# Homework Help: Newtonian - Force, Gravity, and Bungee Jumping

1. Oct 7, 2009

### staycrunchy

1. The problem statement, all variables and given/known data
A person weighing 55kg jumps off of a bridge with a bungee cord attached. The problem seeks to find the bungee cords Force vector when the acceleration is 7.8m/s2 in the downward direction.

m = 55kg
a1 = g = 9.8m/s2

To find:
F1 when a2 = 7.8m/s2 downward.

I've specified my grid as $$\uparrow$$+ $$\rightarrow$$+ so the accelerations are technically negative.
2. Relevant equations
$$\Sigma$$F = ma

3. The attempt at a solution
$$\Sigma$$F = ma
$$\Sigma$$F=55kg(-7.8m/s2)
$$\Sigma$$F = -418N

$$\Sigma$$F = F1 - F2
$$\Sigma$$F = 539N - 418N
Force of bungee cord acting = +121N

2. Oct 7, 2009

### semc

Why did you subtract the forces?

3. Oct 7, 2009

### staycrunchy

The 539N was the Force with F=ma and a = g. I think I forgot to write that.
The -428N was the Force with F=ma and a = 7.8m/s2.

My inclination was to subtract the forces to get the force required by the bungee cord to slow the decent to the 7.8m/s2

4. Oct 7, 2009

### semc

Well since gravitational force is downwards and he is travelling downwards.......