# Frames of reference

• Oct 27th 2008, 01:21 PM
free_to_fly
Frames of reference
I'm very stuck on this question:

In a road accident at cross-roads, a car of mass 1250kg traveling at a speed of 40km/h from west to east hits a truck of mass 4000kg traveling at a speed of 30km/h from North to South. The vehicles lock together and skid off the road.
Consider the situation just before the collision from two different point of views of the pilot of a helicopter:
i)hovering above the intersection
ii) flying parallel to, and keeping pace with the car
In each case, find the direction and speed of the vehicles after the collision.

I wanted to use conservation of linear momentum to solve the question, but the two vehicles are not traveling in a line so I don't know how to adapt the equation, and for the second case I did 30*4000=(1250+4000)v, and v came out to be 22.857km/h, which is the wrong answer (correct answer would have been 38.1km/h at 233 degrees)

Any help would be hugely appreciated.
• Oct 31st 2008, 08:09 AM
CaptainBlack
Quote:

Originally Posted by free_to_fly
I'm very stuck on this question:

In a road accident at cross-roads, a car of mass 1250kg traveling at a speed of 40km/h from west to east hits a truck of mass 4000kg traveling at a speed of 30km/h from North to South. The vehicles lock together and skid off the road.
Consider the situation just before the collision from two different point of views of the pilot of a helicopter:
i)hovering above the intersection
ii) flying parallel to, and keeping pace with the car
In each case, find the direction and speed of the vehicles after the collision.

I wanted to use conservation of linear momentum to solve the question, but the two vehicles are not traveling in a line so I don't know how to adapt the equation, and for the second case I did 30*4000=(1250+4000)v, and v came out to be 22.857km/h, which is the wrong answer (correct answer would have been 38.1km/h at 233 degrees)

Any help would be hugely appreciated.

Momentum is a vector quantity, so what is conserved in the collision is:

\$\displaystyle \bold{p}=m_1 \bold{v}_1+m_2 \bold{v}_2\$

where \$\displaystyle \bold{v}_1\$ and \$\displaystyle \bold{v}_2\$ are the vector velocities of the two vehicles in whatever reference frame is being considered.

CB