Calculate the distance traveled by 2 vehicles(reaching the same point)

[iloveyourspam]

Homework Statement

A car races past a pedestrian crossing at 20 m s-1 (which is well in excess of the speed limit); 10 seconds later a police car traveling at 30 m s-1 passes the pedestrian crossing in pursuit of the offender. Assuming that both cars maintain constant speeds, how far from the crossing will the police car catch the speeding car?

Homework Equations

I was thinking I would use x=vt ('x' being displacement) and solve using a simultaneous equation

The Attempt at a Solution

Speeding car:
x=20*t

Police car:
x=30*(t+10)
x=30t+300

I'm not sure where to go from here

 

[voko]

Why is it (t + 10) for the police car? What time does t = 0 correspond to?

 

[iloveyourspam]

I thought because the police car started 10 seconds after the offender you would do that. when t=0 the offender crosses the pedestrian crossing.

 

[voko]

Substitute t = 0 into the equation for the police car. You get 300 m. Does that look right to you?

 

[iloveyourspam]

Ok I get why you didn't use the +10 but I'm still confused on the working out. How did you know it was 300m?

 

[voko]

Since x = 30*(t + 10), substitute t = 0: x = 30*(0 + 10) = 30*10 = 300. The question is, is it correct that the police car is already 300 m away from the crossing in the positive direction when the offender is at the crossing, also moving in the positive direction?