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# Tag Archives: Momentum

## Momentum

I’ve just created this question for my undergraduated students that are going to apply for an engineering institute. I’m not quite sure if it’s a nice question, so I’d like to know your opinions or suggestions about it (and yours solutions, of course!)A flying saucer with mass 5m and radius R is at rest and preparing to destroy our beloved Earth. An aircraft is therefore sent to prevent it and shoots a missile with mass m and velocity v1 at the same plane of the flying saucer. The UFO reacts and launches an intercepting missile of mass m and velocity v2 that makes a 30o angle with the direction of the first projectile. If both projectiles meet after a distance h from the launching point of the second missile and then keep moving together, find the minimal value of v2 for which the aircraft will not hit the target.PS.: Ignore friction and gravitational forcesPlease check the attached picture.

Marcos Valle asked

## Must Practice portions for class xi physics

Hi Class XI Students!

The following portions are very much essential for obtaining a pass mark in class XI. So please read the portions from your text book well and try to answer the questions which can be asked from them.

- Uses of dimensional analysis
- Types of errors and propagation off errors
- derivation of equations of uniformly accelerated motion graphically.
- Derive expression for the trajectory of a projectile (or show that the trajectory of a projectile is a parabola.
- Derive expression for the horizontal range, time of flight and max. height of a projectile motion
- Derive expression for centripetal acceleration of a body in uniform circular motion.
- State the laws of motion
- Derive F = ma (more…)

## We are not Psychic – It’s Physics Here!

We receive many questions daily where in questions based on dreams, love, relations etc. are asked.

We thought initially that those were posted by spammers. But the emails and posts were not from spam-bots but genuine IPs and persons. On analysis, it was found that those posts were made by people who thought that the site name as “Ask**Psychics**” and thus they asked such questions. Some even misspell Psychic as Physic.

When they don’t get their answers, they come back to the site and start looking at the answered questions to find that the site is **all about Physics** and that there are no **Psychichs** here!

Thanks to our principle that we do not allow questions to be published directly. All questions posted via the Ask a Question link reach the email inbox of the admin and editors of the site. we scrutinize the question and are posted to the site only if it is related to Physics or education.

Once again I would like to announce –

## We are PHYSICS People

## –

# Not Psychics

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## Why is it necessary to bend knees while jumping from a height?

Why is it necessary to bend knees while jumping from a height?

Asked Rahul. (via SMS from our mobile version http://m.askphysics.com )

Answer:

This is a consequence of the impulse momentum principle.

When we bent our knees on landing, the time of impact increases and reduces the average force exerted during the entire process. This prevents the chance of damage to our body.

Impulse momentum principle states that the impulse of a force is equal to the change in momentum produced.

Impulse is measured as the product of force and time interval. So, when we stop ourselves, there is a fixed change in momentum. If we stop ourselves very fast, the time interval is less and the average force will be very large.

But, if we bent our knees during landing, the time involved in the proces of landing will be more and thus the average force will be less and hence the damage will also be less.

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## Questions waiting for answers here!

- If energy is mass-less then why does it cause linear momentum?
- one object is thrown upward with velocity v from height h and another object is thrown downward with same velocity v and other object is thrown horizontally with same velocity v. which of the above will get maximum speed while touching the ground?

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- Conservation of linear momentum – Problem (askphysics.com)
- A Complicated problem from force and motion (askphysics.com)
- Where is the light energy going? (askphysics.com)

## Conservation of linear momentum – Problem

Astronauts Mr.X and Mr.Y float in a zero gravity space with no relative velocity to one another. Mr. Y throws a mass of 5 kg towards X with speed 2 m/s. If Mr. X catches it, the change in velocity of X and Y are (choose the correct option)

(a) 0.21 m/s, 0.80 m/s

(b) 0.80 m/s, 0.21m/s

(c) 0.12 m/s, 0.08 m/s

(d) 0.08 m/s, 0.12 m/s

Please provide me the solution part step by step

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## Rotational Equilibrium – A Numerical Problem

**A father and his son are supposed to carry a load of 500 N. They decided to use a uniform pole that is 4.0 m long and weighs 60 N. The father and his son support the pole at the ends. Where should the load be placed so that the father supports three times as much as his son?**

Posted by Daniel

Answer:

Let’s represent the situation be the folowing diagram

Let the force exerted by son be f

Then the force exerted by father = 3f

Therefore the total force exerted =Total weight = 560N = 4f

Which gives f = 140 N, the force exerted by son

3f=420 N, the force exerted by father

Applying the principle of moments about the point where father holds the rod,

420 x 0 – 500 x *x* – 60 x 2 + 140 x 4 = 0

==> **x**** = 440/500 = 0.88 m** from the end where father holds the load

(If there is any problem you find in the solution given, please post as comment to this post.)