Woman standing on a scale in an elevator accelerating downward

[IceCherryPop]

Homework Statement

A 65 kg women is standing on a bathroom scale while an elevator is accelerating downwards at 2.5 m/s^s.

Relevant Equations

My teacher had told us that when FG>N to add and when FG>N to subtract.

For A, I had multiplied 65 and 9.8 since that’s the force of gravity.
For B, I again multiplied 65 and 2.5 (that’s what the scale says it reads.
*I believe you multiply them for c and d, but I’m not positive for it.
C- I think you multiply then subtract
D- I think you multiply and I’m not sure where to go from there.

 

[PhanthomJay]

Add what to what? Far better to use Newton’s 2nd law, sum of all forces acting on person equals mass times acceleration. Note that gravity acts down on her, and the scale normal force acts up on her (normal forces are pushing forces acting toward the person). Acceleration is always in the direction of the net (sum of forces) force.

 

[IceCherryPop]

Add what to what? Far better to use Newton’s 2nd law, sum of all forces acting on person equals mass times acceleration. Note that gravity acts down on her, and the scale normal force acts up on her (normal forces are pushing forces acting toward the person). Acceleration is always in the direction of the net (sum of forces) force.

So you would just add all the forces needed then multiply her mass?

 

[PhanthomJay]

F_net = ma, where F_net consists of the weight force acting down and the normal force acting up, m is given, and a is given, downward, part A . Since a is downward, the net force must be downward.

 

[Chestermiller]

Let's see your free body diagram. That will help you resolve this. (Did you think you had advanced to the point where you no longer need to draw free body diagrams?)

 

[IceCherryPop]

Let's see your free body diagram. That will help you resolve this. (Did you think you had advanced to the point where you no longer need to draw free body diagrams?)

My teacher never really taught how to draw one but I asked a friend and they showed me how. For B since both of the forces and going downward I just subtracted it then multiplied to find the force.

 

[PeroK]

My teacher never really taught how to draw one but I asked a friend and they showed me how. For B since both of the forces and going downward I just subtracted it then multiplied to find the force.

Assuming gravity is one downward force, what is the other?

What answer did you get?

 

[IceCherryPop]

Assuming gravity is one downward force, what is the other?

What answer did you get?

For B, I got 107.25. One of the forces I put was the tension force.

 

[PeroK]

For B, I got 107.25. One of the forces I put was the tension force.

What tension force? Assuming your answer is in Newtons, then that cannot be correct.

 

[jbriggs444]

For B, I got 107.25. One of the forces I put was the tension force.

The tension force from the glue on the woman's shoes where they touch the scale?

In a free body diagram, one picks a specific body and identifies all of the forces on that body. Which body did you pick? What tension force on that body are you considering?

 

[Chestermiller]

There is no excuse for your teacher not teaching you about free body diagrams. He/she should be ashamed. You will never be able to do physics without this. The proof of this is that you are struggling even with this simplest of problems. Please draw a diagram showing the woman’s body, and draw vector arrows to indicate the forces acting on her body. Label these arrows to indicate the magnitude of the forces. Show us what you get for this free body diagram.