Question

1. Use basic laws of physics to explain the following situations:

a. Spiderweb has a greater tensile strength than steel, yet you can snap a spider web very easily with your hand. Explain why this is so.

b. Use Bernoulli's Principle to explain how the shape of an airplane wing creates a "lift force."

c. Why is it easier to sew with a sharp needle than it is to sew with a dull one?

d. Describe how the speedometer of your car is able to tell how fast the car is going.

Answer 1

a) We know that tensile strength is maximum force we can apply per unit cross sectional area i.e F_max/A

We know that Spiderweb has a greater tensile strength than steel, yet you can snap a spider web very easily with your hand because the cross sectional area of a spider web is very small which results in small maximum force F_max as compared to steel.

b)  What actually causes the lift is introducing a particular shape into the airflow, which curves the streamlines and introduces pressure changes—lower pressure on the upper surface and higher pressure on the lower surface of the airplane wings which ultimately creates a lift force on the wings of the airplane.

c) Sharp needle has very less cross sectional area at its tip which results in very small amount of force needed to penetrate through a cloth in contrary to the dull needle which has a relatively larger surface area at the tip which will larger amount of force to generate the same amount of pressure to penetrate the same cloth.

d) The speedometer of your car counts the number of turns the wheel of your car takes in a unit time which is directly proportional to the speed of the car.