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2

Objects in orbit come pretty close. If you don't mind venting the cabin or taking a walk outside, even air drag can be very nearly eliminated. All you have left are very small forces due to being in a non-inertial reference frame, and drag from the very, very thin atmosphere. Neither would be noticed without some very precise equipment. To reduce these ...


1

Have you heard of superfluidity? It happens when you cool liquid helium below about 2 Kelvin. The helium then will flow freely and without any friction. If you induce a current vortex in liquid helium, it will remain flowing until the end of time (however, you cannot draw energy from it, because the liquid is frictionless) or until it warms up again. So, in ...


3

(Temporarily pretend you are on the second floor of a building.) Jump up and down in one place. After you leave the floor: your displacement is always positive (i.e., above the floor); your velocity is positive (rising), zero (at your highest displacement), then negative (falling back to the floor); and your acceleration is constant and negative -- it's ...


11

Get on a car, turn it on and press the accelerator! (${{a}}>0$)... then press the brake (${{a}}<0$). Until the car is steady, the situation is as you described.


6

Of course. Acceleration is the rate of change of the velocity. For motion in a line, if the object is slowing down, the acceleration is opposite the velocity. If the object is speeding up, the acceleration is in the direction of the velocity. Imagine you're pedalling a bike, gaining more and more speed and then, suddenly, stop pedalling and apply the ...


5

Absolutely. Acceleration is the change in velocity, so when you say that the acceleration reverses in direction, it means that the object is transferring from either speeding up to slowing down, like a skateboard which just passed the bottom of a U-shaped ramp, or from slowing down to speeding up. Think of the skateboard as having just crested a hill. ...


30

A pendulum is a day to day example of this. If you watch a pendulum swinging from left to right as it passes the mid point the velocity and acceleration are: The acceleration always point towards the mid point, so as the pendulum passes through the mid point the acceleration reverses direction but the velocity does not.



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