# Homework Help: Momentum and Energy.

1. Dec 8, 2009

### Ronaldo21

part (1 of 3 ). A 15 metric ton train moves toward the south at 70 m/s.
At what speed must it travel to have five times its original momentum? A metric ton
is 1000 kg.

and

Part (2 of 3 ). At what speed must it travel to have a momentum of 5 × 105 kg · m/s?

and

Part (3 of 3). If there were a speed limit for this train as it traveled through a city, but not a weight limit, what mass must be added to the train to slow it down to 10 m/s while at the same time keeping the momentum the same as in the second part?

THANK YOU!!

2. Dec 8, 2009

### hitmeoff

Part 1: p = mv

So pi= 15000kg*70m/s = 1,005,000 Ns

pf = 5*pi = 5,250,000 Ns

Since we cant change the train's mass, we have to change its speed: 5,250,000 = 15000*v, v= 350 m/s

3. Dec 8, 2009

### hitmeoff

Part 2: 5 x 10^5kg*m/s

So if p = mv, then 5x10^5kg*m/s = 15000*v

so v = 33.3 m/s

4. Dec 8, 2009

### hitmeoff

Part 3:
so
p = 5 x 10^5 Ns
v = 10 m/s

p = mv, m = p/v; m = 50000kg final weight. Train starts out at 15000kg, so you must add 35000kg to have a total weight of 50000kg

5. Dec 8, 2009

### Ronaldo21

wow!! thank you soo much!! this makes so much more sense now!!!!!

6. Dec 8, 2009

sure thing!