Homework Help: Newton's Second law: Tension on Cable

1. Feb 27, 2017

aatari

Hey guys is my solution correct for the question below?

1. The problem statement, all variables and given/known data

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.

2. Relevant equations
Fcable = m.a

3. The attempt at a solution
Fcable = m.a
= 5.0 kg(2.0)
= 10N

2. Feb 27, 2017

kuruman

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

3. Feb 27, 2017

aatari

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

4. Feb 27, 2017

aatari

Or in this situation should it be Fcable +Fgravity

5. Feb 27, 2017

kuruman

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

6. Feb 27, 2017

aatari

Fcable = 10 N
Fgravity = -49N

Fcable + Fgravity = 10N - 49N
= 39 N [down]

Is this correst?

7. Feb 27, 2017

kuruman

How do you figure Fcable = 10 N? It seems that you multiplied the acceleration by the mass and called that Fcable. 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 Fcable.

8. Feb 27, 2017

aatari

I think I get it now.

So Fnet = Fcable + Fgravity
10N = Fcable - 49N
10N + 49N = Fcable
Fcable = 59N

9. Feb 27, 2017

kuruman

Yes, you got it.

10. Feb 27, 2017