# Solving for a specific #

1. Dec 6, 2004

### Rapta3

How do you slove for Mb from this equation (MaVa+MbVb)/Ma+Mb=Vab

2. Dec 6, 2004

### Tide

HINT: Multiply both sides by $M_a + M_b$

3. Dec 6, 2004

### Rapta3

K got it, ty, got another one for you tho.

This one is HARD.

A 13 g bullet traveling 218 m/s penetrates a 2.0 kg block of wood and emerges going 161 m/s. If the block is stationary on a frictionless surface when hit, how fast does it move after the bullet emerges?

4. Dec 6, 2004

### futb0l

Try Conservation of Momentum.

5. Dec 6, 2004

### futb0l

Change the units into kg first...
13g = 0.013kg

so...

$$0.013(218) = 0.013(161) + 2v$$

..

$$v = \frac{0.013(218-161)}{2} = 0.3705 m/s$$

6. Dec 6, 2004

### Rapta3

Yea converting the g to kg was the missing step, thanks.

7. Dec 6, 2004

### futb0l

No problems - I used to make careless mistakes like that all the time.

8. Dec 6, 2004

### Rapta3

A tennis ball may leave the racket of a top player on the serve with a speed of 62.0 m/s. If the ball's mass is 0.0600 kg and it is in contact with the racket for 0.0200 s, what is the average force on the ball?

Would this force be large enough to lift a 60 kg person?
large enough or
not large enough

I got .0600x62=3.72

.0200/3.72=.00537

I dont think that is right because that seems way to small but I dont know what im missing...if it is incorrec that is.

9. Dec 6, 2004

### nolachrymose

Try momentum-impluse theorem:
$$F\Delta t = m\Delta v$$

10. Dec 6, 2004

### dextercioby

You're last calculation is wrong.As smb else said,apply the definition of force wrt to changes in momentum (the integrated form of Newton's second law) correctly.
You should be getting round about 19 kgf which would not be enough to lift 60kg off the ground,since the force needed to do that is obviously 60kgf.
Divagation ( culture useful):
The greatest tennis player ever (to me ond not only,though he didn't win Rolland Garros),Pete Sampras (i don't believe the bull**** they say about Federer),used to have the strings of his racket streched to a tension of 34kgf.It was the most "tensioned" racket in the history of the sport.And even so,it broke frequently.He was famous for breaking an impressive number of rackets (strings) during serve,which meant that (at a maximum speed of 62m/s -he never served more than 220 kmph/137mph) for light balls (the ones used on grass) the time of impact between the racket and the ball was at the order of microseconds.Amazing,really!!That's the reason for those huge and perfect forehands crosscourt and down the line.Alongside genious,of course.
I almost cried when he retired.

Last edited: Dec 6, 2004