let's look back again now the focal pendulum dropped slammed as its wings i think you can see the paint line where the planned trail began the amplitude upswing is decreasing the famed trail is not long now but the important thing to speak is that the plane upswing has been rotating during the half hour that we've been talking to you an inertial frame of reference is one in which the alarm inner cities without all frames of reference moving at a constant velocity with respect to an inertial frame are also inertial frame we use the word as an inertial frame of reference but it is only approximately one a small acceleration with respect to the stars for example the frame of reference other stars is the best we can do when we look for a frame of reference which is for all practical purposes bextra accelerated frame of reference is not a member shall frame and when we are in an accelerated frame we have to introduce forces which we called fictitious forces in order that the law of inertia and the other laws of physics don't change