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conservation of mass---PLEASE HELP.

How do you work these out please?

question 1: A car of mass 1200kg travelling at 30m/s runs into the back of a stationary lorry. The car and lorry move at 4m/s after impact. Determine the mass of the lorry.

question 2: A truck of mass 2kg travels at 8m/s towards a stationary truck of mass 6kg. After colliding the trucks move off together. Calculate their common velocity.

thanks.

v-u/t= acceleration

Original post by Moonbow

v-u/t= acceleration

thanks but i don't know what that means and still don't know how to work those questions out

Reply 3

Moonbow

so v is current speed so 4m/s from 30 m/s so take them away and you get negative 26m/s. It is some collision theory question i think you make mass and speed equal to each other or something, sorry i have not done this for months :")

Reply 4

Stonebridge

Use conservation of momentum.
Have you done the theory for this?
Total momentum of the 2 objects before the collision = total momentum after.

Momentum is mv and you must always take in to consideration the direction of travel.
If you say mv is positive from left to right, for example, then it is negative if the object is moving right to left.