Newton's Second law: Tension on Cable

[aatari]

Hey guys is my solution correct for the question below?

1. Homework Statement
Consider a 5.0 kg watermelon that is being accelerated at 2.0 m/s2 [up] by a cable. Find the tension in the cable.

Homework Equations

F_cable = m.a

The Attempt at a Solution

F_cable = m.a
= 5.0 kg(2.0)
= 10N

 

[kuruman]

Your solution is incorrect. Newton's second law says F_net = ma. F_net is the sum of all the forces acting on the watermelon, not just the force from the cable.

 

[aatari]

Your solution is incorrect. Newton's second law says F_net = ma. F_net is the sum of all the forces acting on the watermelon, not just the force from the cable.

Ok I am confused now. I thought F_net = m.a

 

[aatari]

Or in this situation should it be F_cable +F_gravity

 

[kuruman]

Or in this situation should it be Fcable +Fgravity

Yes, it should be as you say. Can you finish the problem now?

 

[aatari]

Yes, it should be as you say. Can you finish the problem now?

F_cable = 10 N
F_gravity = -49N

F_cable + F_gravity = 10N - 49N
= 39 N [down]

Is this correst?

 

[kuruman]

How do you figure F_cable = 10 N? It seems that you multiplied the acceleration by the mass and called that F_cable. We agreed that mass times acceleration is the net force. Write an expression for the net force as the sum of all the forces, set it equal to mass times acceleration and solve for F_cable.

 

[aatari]

How do you figure F_cable = 10 N? It seems that you multiplied the acceleration by the mass and called that F_cable. We agreed that mass times acceleration is the net force. Write an expression for the net force as the sum of all the forces, set it equal to mass times acceleration and solve for F_cable.

I think I get it now.

So F_net = F_cable + F_gravity
10N = F_cable - 49N
10N + 49N = Fcable
F_cable = 59N

 

[kuruman]

Yes, you got it.

 

[aatari]

Yes, you got it.

Thanks for your help.