Solving Johnny's Swing Height Puzzle: Can You Get 1.4 Meters?

[Speedking96]

Homework Statement

Johnny is swinging (on a swing) at a velocity of 4m/s when he is 56 cm above the ground. What is his maximum height?

Johnny is 45 Kg.

Homework Equations

Pe = mgh
Ke= (1/2)(m* v^2)
Total energy= Pe + Ke

The Attempt at a Solution

I tried setting the Ke equal to the Pe, but I get 0.81 Meters. However, I know that the correct answer is around 1.4 Meters.

I would like to know how to get 1.4 Meters.

 

[CWatters]

At 45cm he has some PE and some KE.
At the max height all of that is converted into PE.

I get 1.37m if I use g = 9.81

I suggest you show your working.

 

[Speedking96]

Okay. So, what you are saying is:

Pe: (9.81)(45)(0.56) + Ke: (1/2)(45)(4^2)

Then you plug all of that in for Pe and solve for height.

Thanks a bunch.

 

[CWatters]

Correct.