Our textbook describes a common model for friction (both static and kinetic) in which the area of contact between interacting surfaces is irrelevant to the size of the frictional force. However, we can't help noticing how wide the tires are on drag racing cars! One explanation for the large size of the tires would be to give a larger contact patch between the tire and the track in search of more traction. But our model says contact area doesn't matter. Clarify this situation in a mini essay of 2 or 3 sentences. Sort out whether contact area matters or not. Can you think of other advantages of really wide tires -- in addition to how cool they look.

You are still fascinated by the process of inkjet printing, as described in the opening storyline for this chapter. you convince your father to take you to his manufacturing facility to see the machines that print expiration dates on eggs. you strike up a conversation with the technician operating the machine. he tells you that the ink drops are created using a piezoelectric crystal, acoustic waves, and the plateau-rayleigh instability, which creates uniform drops of mass m = 1.25 ✕ 10−8 g. while you don't understand the fancy words, you do recognize mass! the technician also tells you that the drops are charged to a controllable value of q and then projected vertically downward between parallel deflecting plates at a constant terminal speed of 20.0 m/s. the plates are ℓ = 2.15 cm long and have a uniform electric field of magnitude e = 6.40 ✕ 104 n/c between them. noting your interest in the process, the technician asks you, "if the position on the egg at which the drop is to be deposited requires that its deflection at the bottom end of the plates be 0.17 mm, what is the required charge on the drop (in c)? " you quickly get to work to find the answer. (neglect the force of gravity.)