# Measuring weight with weighing scale doing dumbbells

I am on a weighing scale that measures 'weight' (not mass). And I'm performing bicep curls with two 5Kg dumbbells in each hand.
The body is kept straight during the same.

I was asked to plot the reading of the weighing machine as a function of time.

Being a student I thought the normal force exerted on the scale would probably not change while doing the curls. so I drew a straight horizontal line in the graph.

This turns out not to be the correct answer. I would like to know why? And also the correct answer.

• Hint: Newton's first and third laws come into play here. The dumbbells have mass and therefore inertia. In doing curls with them you must alter the dumbbells' inertia, which requires applying a force against them. When you apply that force, there must, therefore, be a reactive force. – BillDOe Feb 6 '18 at 19:14

If your body and the dumbbells were an isolated system at rest in some inertial coordinate system, the position of the center of mass ($\vec{y}_c$) of that system wouldn't change, even if you did your curls. Some external force would be needed in order to begin to change $\vec{y}_c$, or to slow it down.