Is anyone here familiar with the program masterphysics?

[Crusaderking1]

Homework Statement

Ok, so masteringphysics has rejected an answer I think is right. The question is set up with "insert value" and then "insert units"

Heres the question.

A 5.00 kg crate is suspended from the end of a short vertical rope of negligible mass. An upward force F(t) is applied to the end of the rope, and the height of the crate above its initial position is given by y(t) = (2.80 m/s)t +(0.61 m/s^3)t^3

What is the magnitude of the force F when 3.60 s?

F= (value)(units)

I inserted 65.9 and N

It rejected it.

I tried 65.9 and kg*m/s^2

Rejected again.

I have one try left, and I don't understand what is wrong. Thanks.

Homework Equations

The Attempt at a Solution

 

[bbbeard]

I inserted 65.9 and N

Maybe you should put in all the decimal places (65.88), not just the significant digits.

 

[Crusaderking1]

Maybe you should put in all the decimal places (65.88), not just the significant digits.

Usually if my answer is to the wrong amount of digits, it will tell me in the feedback.

 

[Liquidxlax]

Lol i had that same question 3 years ago, sorry i can't remember what i entered, but i know the answer is 65.99N. I'd say talk to you prof/teacher before you use up all your chances. That is the major flaw in those internet based assignments like masteringphyics and masteringchemsitry.

 

[Crusaderking1]

Lol i had that same question 3 years ago, sorry i can't remember what i entered, but i know the answer is 65.99N. I'd say talk to you prof/teacher before you use up all your chances. That is the major flaw in those internet based assignments like masteringphyics and masteringchemsitry.

Haha, 3 years and they still didn't fix it. Alright, I will send my prof. an e-mail.

Its a very frustrating flaw.

 

[Doc Al]

Realize that you are asked to find the force F, which is not the net force.

 

[Crusaderking1]

Realize that you are asked to find the force F, which is not the net force.

Do I just plug 3.66 into the original equation for the answer of 40.2 N?

 

[Doc Al]

Do I just plug 3.66 into the original equation for the answer of 40.2 N?

No.

Draw a Free Body Diagram and set up Newton's 2nd law. What's the acceleration at the specified time?

 

[Crusaderking1]

No.

Draw a Free Body Diagram and set up Newton's 2nd law. What's the acceleration at the specified time?

F = ma

acceleration is 13.176 m/s^2 (3.66*3.60)

m = 5.00

F = 65.9 N?

 

[Doc Al]

F = ma

That should be ƩF = ma.

What forces act on the crate? The applied force 'F' is just one of the forces acting.

acceleration is 13.176 m/s^2

Good.

m = 5.00

F = 65.8 N?

No, that's the net force.

 

[Crusaderking1]

That should be ƩF = ma.

What forces act on the crate? The applied force 'F' is just one of the forces acting.


Good.


No, that's the net force.

Would tension be a factor?

 

[Doc Al]

Would tension be a factor?

The applied force 'F' is the tension. Recall that it's "applied to the end of the rope".

What other force acts?

 

[Crusaderking1]

The applied force 'F' is the tension. Recall that it's "applied to the end of the rope".

What other force acts?

Weight(49 N)
Normal force
65.88 N - 49 N = 16.88 N?

 

[Doc Al]

Weight(49 N)

Right.

65.88 N - 49 N = 16.88 N?

No.

Do it systematically:

What forces act? (One of them will be the force 'F' which you're solving for.)
Write an expression for the net force.
Set the net force equal to 'ma'.
Solve for the applied force F.

 

[Crusaderking1]

Right.

No.

Do it systematically:

What forces act? (One of them will be the force 'F' which you're solving for.)
Write an expression for the net force.
Set the net force equal to 'ma'.
Solve for the applied force F.

Ok I will try to figure this out. Thanks!

 

[Doc Al]

Ok I will try to figure this out. Thanks!

You're almost there. You just need to organize your approach. Don't give up!

 

[Crusaderking1]

T-W=ma
T-49 N = 65.88 N
T = Fapplied = 114.88 N

 

[Doc Al]

T-W=ma
T-49 N = 65.88 N
T = Fapplied = 114.88 N

Perfecto!

 

[Crusaderking1]

Perfecto!

I don't think you know how much you have helped me. Extremely helpful! Thanks.