Force does not tell us whether something is moving, it tells how the motion of an object is changing. The friction force being greater tells us that the motion of the box is slowing down. If the friction force was lesser that would tell us the box is speeding up. If the friction force was equal that would tell us the box is moving with constant velocity. Hope this helps!
Yes acceleration due to gravity is 9.8 m/s/s. However, this video uses 10 m/s/s for g to make things a little easier. Some physics classes use 9.8 but the AP Physics tests use 10 which is why I use 10 in this video.
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Glad it was helpful!
what if the applied force isn't given?
But if the friction force is greater than the horizontal applied force, shouldn’t it not move at all?
Force does not tell us whether something is moving, it tells how the motion of an object is changing. The friction force being greater tells us that the motion of the box is slowing down. If the friction force was lesser that would tell us the box is speeding up. If the friction force was equal that would tell us the box is moving with constant velocity. Hope this helps!
In the second problem, in find the normal force, where did the 10 come from? isn't gravity=9.8?
Yes acceleration due to gravity is 9.8 m/s/s. However, this video uses 10 m/s/s for g to make things a little easier. Some physics classes use 9.8 but the AP Physics tests use 10 which is why I use 10 in this video.
Dynamics 2 unit 3 review question