Question

A kayak is a relatively streamlined object. As a first approximation calculate the drag force under the assumption that it acts as a 4-m long flat plate and 0.55-m wide. Determine: a) The drag force when the speed is 5 m / s, b) The power required to maintain speed. c) The drag coefficient. Note: For this problem you can only use the experimental information reported to below, and assuming that the water density is 997 kg / m3 v (m/s) 0 0.5 1 1.5 2 F(N) 0 1.35 5.3 12.5 22 2.5 34

Answer 1

We will determine the average value of drag coefficient (Cd) from the experimental values using the formula

Cd = F * 2 / (Rho * v^2 * A)

Where, F - Drag force

Rho - Density of water

v - Velocity

A - Surface area of the plate

We will evaluate the value of drag coefficient for all the 5 experimental inputs (ignore v=F=0) using the above formula and then, we will take the average of those 5 values. I did those calculations using in MS Excel and found the average value of Cd as 0.005. This is the answer to part (c).

We will use the average value of Cd to find F when v = 5 m/s.

F = Cd*Rho*v^2*A/2 (Rearrange the above formula)

= 0.005*997*(5^2)*4*0.55/2

= 135.9 N (Ans (a))

Power = Force * Velocity

= 135.9 * 5

= 679.8 W (Ans (b))