Question

#1. A water skier is pulled by a boat traveling with a constant velocity. Which one of the following statements is false concerning this situation?

A) The water skier is in equilibrium. ; B) The net acceleration of the skier is zero [m/s]. ; C) The net force on the skier is zero [N]. ; D) There is a net horizontal force on the skier in the direction the boat’s velocity. ; E) The net vertical force on the skier is zero [N].

#2.

A marble column of cross-sectional area 1.4 [m2] supports a mass of 25000 [kg]. What is the strain within the column? Young’s modulus for marble is 50 × 10^9 [N/m2].

A) less than 0.001 ; B) from 0.001 to 0.1 ; C) from 0.11 to 1 ; D) from 1 to 100 ; E) greater than 100

Answer 1

#1) The false statement is

D) There is a net horizontal force on the skier in the direction the boat’s velocity.

Explanation:

This is because, according to Newton's First law of motion, when an object which in this case is the skier tends to be in equilibrium because it travels at a constant velocity, there is no net acceleration of the skier, and this causes the net force on the skier to be equal to zero.

#2) Option A) less than 0.001

Stress = F/A  = (25000 x 9.8)/1.4 = 1.75 x 10^5 N/m^2

Strain = Stress/Young's modulus

Strain = 1.75 x 10^5/5 x 10^10  = 3.5 x 10^-6