GS-1 Physical Sciences Question 32

[rsonia167876]

I am confused about 32, since in 31 we find that collision is inelastic. Then the final velocity of the car and truck should be the same? 3? Since we are told in the passage that after the collision the car moves with the velocity of 3. Very confuse pleae help!

[JerryWTu7580]

In a "completely" inelastic collision, the cars would stick together and the final velocity would be the same. But the problem only says that the collision is inelastic, meaning some energy is lost as heat/crumpling/etc. I hope that helps.

[rsonia167876]

Thank you Jerry, it make sense now.

[nedaa.asba6809]

Since both objects move in different directions after the collision (not as one mass), the truck will have a different change in velocity than the car and by using the conservation of momentum we can calculate the truck's new velocity.

[ozairmh8911]

Is there a quick way to do this question without calculation?

[jellywing_2058]

No, you need to use the formula.

[Berman878951]

Actually, there is a much simpler way to do this problem without any calculations.

Using physical intuition, this is an easy problem.

Both the car and truck are initially moving in the same direction. Once the car rear-ends the truck, the truck is going to gain some speed. Now let's look at the answer choices:

We can immediately cross out 1.1 m/s as the truck is obviously not going to slow down.

We can also cancel out 3 m/s because the truck must gain some speed. It was just rear-ended by a 1200kg car.

We can also cross out 7.5 m/s because there is no way that the car (which is light compared to the truck) would be able to make the truck gain 4.5 m/s and assume the same speed as the car was initially traveling.

After using the process of elimination, we can confidently chose 3.7 m/s -- meaning that the truck speeds up a little bit after the rear impact.

Hope that helps.