Caitlyn L.

asked • 03/16/17

Impulse and force

I've been stuck on this question forever and I keep getting it wrong :( Please help!
 
A truck of mass 3 tonnes is travelling at 105km/h when it hits a car of mass 1.5 tonne travelling in the opposite direction at 100km/h. The collision takes place for 0.3 seconds and after it both the car and the truck come to rest.
 
a) Calculate the Impulse felt by the truck and the car
*For this question, the hint given to me was that the impulse for both needed to be calculated separately
 
b) Calculate the force felt by the car.
 
Thank you so much!

Arturo O.

I have a difficulty with the problem statement.  The collision of the truck and car involves internal forces between them, so the initial and final total momentum should be the same.  However, the truck starts with 315 ton·km/h (assume to the right) and the car starts with -150 ton·km/h (assume to the left).  Then the initial momentum is (315 - 150) ton·km/h = 165 ton·km/h.  But the final momentum is given as zero, since both come to rest.  In addition, the forces on the truck and car should be equal in magnitude but opposite in direction, but they do not come out that way (divide the change in momentum of each by the time interval).  It seems to me the conditions given in the problem statement are not compatible.  
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03/16/17

Steven W.

tutor
I thought the same thing, Arturo.  But then I realized that they never said the car and truck came to a stop directly after colliding.  I am presuming that they collided, and then slid to a stop (via friction and other resistive forces), and the question is asking for the net impulse applied to each over the entire event.
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03/16/17

Caitlyn L.

Hi Arturo! Thanks for your feedback - but unfortunately, the question is as stated and I think Steven is correct here - the car and truck 'slide' after they collide for, as stated, 0.3 seconds. 
 
Cheers for your comments! Much appreciated :)
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03/17/17

2 Answers By Expert Tutors

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Arturo O. answered • 03/16/17

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