# Thrust and Kinetic Friction

1. Nov 8, 2007

### kraaaaamos

1. The problem statement, all variables and given/known data
Sam, whose mass is 75 kg, takes off across level snow on his jet-powered skis. The skis have a thrust of 200N and a coefficient of kinetic friction on snow of 0.10. Unfortunately, the skis run out of fuel after only 10 s.

a. What is Sam’s top speed?
b. How far has Sam traveled when he finally coasts to a stop?

2. Relevant equations

3. The attempt at a solution

Fnet = F(thrust) - Fk
where Fk = (coeff)mg
= (0.1)(75)(9.8)
= 73.5N
plug in to Fnet = F (thrust) - Fk
= 200 - 73.5
= 126.5N

To find acceleration:
Fnet = ma
126.5 = 75 a
a = 126.5/75
a = 1.69 m/s2

To find top speed:
Vf = Vi + a (deltaT)
Vf = 0 + 1.69(10)
= 16.9m/s

to find b... i have no idea.
i knwo that:
vi would be 16.9m/s
vf would be 0m/s
i dont know what the decelleration value is for when it is coasting
and i knwo that i use the equation: Vf^2 = Vi^2 + 2a (deltaX)
wehre delta (X) is change in position (displacment)

someone help!

Last edited: Nov 8, 2007
2. Nov 8, 2007

### rl.bhat

Now the frictional force is the retarding force. From that find the decelleration.

3. Nov 8, 2007

### physicsbhelp

In February 1955, a paratrooper fell 370 m from an airplane without being able to open his chute but happened to land in snow, suffering only minor injuries. Assume that his speed at impact was 58 m/s (terminal speed), that his mass (including gear) was 85 kg, and that the force on him from the snow was at the survivable limit of 1.2 105 N.

(a) What is the minimum depth of snow that would have stopped him safely?

?m

(b) What is the magnitude of the impulse on him from the snow?

? kg·m/s