Physics Help!

[Lefty RC]

Hey
I have a 50 question problem set for physics and I don't no how to do 2 of them.
If someone could help me out that would be great

1. A person weighing 0.8kN rides an elevator that has a downward acceleration of 1.5m/s/s. The acceleration of gravity is 9.8 m/s/s. What is the magnitude of the force of the elevator floor on the person.
Answer in units of kN.

2. Figured out Number 2

Any type of help would be awesome!
Thanks

 

[Koot]

Don't take my advice as truth because my physics is a bit rusty.

For 1, you would first find the mass of the person then since the accelerations are in opposite directions you subtract and find the force again using the new acceleration.

For 2 it's pretty much the same thing, but the accelerations are in the same direction.

Again, I haven't taken physics in 2 years so I have no idea if what I did is correct. Someone can verify or something.

 

[Physics213]

I'd suggest drawing a free body diagram for the elevator problem, they REALLY help!

 

[Koot]

With a name like Physics, I assumed you would have had a definite answer.

 

[Physics213]

With a name like Physics, I assumed you would have had a definite answer.

How would giving him/her a direct answer help them? Leading them in the right direction will help them more often I believe!

 

[Koot]

That's true but I assumed that free body diagrams the first step people take to solve these problems.

Looks like OP is doing fine.

 

[Kanchu]

The force acting on a mass is defined as its mass times the acceleration acting on the object. With the mass being the elevator + the person inside.

Now with that in the back of your mind, can you calculate the mass?
You know the force that is acting on the mass and you know its acceleration.

When you've calculated the mass, you need to calculate the "upward force".
What are the two forces acting on the object? There is always one clear force acting on a mass. The other force is the "upward" force.

Now the upward force is unknown, however you do know the resultant force. What is the acceleration of the mass where this resultant force is acted upon or what is the sum of the forces?

If you know the resultant force and you know the .... force that is acted on the object + the upward force is equal to the resultant force, surely this must not give any more problems.

I uploaded my own answer plus calculations if you need to check.
http://img16.imageshack.us/img16/3261/img005fk.jpg

Please note that I calculated the "upward force" , that is how I interpreted the question. and.....Can I haz vouche hehe (see threads started (A))

 

[DtheK]

1. A person weighing 0.8kN rides an elevator that has a downward acceleration of 1.5m/s/s. The acceleration of gravity is 9.8 m/s/s. What is the magnitude of the force of the elevator floor on the person.
Answer in units of kN.

1. Draw the FBD of the person. There is the Normal Force of the elevator on the person going up and the Force = to mg going down.

F = ma
F_N-mg=ma
F_N=ma+mg=(800N/(9.8m/s^2))(-1.5m/s^2)+800N
F_N = 677.6N = .6776kN

The force acting on a mass is defined as its mass times the acceleration acting on the object. With the mass being the elevator + the person inside.

I'm not really sure that the mass of the elevator is necessary because both the person and the elevator are accelerating at the same rate and you are only looking for the normal force