Conceptual Questions

5.1 Friction


Define normal force. What is its relationship to friction when friction behaves simply?


The glue on a piece of tape can exert forces. Can these forces be a type of simple friction? Explain, considering especially that tape can stick to vertical walls and even to ceilings.


When you learn to drive, you discover that you need to let up slightly on the brake pedal as you come to a stop, or the car will stop with a jerk. Explain this in terms of the relationship between static and kinetic friction.


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.

5.2 Drag Forces


Athletes such as swimmers and bicyclists wear bodysuits in competition. Formulate a list of pros and cons of such suits.


Two expressions were used for the drag force experienced by a moving object in a liquid. One depended upon the speed, while the other was proportional to the square of the speed. In which types of motion would each of these expressions be more applicable than the other one?


As cars travel, oil and gasoline leaks onto the road surface. If a light rain falls, what does this do to the control of the car? Does a heavy rain make any difference?


Why can a squirrel jump from a tree branch to the ground and run away undamaged, while a human could break a bone in such a fall?

5.3 Elasticity: Stress and Strain


The elastic properties of the arteries are essential for blood flow. Explain the importance of this in terms of the characteristics of the flow of blood (pulsating or continuous).


What are you feeling when you feel your pulse? Measure your pulse rate for 10 seconds and for 1 minute. Is there a factor of 6 difference?


Examine different types of shoes, including sports shoes and thongs. In terms of physics, why are the bottom surfaces designed as they are? What differences will dry and wet conditions make for these surfaces?


Would you expect your height to be different depending upon the time of day? Why or why not?


Why can a squirrel jump from a tree branch to the ground and run away undamaged, while a human could break a bone in such a fall?


Explain why pregnant women often suffer from back strain late in their pregnancy.


An old carpenter's trick to keep nails from bending when they are pounded into hard materials is to grip the center of the nail firmly with pliers. Why does this help?


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.