1:[[I release the object and let it fall]
I release the object and let it fall,and it moved backward,I wanna know if there's a force make it move?
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Whenever something changes its velocity, there is a force responsible for this change. I reckon you know that the force is due to gravity. You need to decompose this force ... – Semoi Jul 29 '23 at 14:28
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What does your red arrow mean, is it the direction the thing moved? – trula Jul 29 '23 at 14:38
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This might help, though it is for a slightly different problem - Toppling of a cylinder on a block – mmesser314 Jul 29 '23 at 15:30
2 Answers
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There are three forces acting on the object.
Its weight $Mg$ acting at its centre of mass $C$, a normal force, $N$, and a frictional force $f$ due to bottom sheet of paper.
Suppose that the frictional force is zero, $f=0$ then there are no horizontal forces acting on the object thus the centre of mass does not move horizontally and only accelerates downwards with an acceleration $a$ given by $M\,g-N=M\,a$.
There is a counterclockwise torque about $C$ which causes the object to rotate counterclockwise so the end $B$ moves to the right.
With friction the centre of mass moves to the left but if that force is sufficiently small the leftward movement of the centre of mass will be smaller than the rightward movement of end $B$.
In general, eg with a 30 cm ruler on a bench, it hard to show the effect but with a metre rule on a shiny floor the effect is quite pronounced as you can see below.
Farcher
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The card has to rotate to fall, because one end of it is supported by the table. If the force of static friction between the card and the table is insufficient to accelerate the center of mass of the card to the left, then the counterclockwise rotation of the card will make the bottom end of the card go to the right and up relative to the center of the card, while the top end of the card goes to the left and down. Since the center of the card is going down, "right and up" of it ends up being just to the right, while "left and down" ends up being left and faster-down.
On a frictionless table, the center of mass of the card would fall straight down while the card rotated around it. In real life the center of mass of the card will still move somewhat to the left as it falls and rotates because the force of kinetic friction between the moving point of contact and the table points left, opposite the direction of movement of the point of contact.
g s
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