Velocity problem (1 Viewer)

[davidbarnes]

"An unfortunate driver of mass 50 Kg was travelling on an icy road in her 1200 Kg car, collides with a stationary police car of total mass (including occupants) of 1500 Kg. The tangled wreck moves off after the collision with a speed of 7.0 m s^-1. The firctional force on both cars can be assumed to be neglible"

At what speed was the unfortunate driver travelling before her car hit the police car?

I was doing aheap of questions liek these earleir, and can do them, althoguh just couldn't seem to solve this one. I kept getting v = 12.4 m s^-1 and the answer is v = 15.0 m s^-1.

 

[BlackJack]

Because the answers are wrong. I got

v = (1250+1500)* 7 / 1250 = 15.4.

 

[xiao1985]

it's a conservation of momentum question...

note that m1v1 = m2v2

1 = before collision
m1 = mass of female driver + car
v1 = velocity at which the car and her is travelling before collission

2 = after collision
m2 = mass of female driver + her car + police car + very angry police officers
v2 = velocity at which the cars and occupants are travelling in after collision.

which is how you get blackjack's answer.

/wave @ blackjack: how's things?

 

[BlackJack]

I'll express this in an equation:
Research + teaching labs I studied for the year before + sitting around waiting for simulations to finish (because they so slooooww..) + visiting friends at many churches + picking up guitar again + everything else that is happening = magnificent!

You are well?

 

[xiao1985]

except cell work going insane, which means end of the world (honour) for me =/