A 90 kg woman jumps from a ledge that is 1.4 m above concrete and prepares to become internet famous.

Question

(a) What is her speed just before her feet strike the pavement?

(b) The only cushion for her feet is the 0.50 cm in the pads of her feet. So, she displaces only 0.50 cm during the landing. Calculate the average force the ground exerts on her in this situation.

Answer 1

Hello Charlotte;

Although it is not a great way of becoming famous, still, let's try to understand what this lady wants to do and tell her about the dire consequences of what might await her:

1.4m is going to be used to determine the speed of impact. (when she reaches ground/fame)

a) V_final ² = V_initial ² + 2gh (just using speed with no direction/signs)

V_final ² = 0 + 2 x 9.8 x 1.4 ==> V_final ≅ 5.2 m/s

b) So, she will slow down to zero speed padded by only 0.5cm thick cushion. Let's see how much force is involved during the infamous injurry although she gets famous online putting her knees on the line.

V_final ² = V_initial ² + 2gh

0 = 5.2² + 2 x a x 0.005m (Average (de)acceleration with the cushion)

a = -2704 m/s² (Prettty nasty deceleration rate, of course a rough average)

Now, let's see what Mr Newton will tell us about the force with his 2^nd law.

Fingers crossed, but her mass is dangerous. Why?

F = ma (we have both a when slowing down and mass, we wish she lost some mass during the fall :-))

F = 90 x (-2704)

F = -243,360N (Orthopedist recommended)