In this unit we use the example of holding a 50 lb weight in the hand to learn more about various forces acting within the body, how they are generated, and how to determine their magnitude and direction.
The elbow joint flexed to form a 60° angle between the upper arm and forearm while the hand holds a 50 lb ball . Image Credit: Openstax University Physics
The tension force in the biceps is trying to lift and rotate the forearm around the elbow joint. The force of gravity is pulling down on the ball and the forearm, causing rotation in the opposite direction to that caused by the biceps. Our intuition tells us that the muscle tension needs to somehow counteract the weight of the ball with the forearm acting as a sort of lever. That’s where we’ll start to solve this example.
We know the ball weighs 50 lbs. If we wanted to include the effect of the weight of the forearm in our example problem we could look up a typical forearm weight and also look up where the center of gravity of the forearm is located. Instead let’s take this opportunity to practice making justified assumptions. We know that forearms typically weigh only a few pounds, but the ball weight is 50 lbs, so the forearm weight is about an order of magnitude (10x) smaller than the ball weight. Therefore, let’s assume the forearm weight doesn’t matter much and continue solving our biceps force problem only including the weight of the ball, which acts at the center of gravity of the ball.
- OpenStax University Physics, University Physics Volume 1. OpenStax CNX. Jul 11, 2018 http://firstname.lastname@example.org. ↵
- "Weight, Volume, and Center of Mass of Segments of the Human Body" by Charles E. Clauster, et al, National Technical Information Service, U.S. Department of Commerce↵