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Mechanics (M1) - Momentum

I don't fully understand how you account for a force in a momentum question:

A car of mass 1.2 tonnes collides with a stationary van of mass 2.4 tonnes. After the collision the two vehicles become entangle and skid 15m before stopping. Police accident investigators estimate that the magnitude of the friction force during the skid was 2880N. Assume the road is horizontal and that all the motion takes place in a straight line.
a) Find the speed of the vehicles just after the collision.
b) Find the speed of the car before the collision.

Thanks :smile:
Original post by anil10100
I don't fully understand how you account for a force in a momentum question:

A car of mass 1.2 tonnes collides with a stationary van of mass 2.4 tonnes. After the collision the two vehicles become entangle and skid 15m before stopping. Police accident investigators estimate that the magnitude of the friction force during the skid was 2880N. Assume the road is horizontal and that all the motion takes place in a straight line.
a) Find the speed of the vehicles just after the collision.
b) Find the speed of the car before the collision.

Thanks :smile:


The force only comes into play when the combined mass is decelerating. Start by working out the deceleration, and then the speed of the vehicles after the collision, and then use momentum to find the speed before collision.
Reply 2
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anil10100
OP
Original post by ghostwalker
The force only comes into play when the combined mass is decelerating. Start by working out the deceleration, and then the speed of the vehicles after the collision, and then use momentum to find the speed before collision.


How would I work out deceleration ?
Original post by anil10100
How would I work out deceleration ?


F=ma

It's the force of friction that's slowing the mass.
Reply 4
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anil10100
OP
Original post by ghostwalker
F=ma

It's the force of friction that's slowing the mass.


Got the deceleration as -0.8m/s^2 but I don't know what to do now..
Original post by anil10100
Got the deceleration as -0.8m/s^2 but I don't know what to do now..


For the period after the collision, you've got the distance travelled, the final speed, the acceleration, and you want to know the initial speed (that is the speed immediately after the collision).

What does that suggest?
Reply 6
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anil10100
OP
Original post by ghostwalker
For the period after the collision, you've got the distance travelled, the final speed, the acceleration, and you want to know the initial speed (that is the speed immediately after the collision).

What does that suggest?


Oh yeah suvat equations, thanks, got the right answers now 4.90m/s and 14.7m/s.
Now hopefully I should be able to apply this to the rest of the questions :biggrin:
Reply 7
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Sid.
9
Original post by anil10100
Oh yeah suvat equations, thanks, got the right answers now 4.90m/s and 14.7m/s.
Now hopefully I should be able to apply this to the rest of the questions :biggrin:


How have you got 4.90? what is the final speed?

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