# Getting wrong answer in an (angular) impulse momentum problem

• Divya
In summary, The forum rules state that images are not allowed and working should be typed in. The issue is with finding the mass centre of the rod arrangement, which may be interpreted as having a free joint. This approach gave the correct answer.
Divya
Homework Statement
Three masses m, 2m and 3m are connected by two massless and rigid rods
of length l which are currently perpendicular to each other, as shown in the
ﬁgure. If the masses initially travel at velocity u towards a vertical wall and
mass m undergoes a collision with the wall, determine the impulse delivered
by the wall to mass m if the ﬁnal horizontal velocity of mass m is zero. There
is no friction between the wall and mass m. Assume that the tensions in the
rods are strictly longitudinal (because they are massless).
Relevant Equations
impulse = change in momentum
angular impulse = change in angular momentum

I have tried this same approach three times and I got the same answer. I can't figure out what's wrong. Btw answer is 12mu/(3+cos2α)
And yes, sorry for my shitty handwriting. If you can't understand the reasoning behind any step then please let me know.

Divya said:
sorry for my shitty handwriting
Forum rules state that images are for diagrams and textbook extracts. Please type in your working, preferably in LaTeX.
Also, it helps if you provide some explanation of your approach and define all variables.

It looks to me as though you are taking the rod arrangement as rigid. It only says initially a right angle. I would take it as having a free joint.

In my experience, it rarely gains anything to bother finding the mass centre of such an assemblage anyway.

Divya
I am really sorry for not seeing the rules of forum before posting ( I am new here). Also thanks for your help. The 'free joint' interpretation gave the right answer.

PeroK
Divya said:
I am really sorry for not seeing the rules of forum before posting ( I am new here). Also thanks for your help. The 'free joint' interpretation gave the right answer.
well done

## 1. Why am I getting the wrong answer in my angular impulse momentum problem?

There could be several reasons for getting the wrong answer in an angular impulse momentum problem. Some common mistakes include using the wrong formula or equation, not considering all the forces acting on the system, and not properly accounting for the direction of angular momentum. It is important to double check all calculations and make sure all relevant factors are taken into account.

## 2. How can I check if my answer is correct in an angular impulse momentum problem?

One way to check if your answer is correct in an angular impulse momentum problem is to use the principle of conservation of angular momentum. This means that the initial angular momentum of a system should be equal to the final angular momentum. You can also compare your answer to the expected outcome based on the physical situation described in the problem.

## 3. What are some common mistakes to avoid in solving an angular impulse momentum problem?

Some common mistakes to avoid in solving an angular impulse momentum problem include using the wrong units, not properly identifying the direction of angular momentum, and not considering all the forces acting on the system. It is important to carefully read and understand the problem, use the correct equations and units, and double check all calculations.

## 4. Can I use the same approach to solve all angular impulse momentum problems?

No, the approach to solving an angular impulse momentum problem may vary depending on the specific situation and the information given in the problem. It is important to carefully read and understand the problem before determining the appropriate approach and equations to use.

## 5. How can I improve my understanding and problem-solving skills in angular impulse momentum?

To improve your understanding and problem-solving skills in angular impulse momentum, it is important to practice solving different types of problems, review and understand the relevant equations and principles, and seek help from a teacher or tutor if needed. It can also be helpful to break down a problem into smaller steps and carefully consider all the forces and factors involved.

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