Finding tension in a pendulum string at the bottom of the swing

[JJones_86]

Homework Statement

A pendulum is 0.8 m long and the bob has a mass of 1.0 kg. At the bottom of its swing, the bob's speed is 1.3 m/s. What is the tension in the string at the bottom of the swing?

Homework Equations

F=m((v^2)/r)

The Attempt at a Solution

F= 1.0kg((1.3 m/s^2)/.08)
F= 2.1125

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I must be using the wrong equation or numbers.. Any help is appreciated.

 

[Gear300]

The velocity is squared so your force would be F= 1.0kg((1.3 m/s)^2/.08)

 

[JJones_86]

Oh, i meant 21.125, but I am still getting the wrong answer.

 

[Gear300]

I'm getting F= 2.1125N...maybe its trying to keep track of significant figures...in that case I'm thinking it would be F = 2N

 

[JJones_86]

Yeah, I've tried all those answers, am I even using the right equation? Would the radius really be 0.8 m since the string is 0.8 m and not perpendicular to the Earth's gravitational pull?

 

[Gear300]

yup...the problem says the string is .8 m. Looks like I missed something too. The centripetal force is 2N, which is the force of the object's "inertia" on the string. At the bottom the object's weight also contributes to the tension...so you'll have to add the two.

 

[JJones_86]

So it would be 11.9225N?

 

[Mattowander]

The centripetal force will be the Tension minus the weight. You should then be able to solve for the tension.