# momentum problem

• Nov 14th 2007, 02:54 PM
rcmango
momentum problem
In a football game, a receiver is standing still, having just caught a pass. Before he can move, a tackler, running at a velocity of +4.0 m/s, grabs him.

The tackler holds onto the receiver, and the two move off together with a velocity of +3.0 m/s.

The mass of the tackler is 110 kg. Assuming that momentum is conserved, find the mass of the receiver.
in kg.

i tried to solve this like this:

3.0 m/s = m1 * 4.0 m/s divided by 110 kg

and i got 82.5 which is not correct.

• Nov 14th 2007, 04:46 PM
Spimon
Do you have the correct answer? My answer seems too low..unless this particular game of football is between large men and small children... I got 36.6kg

Momentum = Mass * Velocity

M1 = 110 * 4 kg/ms
M1 = 440

Law of conservation of energy in a closed system means momentum must remain constant.

M2 = M1
440 = M2 * V2
440 = 3(x+110) where x is the unknown players mass)
x = 36.6kg
• Nov 15th 2007, 08:28 PM
rcmango
hey 36.6 kg was correct.

it must be small men playing.

thankyou for the equations and solution.
• Nov 16th 2007, 07:33 AM
topsquark
(Puke)

Quote:

Originally Posted by Spimon

Momentum = Mass * Velocity

M1 = 110 * 4 kg/ms
M1 = 440

What??

What is M1? Surely not the mass. The only thing I can think of is that you are referring to the total momentum, but the standard symbol for that is "P."

Quote:

Originally Posted by Spimon
Law of conservation of energy in a closed system means momentum must remain constant.

Again, what???

The total momentum of a system is conserved if there is no net external force acting on the system. Done. Finito. It has nothing to do with energy conservation.

Quote:

Originally Posted by Spimon
M2 = M1
440 = M2 * V2
440 = 3(x+110) where x is the unknown players mass)
x = 36.6kg

Again, again, what???

In line 1 above you seemed to be using M as a total momentum. But in line 2 you are using it as a mass?

This whole post is sloppy and barely coherent. PLEASE make sure you know how to say what you mean to say before you post a solution! The fact that you got the correct answer means you know what you are talking about, which is good. But you need to be able to communicate what you are doing clearly.

-Dan
• Nov 16th 2007, 07:38 AM
topsquark
Quote:

Originally Posted by rcmango
In a football game, a receiver is standing still, having just caught a pass. Before he can move, a tackler, running at a velocity of +4.0 m/s, grabs him.

The tackler holds onto the receiver, and the two move off together with a velocity of +3.0 m/s.

The mass of the tackler is 110 kg. Assuming that momentum is conserved, find the mass of the receiver.
in kg.

Let the +x axis be in the direction that the tackler is initially moving in. Initially the receiver of mass $m_r$ is not moving, so his/her momentum before the collision is 0 kg/m s. So the tackler has all of the momentum at the beginning of the problem. After the collision, they move together as one object at the same velocity.

So. No net external forces acting on the players, so the momentum of the system of two players is conserved:
$P = P_0$

$(m_r + m_t)v = m_tv_t$

$(m_r + 110)(3) = (110)(4)$

Solve for $m_r$. I got 36.7 kg.

-Dan
• Nov 16th 2007, 08:51 AM
rcmango
thanks for the solutions, seeing this multiple ways just makes it easier for me to understand it more thoroughly.

Thanks again.
• Nov 16th 2007, 10:00 AM
topsquark
Quote:

Originally Posted by rcmango
thanks for the solutions, seeing this multiple ways just makes it easier for me to understand it more thoroughly.

Thanks again.

My method and Spimon's method are the same. It is just that my work was far more organized and lacked the ambiguity of his.

-Dan