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

Question relating Newton's First Law of motion!

  1. Oct 16, 2012 #1
    Hi there! I kept on reading my textbook and i found one simple confusion in Newton's First law of motion. I want to consider a situation where a space shuttle is moving in the outerspace (assume there is no gravity) under the influence of the engines. What will happen when the engine stops functioning? It is given that the space shuttle will move with a constant velocity. Now, my opinion would be that there was an initial forward force given by the engine, no it is taken off. Is is like pulling the space shuttle. So it ultimately must end up by a stop! This is confusing me. I would really be thankful to whoever helps me in clarifying my confusion. Thanks in advance. I need elaborate answers if u please :)
  2. jcsd
  3. Oct 16, 2012 #2
    The intuition that once you stop the force the object will stop is an illusion produced
    by the fact that we grow up in an environment where there is friction everywhere. When we stop pushing an object, it may only move a short distance before friction slows it down to a stop. But friction is also a force. If no force at all, including friction, acts on an object, it will move with a constant velocity forever. The ancient people, even the most educated, believed the idea that seems like common sense but is incorrect -- until Galileo figured out the real answer in the 1600s.
  4. Oct 16, 2012 #3
    space is a fun place to do physics because we are allowed to make up situations that we normally couldn't do on earth.

    So lets say you are going in your shuttle off earth into space and at t=20 your engine turns off, and you get past earth's gravity, you are in free floating space. you could then just keep on going that same speed for ever and ever until you bumped into something, or near something real big. (I think we have satelights pointed at us sending information that use no fuel to keep on going. Because there is no need)

    Thinking it would slow down is common sense nowadaysz because if your rolling down the street on a scooter eventually your going to slow down. There are so many different frictions to think about when on a scooter that we take for granted. The fact that there is air that is trying to get around us slows us down, plus friction between the wheel and the ground, and then the wheel bearing.
  5. Oct 16, 2012 #4


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    yup it sure is we can only sorta create the conditions of Newtons first law of every action has an equal and opposite reaction

    At NASA they use smooth floors and have air cusioned units that will glide over the surface with a minimal amount of friction its a very good approximation :)

    very difficult to get away from the effects of Earth's gravity. You much be a VERY long way away. Remember that even in orbit satellites, shuttle and its astronauts are in a constant state of freefall due to gravity

    Satellites do have small rocket thrusters ( usually just gas exhausts) these are there to periodically boost the satellite's orbit. Most satellites (in lower earth orbit regions) are not that far above the Earth's atmosphere and the small amount of resistance from that and loss of orbital velocity means they do slowly loose orbital height and need the occassional boost to get them back in position.

    The life of the satellite comes to an end when there is no more propellant on the satellite to do the boosts in orbit and then they are on that inevidable spiral back into the atmosphere and burn up

  6. Oct 17, 2012 #5
    When you stop accelerating an object, it will continue to move at the velocity it had before.
    For example, say I had a rock and attached a string to it, then went and spun the rock with the string (Think David and Goliath),after a while the string snapped and ceased to pull on the rock, the rock will fly out tangent to the circular trajectory it used to be on.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook