All Questions
108
questions
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How does the direction of the change in momentum of the object change during the motion?
In the case of an object's movement resembled in this graph, as the gradient is decreasing, a decrease in velocity occurs.
According to the formula
$$p = mv$$,
The momentum is directly proportional to ...
-1
votes
2
answers
65
views
Total momentum of a fluid in a pipe [closed]
Suppose we have a cylindrical pipe let's say length L = 10
m and radius R = 1 m through which water is flowing. The velocity distribution is given by Poiseuille's Law: $v(r)=\frac{\Delta P}{4L\eta}\...
0
votes
3
answers
71
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How do mass and velocity relate to momentum? [duplicate]
So this is more of a clarifying question. A lot of online definitions state that momentum is a measure of how hard it is to stop or swerve an object, which makes sense. However, the formula for ...
- 539
0
votes
1
answer
69
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Changing variables from $\dot{q}$ to $p$ in Lagrangian instead of Legendre Transformation
This question is motivated by a perceived incompleteness in the responses to this question, which asks why we can't just substitute $\dot{q}(p)$ into $L(q,\dot{q})$ to convert it to $L(q,p)$, which ...
- 7,398
0
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2
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161
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If falling coconut has more momentum than a bullet then why bullet is more deadly? [closed]
Imagine a coconut of mass 1kg is falling from a 10m tall tree, the momentum is around 14kg.m/s (I don't know how to format my work but just trust me)
and then the momentum of a bullet of mass 0.01kg ...
- 13k
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2
answers
75
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Can generalised momentum be defined as mass times generalised velocity？ [closed]
The generalised momentum is defined by the partial derivative of Lagrangian. Can we think of this as a mass times a generalised velocity?
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1
vote
1
answer
696
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What is Wald talking about here in his book "General Relativity"?
I'm reading General Relativity by Robert Wald and his notation is confusing me.
On page $61$ of the book, Wald claims that by setting $c=1$ we can derive $E=mc^2$ from the following information.
(1) $...
- 113
2
votes
5
answers
1k
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Why do we have momentum?
Momentum is the defined as the product of mass and velocity and can be thought as measuring how much motion something has. However, it is not clear to me why we need momentum and why force is not ...
5
votes
3
answers
1k
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What is the relationship between linear momentum and kinetic energy?
Okay y'all. I cannot be the the first person to notice that $$ \frac{dE}{dv} = p$$ (or, alternatively, that $\int pdv = E$). I futzed around with the equations a little bit to see if replacing ...
- 125
2
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30
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Why is the energy of a particle with $4$-momentum $p^a = mu^a$ measured by an observer with $4$-velocity $v^a$ given by $E=-p_av^a$? [duplicate]
On page 61 of his book “General Relativity”, Robert Wald says that the energy of a particle with $4$-momentum $p^a = mu^a$ measured by an observer with $4$-velocity $v^a$ is given by $E=-p_av^a$. How ...
- 669
4
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1
answer
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Wald's definition of energy of a massive particle
In R.Wald's book on "General Relativity" appears on p.61 (section 4.2) a rather peculiar definition of the energy of a massive particle:
$$ E = -p_a v^a \tag{4.2.8}$$
I guess the minus sign ...
- 9,778
0
votes
6
answers
1k
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Calculating momentum change?
A 100 g ball with a speed of 5 m/s hits a wall at an angle of 45 degrees. The ball then bounces off the wall at a speed of 5m/s at an angle of 45 degrees. What is the change in the momentum of the ...
4
votes
3
answers
236
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How to show the velocity of free motion is constant in Galileo's relativity principle?
Picture below is from Landau & Lifshitz's Mechanics. How to get the red line from green line?
- 351
6
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3
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845
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What happens to the velocity of a radioactively decaying object?
Suppose that I have a lump of a radioactive material, like Uranium-235. I put it in an ideal box, which perfectly isolates the inside from the outside - no radiation escapes the box and the outer ...
- 219
4
votes
4
answers
258
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Intuition behind $p = mv$ and $F = ma$
I am only familiar with physics at an elementary level; and while that is true, it would be nice if I could clear up some clashes with intuition I'm having before progressing further. What is the ...
- 197