Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Two energy problems I do not understand

  1. Mar 16, 2010 #1
    1.) In the 1950s, an experimental train that had a mass of 2.50x10^4kg was powered across a level track by a jet engine that produced a thrust of 5.00x10^5 N for a distance of 509m.

    A-Find the work done on the train.
    B-Find the change in Kinetic Energy.
    C-Find the final kinetic energy of the train if it started from rest.
    D-Find the final speed of the train if there were no friction.

    B,C, and D I cannot understand. For part b, I believe you use KE=1/2mv^2, but I have no velocity..., if I had a quantity for velocity, I would be able to find ΔKE by doing K1=...,K2=.... and finally...W=K2-K1=.....J.

    For c and d, I'm totally lost.
    2.) A 14,700N car is traveling at 25m/s. The brakes are applied suddenly, and the car slides to a stop. The average braking force between the tires and the road is 7100N. How far will the car slide once the brakes are applied?
    Well, I know this:
    F=14,700 N
    Vi=25 m/s
    Vf=0 m/s
    avg. braking force=7100 N
    and I'm assuming that I need to find d=......?

    What formulae would I use to find the answer? I know these:

    W=fd. I'd be able to solve for d, but how would I go about finding the value for W?
    Last edited: Mar 16, 2010
  2. jcsd
  3. Mar 16, 2010 #2
    BUMP. Please help me.
  4. Mar 16, 2010 #3


    User Avatar
    Homework Helper
    Gold Member

    True, the velocity was not given. But you know the amount of work put into the system (part A). Work is a measure of energy. If you ignore friction, where does this work go? (Hint: kinetic energy is also a measure of energy :wink:) I'm assuming you need to ignore friction for part B.

    Part C is dependent upon your answer for B. What is the kinetic energy of a train at rest?

    Once you have the answer to part C, you already have the information and the equation you need to find the velocity.

    Now you're thinking. :approve: Both work (caused by the breaking force) and KE = 1/2mv^2 are measures of energy. In this problem, the initial kinetic energy is converted to "work" done on the breaks. Since you know the force on the breaks, it shouldn't be too tough to calculate the distance.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook