Postby **MattChina** » February 27th, 2018, 2:12 pm

Can anyone help with these two physics questions?

**1. With what constant velocity can a 4.00 hp motor raise a mass of 125 kg?**

The answer key says 2.76 s.

This is what I did, but it's giving me the wrong answer:

v = 4.00 hp * 746 W/hp / 9.8 m/s^2 / 125 kg = 2.44 m/s

**2. A train car of mass 1500 kg. moving with a velocity of 1.2 m/s collides with a 3000. kg car that is stationary. Assuming no loss of energy to friction, calculate the subsequent velocity of the stationary train car assuming the collision is perfectly elastic.**

I calculated the 1500 kg car to have a velocity of -.4 m/s and the 3000 kg car to have a velocity of 0.8 m/s, so my answer was 0.8 m/s. The key said -0.4 m/s. Is it just me, or does it not make sense for the stationary car to go backwards after the collision?

Sorry if these questions seem dumb, I just want to be sure.

For the first one, I also got the same answer and did it the same way as you did so Idk, maybe the answer key is wrong

For the second one, the velocity of the stationary train should have a positive velocity it doesnt make sense that the answer is negative. I also got 0.8 m/s. Idk maybe the answer key is extremely wrong.

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