When you push a piece of chalk across a chalkboard, it sometimes screeches because it rapidly alternates between slipping and sticking to the board. Describe this process in more detail, in particular explaining how it is related to the fact that kinetic friction is less than static friction. (The same slip-grab process occurs when tires screech on pavement.)
Kinetic friction acts when there is a relative motion between the surfaces. Static friction works if there is tendency of motion but no relative motion between the surfaces.
Kinetic friction acts when we are writing on the board while static friction works if we stop writing.
Kinetic friction is the friction that occurs when two or more things are moving in opposite directions.
Chemical bonding occurs between both the surface and the chalk when you push a piece of chalk throughout a chalkboard.
This bonding is responsible for static friction as well as kinetic friction. The friction force depends upon the nature of surface. Normally the coefficient of kinetic friction is less than static friction. The friction force is determined by the type of surface. Kinetic friction normally has a lower coefficient than static friction.
Therefore, value of kinetic friction is less the value of static friction.
While writing on the chalkboard there is a relative motion between the surfaces so kinetic friction works and lesser force is required to move the chalk.
But when the chalk sticking on the board static friction acts between the surfaces and it oppose relative motion so you require slightly greater force to write on the board.
When a glass bottle full of vinegar warms up, both the vinegar and the glass expand, but vinegar expands significantly more with temperature than glass. The bottle will break if it was filled to its tightly capped lid. Explain why, and also explain how a pocket of air above the vinegar would prevent the break. (This is the function of the air above liquids in glass containers.)
Calculate the deceleration of a snow boarder going up a , slope assuming the coefficient of friction for waxed wood on wet snow. The result of Exercise may be useful, but be careful to consider the fact that the snow boarder is going uphill. Explicitly show how you follow the steps in Problem-Solving Strategies.
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