I'm back once more because of another problem that is driving me nuts. I've been trying to do this one for a long time. I'm posting the previous problem because it has relevant information and then bold typing the problem I'm confused on.(adsbygoogle = window.adsbygoogle || []).push({});

----Find the speed at which Superman (mass = 79 kg) must fly into a train (mass = 16351 kg) traveling at 80 km/hr to stop it.Calculate the time it takes Superman to stop the train, if the passengers experience an average horizontal force of 0.55 times their own weight.------

And the hint they give for the problem is:Hint: The average horizontal force tells you the maximum acceleration of a passenger. With the change in speed in a given time, you can find the acceleration. Careful with units. Be sure to differentiate between weight and mass.

Ok....so I need to find the time it takes for Superman to stop the train. What I did was find the weight of the train...16351 kg x 9.8 m/s^2....and then multiplied that answer by 0.55 to the get the horizontal force, which I calculated to be 88131.89 N. I know that F=ma...so 88131.89 N = 16531 x A.

Acceleration is the change in velocity over the change in time. So I solved for A and got 5.33 m/s^2....the change in velocity is 1.66E+04 km/hr (which I found in the previous problem) - 80 km/hr = 16520 km/hr.....

So 5.33 = 16520/T (change in time)....and I get 3099.44 s or hr...that I'm not sure of. Either way, the answer is wrong and I really don't know what I'm doing wrong.

If anyone can make any sense of what I wrote and see what I did wrong, I'd be grateful to know. Thanks so much. :)

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Superman problem

**Physics Forums | Science Articles, Homework Help, Discussion**