#### Question

# I need help in this question about motion !?

A person can lift his feet 0.2 m off the ground by jumping straight up into the air on the surface of the Earth.

i)Calculate the speed with which his feet leave the ground.

ii) Find the impulse on his body given by his feet if his mass is 75 kg. What are the units of impulse?

iii) Assuming the same initial velocity as in i) , calculate how high he would jump if the person is transported on to the moon where the surface gravity is 1.6 m/s.

iv) How long would the jump last if performed on the moon's surface?

Can someone help me please?

#### Answers

i) Use the kinematic equation,

v^2 = u^2 - 2 g s

0 = u^2 - 2 g s

u = sqrt ( 2 x 9.8 x 0.2) = 1.98 m/s

Then find t from

v = u - g t

0 = u - gt

t = u / g= 1.98 / 9.8 = 0.2 s

ii) Impulse = Force x time

I = F x t

...= m g x t = 75 x 9.8 x 0.2 = 148.5 N - s

iii) 0 = u^2 - 2 g' s

s = u^2 / 2 g' s = 1.98^2 / 2 x 1.6 = 1.225 m

iv) Again ,

t' = u / g' = 1.98 / 1. 6 = 1.2375 s

The total time for the up and down motion is

2 t' = 2.475 s