A man (weighing 915 N) stands on a long railroad flatcar (weighing 2805 N) as it rolls at 18.0 m/s in the positive direction of an x axis, with negligible friction. Then the man runs along the flatcar in the negative x direction at 40.00 m/s relative to the flatcar. What is the resulting increase in the speed of the flatcar?
For Further Reading
* Physics - bobpursley, Sunday, February 25, 2007 at 5:25pm
Use conservation of momentum. I will be happy to critique your thinking.
* Re: Physics - COFFEE, Sunday, February 25, 2007 at 11:19pm
Ok, so I use mv = m1v1 + m2v2??? And I would get m1 and m2 by dividing the weights by 9.8 m/s^2? Please explain.
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* RE: PHYSICS - bobpursley, Monday, February 26, 2007 at 6:10pm
The original momentum is..
(915+2805)g18
That is equal to the final momentum
915g*(-40+18) + 2805g(V+18)
solve for V.
I did:
(915+2805)g(18)=(915)g(-40+18)+(2805)g(v+18)
g cancels out on both sides.
I ended up with 13.04 m/s and this is the wrong answer. Why???
For Further Reading
* Re: PHYSICS - bobpursley, Tuesday, February 27, 2007 at 7:26am
Is there anything wrong with my thinking?
* Re: PHYSICS - COFFEE, Tuesday, February 27, 2007 at 9:58pm
Not that I can see but it's telling me that it's the wrong answer :( I don't know how that can be, your reasoning makes sense to me!
for momentum after, the man has a momentum of his own that is -40m/s. but there is also the combined mass of the man and cart which has its own momentum.
To solve this problem, you can use the principle of conservation of momentum. The total momentum before the man starts running is equal to the total momentum after he starts running.
Let's break it down step by step:
1. Calculate the initial momentum of the system before the man starts running:
Momentum1 = (mass of the man + mass of the flatcar) * velocity of the flatcar
In this case, it is given that the weight of the man is 915 N and the weight of the flatcar is 2805 N. To convert the weights to mass, we divide them by the acceleration due to gravity, which is approximately 9.8 m/s^2:
Mass of the man = weight of the man / 9.8 = 915 / 9.8 ≈ 93.4 kg
Mass of the flatcar = weight of the flatcar / 9.8 = 2805 / 9.8 ≈ 286.2 kg
Plugging the values into the equation, we have:
Momentum1 = (93.4 kg + 286.2 kg) * 18.0 m/s
2. Calculate the final momentum of the system after the man starts running:
Momentum2 = (mass of the man) * (velocity of the man relative to the flatcar) + (mass of the flatcar) * (velocity of the flatcar + velocity of the man relative to the flatcar)
In this case, the man is running in the negative x direction at a speed of 40.0 m/s relative to the flatcar. Since the flatcar is moving in the positive x direction, the velocity of the man relative to the flatcar is 40.0 m/s in the negative x direction.
Plugging in the values into the equation, we have:
Momentum2 = (93.4 kg) * (-40.0 m/s) + (286.2 kg) * (18.0 m/s + (-40.0 m/s))
3. Equate the initial momentum to the final momentum and solve for the unknown velocity of the flatcar:
Momentum1 = Momentum2
Plugging in the calculated values, we have:
(93.4 kg + 286.2 kg) * 18.0 m/s = (93.4 kg) * (-40.0 m/s) + (286.2 kg) * (18.0 m/s + (-40.0 m/s))
Solve this algebraic equation to find the value of the unknown velocity of the flatcar.
By following these steps and performing the calculations correctly, you should be able to determine the resulting increase in the speed of the flatcar.