# Spring balance force problem

1. Apr 8, 2008

### lsufan1988

ok so i am completely lost on all this force stuff....help....please

An object is hung from a spring balance attached to the ceiling of an elevator cab. The balance reads 65 N when the elevator is standing still.

(a) What is the reading when the elevator is moving upward with a constant speed of 7.7 m/s?

(b) What is the reading when the elevator is moving upward with a speed of 7.7 m/s while decelerating at a rate of 1.8 m/s2?

and this one as well

A firefighter with a weight of 725 N slides down a vertical pole with an acceleration of 2.92 m/s2, directed downward.

(a) What are the magnitude and direction of the vertical force (use up as the positive direction) exerted by the pole on the firefighter?

(b) What are the magnitude and direction of the vertical force (use up as the positive direction) exerted by the firefighter on the pole?

2. Apr 8, 2008

### ChillyWilly

Let me give you a hint from what I read on the post. I will not offer an asnwer or steps to the unswer because I'm a bit rusty on this subject, but Ihope this helps you.

If there was no friction then the fire figther would be freefalling at the rate of gravitational pull (gravity) he is going slower so he is exerting a force on the pole and thus the pole is also exerting a force on the firefigther.

Use your conservation equtions (force, momentum, etc.)

Again I hope this helps.

3. Apr 8, 2008

### PhanthomJay

These problems deal with Newton's 3 laws,
F_net =0,
F_net = ma, and
F_(a on b) = F_(b on a),
respectively. Please show your attempt at a solution, using these laws as applicable for the given data.

4. Apr 8, 2008

### edziura

Use F= m a

From the given info, what is the weight (W) of the object?

Then

$$\Sigma$$F = m a

For part (a), a = ?

What does this mean for $$\Sigma$$ F?

Then, what does the SB read?

Assume up is + and down is -.

For part (b), a = ? (include the appropriate sign)

What forces act ON the object?

Substitute into $$\Sigma$$F = m a and solve.

Oh; don't forget, m = W / g