This boulder sits on an inclined rock face near Mamalapuram, India. When you stand by the boulder, it seems as if even a small shove would send it tumbling down the hill. So why doesn't the boulder slide or roll downwards?
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This boulder sits on an inclined rock face near Mamalapuram, India. When you stand by the boulder, it seems as if even a small shove would send it tumbling down the hill. So why doesn't the boulder slide or roll downwards? |
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Why doesn't the boulder slide? The picture at right shows all the forces acting on the boulder. Our x-y axis has been set according to the angle of the incline (ø), so the force of gravity (FG = mass x gravity) pulling the boulder straight down has to be broken into x and y components of mgsinø and mgcosø, respectively. In order for the rock to slide, FG in the positive x direction must overcome Ff in the negative x direction. Since the rock does not move, we must assume that the magnitude of Ff equals the magnitude of mgsinø. |
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Why doesn't the boulder roll?
Rolling motion is different from sliding motion, and it is much more complex. However, we can see from the shape of the rock that the center of mass of the rock is located above the portion of the rock contacting the ground (the pivot point). As long as that center-of-mass point does not pass the pivot point, then there is not enough force (torque) on the rock to cause is to turn.
© 2004 Ana Marx