Question

The Achilles tendon connects the muscles in your calf to the back of your foot. When you are sprinting, your Achilles tendon alternately stretches, as you bring your weight down onto your forward foot, and contracts to push you off the ground. A 70 kg runner has an Achilles tendon that is 15 cm long and has a cross-section area of 110 mm² typical values for a person of this size. 1. By how much will the runner's Achilles tendon stretch if the force on it is 8.0 times his weight? Young's modulus for tendor is 0.15 x 10¹⁰N/m². Express your answer to two significant figures and include the appropriate units. 2. What fraction of the tendon's length does this correspond.

Answer 1

A) 0.5cm B) 1/30

Step-by-step explanation:

The weight of the man = mass * acceleration due to gravity where the mass is 78kg and acceleration due to gravity is 9.81m/s^2

W = m * g = 78 * 9.81= 686.7 N

The force acting on the tendon is 8 times of the weight

Force = 8 * weight of the body = 8 * 686.7 = 5493.6 N

Young modulus of the tendon(e) = (F/A)/ (DL/L) where A is the cross sectional area in square meters, DL is the change in length of the tendon in meters and L is the original length of the tendon

e = (FL)/(ADL) cross multiply and make DL subject of the formula

DL = (FL) / (AL)

Convert the cross sectional area A into square meters and the length also

A = 110 / 1000000 since 1/1000 m = 1mm, 1/1000000 m^2 = 1 mm^2 and 1/100m = 1 cm

A = 0.00011 m ^2 and L = 0.15m

Substitute the values in the derived equation

DL = (5493.6 * 0.15)/ (1.5 * 10^ 9 * 1.1* 10^-4)

DL = 824.04 / 1.65 * 10^ 5

DL = 499.42 * 10^-5 = 499.42 *10^ -5 / 100 to convert it to meters

DL = 0.49942cm approx 0.5cm

B) fraction of the DL to L = 0.5 / 15 = 1/30