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# Category Archives: Mechanics

## 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?

Asks Atanu PAUL

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.

The following presentation will help you understand the concept better.

A **PowerPoint Presentation** on **projectile_motion**

###### Related articles

- A Few problems from Projectile Motion (askphysics.com)
- Uniform Circular Motion in Horizontal Plane (askphysics.com)
- Gravity, Balance and some doubts (askphysics.com)

## Uniform Circular Motion in Horizontal Plane

In uniform horizontal circular motion, how are the vertical forces balanced?

Asked Arjun M Nair

Answer:

If you are considering an object tied to a string and whirled with uniform speed in a horizontal plane, the object will be moving in a horizontal plane but the string has to make a certain angle with the horizontal so that the tension can be resolved in such a way that the horizontal component provides the centripetal force and the vertical component balances the weight of the body.

The motion will become almost entirely in a horizontal plane when the centripetal force is much larger than the weight of the particle.

Refer:

http://cnx.org/content/m14090/latest/

The case of rid body structures the situation is complex, but as long as there are no vertical accelerations, the vertical components of forces will be balanced.

**Comments are welcome from students and teachers WHO THINK!**

## Gravity, Balance and some doubts

Nassar posted: “I have no explanation why did the scale get balanced when there are twe equal masses on both sides ???

Here is the problem:

Suppose that a have a scale and I put 1kg on one side , so this side will get down – no problem with that-

but,

if I put another 1kg but on the other side of the scale…now imagine this,

the first 1kg was down and at this very moment ( I mean before the scales started to get balanced) the new 1kg will be at top , now according to the law of Gravity , the gravitaional force affecting the lower mass is greater than that affecting the top mass (I mean the mass at the bottom is closer to the center of the earth)…

Here is my question:

What force make the mass at the buttom to rise up against the gravity???

All explanations said that its in order to equalaze the gravitational force on both sides , but this makes no sense,First why to equalize? ,

and second ,

What force make the mass at the bottom to rise ?

For me it is not silly question..

Thank you

*This is the type of doubt which we expect from students and visitors. The doubts which we encounter when we think on what we learn.*

Answer:

First of all let us talk about the force of gravity and its (more…)

## What is pseudo force

What is pseudo force when it is applied ?

Asked Niharika Dinkar

Answer:

Pseudo force is also called fictitious force.

A fictitious force arises when a frame of reference is accelerating compared to a non-accelerating frame.

For example if you consider a person standing at a bus stop watching an accelerating car, he infers that a force is exerted on the car and it is accelerating. Here there is no problem and the pseudoforce concept is not required

But, if the person inside the accelerating car is looking at the person standing at the bus stop, he finds (more…)

## The Physics of Breaking Stuff – The Physics of Martial Arts

How is Physics applied in Breaking things in Karate Demonstrations?

Answer:

The following video explains it well. If you have further doubts, please post them as comments to this post.

## Passenger in a moving train

A passenger in a moving train tossed a coin which falls behind him .State the type of motion of the train ?

**Answer:**

The train is in accelerated motion.

**Explanation**

If the train were moving with uniform velocity, the coin would fall back to the passenger’s hand (This is due to the inertia of motion of the coin. When it was thrown ‘vertically’ up, it had the same horizontal velocity as that of the train. Therefore, it will move alongwith the train and fall back to the thrower’s hand)

If the train is accelerated, the horizontal velocity of the train would be more than that of the coin (which remains constant through out its flight) by the time the coin returns. Therefore, the train travels a greater horizontal distance than the coin and the coin falls **behind the passenger.**

## Kinetic Energy and Momentum comparison

A light body and a heavy body have the same momentum which of the two bodies will have greater kinetic energy ?

Sumanta Das requested to explain the answer to this question.

Answer:

As we know,

The momentum of a body, **P = mv** whee m is the mass and v is the velocity

and Kinetic Energy of a body, **E = (1/2) mv ^{2}**

Since the velocity term comes twice in E, the lighter body will have greater KE

Alternately,

P = mv

P^{2} = m^{2}v^{2}

p^{2}/2m = (1/2) mv^{2} = E

which shows that E is inversely proportion to mass for bodies with same momentum.