title: Allowing user to choose an input
body: <p>I am trying to sharpen my python programming skills by coding solutions to basic physics problems.</p>
  
  <p>The problem states that a spaceship is traveling a distance x at a velocity v towards a destination from Earth. I am to find the time elapsed by a stationary observer on Earth and the time elapsed by a passenger aboard the spaceship. Rather than assigning values to x and v, I am to allow the user to input values for x and v. But, I would like to allow the user to choose how they would like to input their data. For example, the user could put v = some number in meters/sec or the user could put v = a*c where 0 ≤ a ≤ 1 and c = speed of light; I would like to let the user decide which input is preferred.</p>
  
  <p>I understand how to ask the user to input a value.
  ex: 
      x = float(input("What distance has the spaceship traveled:  "))</p>
  
  <p>But how can I let the user decide which v they would like to input?</p>