1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Information regarding acceleration

  1. May 12, 2015 #1


    User Avatar

    Hello guys,

    The units for acceleration is ms-2, does this mean that for every second the speed is increasing by x ms-1?

    Also can acceleration be used for speed and velocity since speed is distance/time while velocity is displacement/time?

  2. jcsd
  3. May 12, 2015 #2


    User Avatar
    Science Advisor

    If the acceleration is in the same direction as the velocity, yes. If the acceleration were in the opposite direction then speed would instead be decreasing by x meters/second every second. If the acceleration were in the perpendicular direction then speed would not be changing at all.

    The unit for acceleration are the same as the unit for the rate of change of speed over time. So yes it is perfectly acceptable to use units of acceleration when talking about a rate of change of speed. The term "tangential acceleration" is often used to make it more explicit.
  4. May 13, 2015 #3


    User Avatar

    Hello, thanks for replying,

    What do you mean by direction? Is it the graph?

    Also my understanding of velocity is displacement/time, is this correct?

  5. May 13, 2015 #4
    Velocity is a vector (a quantity with both magnitude and direction), speed is just a magnitude regardeless of direction of propogation. So if you're traveling "forward" with a speed ##y\frac{m}{s}##, and there is an acceleration backwards with magnitude ##x\frac{m}{s^2}## then every second, you will have a change in velocity of ##x\frac{m}{s}## backwards, which means a total change in velocity of ##(y-tx)\frac{m}{s}##, where t is time. Hope this helps.
  6. May 13, 2015 #5


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    If you are travelling in a straight line in one direction only, then essentially speed is the same as velocity. But, there are two cases when you must be careful not to mix the two up:

    a) If you travel in a straight line both forwards and backwards, then you have to choose which direction is +ve and which is -ve. Speed is then the magnitude of velocity, which can be positive or negative. If you choose moving to the right as positive, then you would have a velocity of, say, +5m/s if you are moving to the right and -5m/s if you are moving to the left. In both cases the speed is 5m/s.

    b) If you are moving in a curve (a circle, for example), then you may be changing both your speed and direction. For example, if you are moving in a circle at constant speed, then you are accelerating. This is because you are constantly changing direction. And, to move in a circle, you need a force to be constantly pushing you inwards. So, in fact, the acceleration (which is also a vector) is directed towards the centre of the circle.

    In fact, for any motion, you need to be thinking in terms of velocity, with speed being the magnitude of the velocity. And acceleration being the rate of change of velocity; never the rate of change of speed.

    Finally, velocity is the "change in displacement/time", which is the "rate of change of displacement". Think of the circular motion again: the change in displacement is around the edge of the circle.
    Last edited: May 13, 2015
  7. May 14, 2015 #6


    User Avatar

    Ok thanks everyone replying and helping me out. I think I get it.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook