant throughout.

Then where will the two pebbles land?

(All are invited to answer this)

## A Few problems from Projectile Motion

1. If R and H are the horizontal range and maximum height attained by a projectile, than what is its speed of projection? please explain .
2. A particle projected from ground at angle 45 degrees with horizontal from distance ‘d1’ from the foot of a pole and just after touching the top of pole it falls on ground at a distance ‘d2’ from pole on other side, then what is the height of pole?please explain.
3. A particle projected from ground moves at angle 45 degrees with horizontal one second after projection and speed is minimum two seconds after the projection.What is the angle of projection of particle?(neglect the effect of air resistance) please explain.
4.A particle is moving on a circular path with constant speed v. It moves between two points A and B, which subtends an angle 60 degree at the centre of the circle. What are the magnitude of change in its velocity and change in magnitude of its velocity during motion from A to B respectively? please explain

1.

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