Maths - mechanics

Lebkuchen

sounds silly but what does it mean for an object to be at the point of moving in a practical sense?

Reply 1

Lebkuchen

another question is how can F max take both the limiting and maximum value?

Reply 2

happygirl02

Original post by Lebkuchen

sounds silly but what does it mean for an object to be at the point of moving in a practical sense?

if the object is about the point of moving, it means it is just about to move. any force that acts on it will cause it to move. you can kind of think of it as being in equilibrium currently, so unmoving, but the second the forces are unbalanced, it will no longer be at a point of moving. hope this helps.

Reply 3

mqb2766

Original post by Lebkuchen

sounds silly but what does it mean for an object to be at the point of moving in a practical sense?

Similar to the previous replay, it makes sense if you consider a resistive force acting on a block. So imagine a block on a flat, initially horizontal, rough plane. Its in equilibrium and the resistive force is zero. As you slowly start to rotate the plane so that it makes an angle theta with the horizontal plane, the resistive force (up the plane) will increase to counter the resolved gravitational force acting down the plane. The object will remain in equilbrium until the plane is rotated such that the frictional force reaches its limiting (max) value. At that point, the block is on the point of moving as any further rotation increases the gravitational force down the plane but the frictional force is unable to increase to stop it moving, so there is a net force acting down the plane which causes an acceleration.

The key thing is to think of friction like a sponge which can absorb/oppose other forces acting on an object until it hits a maximum value (ignoring sign/direction).