Finding T in an Elevator Moving with Constant Velocity

[Effitol840]

Im having trouble with a part of a problem. The problem is:

There is an elevator with a mass of 1200$$kg$$ accelrating upward at 2.1$$m/s^2$$. Find T.

For that what I did was $$1200 \times 9.8m/s^2$$ which gave me 11,760. I then plugged this into $$F=ma$$ and got 14,280.

The second part of the equation says:

The elevator now moves with a constant upward velocity of 10$$m/s$$. Find T.

This is where I get stuck. I don't know how to factor in velocity.

 

[stunner5000pt]

what you've done is plug mg = ma, that isn't right
the gravity is not pulling the elevator down, the elevator is moving upward
that means thhere is some force being exerted by the tension in the cable

taking the upward direction to be postiive
T - mg = ma = Net Force
T = ma + mg

When there is constnat velocity what is the acceleration? So what is the tension using the above equation?

 

[zwtipp05]

Velocity doesn't affect the force. What you want to think about is acceleration. If it's at a constant velocity, what does that tell you about the net force?

 

[stunner5000pt]

my mistake, your answer is correct.. its just the way you said it sounded like you did something wrong. The first part is correct, follow the advice of finding the acceleration in second part to figure out the tension

 

[hotvette]

Here is the approach I would take:

1. Construct free body diagram of the elevator. Cable tension T is up and elevator weight mg is down

2. Write the equation of motion $\Sigma F = m a$

3. Case 1: a = fixed value

4. Case 2: if velocity is constant, what can you say about acceleration?

 

[Effitol840]

ok so if there is a constant velocity there is no acceleration? Which would just make my answer 11760?

 

[Doc Al]

There is an elevator with a mass of 1200$$kg$$ accelrating upward at 2.1$$m/s^2$$. Find T.

I assume that T is the tension in the cable that pulls the elevator up.

For that what I did was $$1200 \times 9.8m/s^2$$ which gave me 11,760. I then plugged this into $$F=ma$$ and got 14,280.

To apply F = ma properly, you need to use the net force on the elevator. There are two forces acting on the elevator: The cable tension pulling up and the weight (mg) pulling down. Find the net upward force (in symbols) and set that equal to ma. Then you can solve for T.

The second part of the equation says:

The elevator now moves with a constant upward velocity of 10$$m/s$$. Find T.

Hint: If the elevator moves at a constant velocity, what must be the net force on it?

Looks like several others beat me to it!

 

[hotvette]

As long as your units are consistent, I'd say yes.

 

[Effitol840]

Yea the answer was 11760. Thanks for the help.