Easy problem that I'm a bit confused about.

In summary, Joe and John are playing catch in space with Joe throwing the ball at a velocity of 7.5 m/s. Joe's mass is 60.9 kg, John's mass is 79.8 kg, and the ball's mass is 6.5 kg. The homework question asks to find the velocities of Joe and John after the catch. Using the equations for sum of external forces and Psys, the velocity of Joe after throwing the ball is calculated to be -0.72329 m/s, and the velocity of John after catching the ball is 0.56489 m/s. The problem is set up correctly.
  • #1

Homework Statement

You have Joe and John playing catch in space. Joe throws the ball to John with a velocity of 7.5 m/s.

Given Variables
Joe's mass is 60.9 kg
John's mass is 79.8kg
Ball's mass is 6.5kg

A) Joe's velocity after he throws the ball.
B) John's velocity after he catches the ball.

Homework Equations

Sum of the external forces = 0; Psys=0

For A, I used:
(Mass of ball+ Mass of Joe) (Velocity of Joe after he throws the ball) + (Mass of ball)(Velocity of Ball) = 0

I ended up with: Velocity of Joe after he throws the ball = - [(Mass of ball)(Velocity of ball)]/(Mass of Ball + Mass of Joe9)

After substituting in the numbers, the velocity of Joe is: -.72329 m/s

For B, I used:

(Mass of Ball)(Velocity of Ball) = (Mass of John + Mass of Ball)(Velocity of John after he catches the ball)

The Velocity of John after he catches the ball is: .56489m/s

Did I set this problem up properly?
Any suggestions or clarification would be greatly appreciated. Thanks.
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  • #2
Looks good to me.
  • #3

Yes, you set up the problem correctly. To find the velocities after the ball is thrown and caught, you must use the principle of conservation of momentum, which states that the total momentum of a closed system remains constant. In this case, the system is closed because there are no external forces acting on it.

Your equations for A and B are correct. However, there is a slight error in your calculations for A. The correct formula should be:

(Mass of ball x Velocity of ball) = (Mass of ball + Mass of Joe) x Velocity of Joe after throwing the ball

When you substitute the numbers, the velocity of Joe after throwing the ball should be 0.72329 m/s, not -0.72329 m/s. This means that Joe will have a velocity in the same direction as the ball after he throws it.

Overall, your approach and equations are correct. Just be careful with your calculations and make sure to double check them to avoid any errors. Good job!

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