Lab 3 Center of Mass of the Body Kin247 PDF

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Lab 3 Center of Mass of the Body entire lab....

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Section Number: Team Number:

Center of Mass of the Body Laboratory #3

Overview In this lab you will determine the center of mass of your body. For any object the center of mass is the point at which the force of gravity acts of the object. For this reason the center of mass is sometimes referred to as the center of gravity. We will use the terms interchangeably. In class we began solving statics ( and ) problems in which we had to consider the force of gravity acting on an object. Recall that gravitational force (weight) can exert a torque on an object just like any other force. One problem you solved involved a person doing a push up.

In this problem the position of the center of mass (cg) relative to the person’s feet is given. But what if you don’t know where the center of mass is?

Part 1 - Force Plate In this first part we will measure the forces on the body using a device called a force plate. The force plate is essentially a bathroom scale that can read and record the normal force applied to it. In this case the plate will record data 100 times per second. We will use the forces measured by the force plate to determine the torques on the body. Remember a torque is a force acting at a place on the object. If the object is static then all the torque add vectorially to zero.

Set up the force place:

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Connect the force plate to the USB link, then connect the USB link to one of the classroom laptops. Log onto to one of the classroom laptops. Open a browser and log onto Moodle. In the assignment for Lab 3 - Center of Mass of the Body, download the Capstone file Center_of_Mass.cap to the laptop connected to the force plate. Launch the Center_of_Mass.cap file. On the force plate press the Tare button on the side near the green LED to zero the force plate. Test the force plate by having one of your teammates stand on the plate. Click Record in the lower right corner of Capstone to record your teammates weight in Newtons. Record the weight for a few seconds, then click Stop.

Part 2 - Torques on the Body Now let’s figure out the torques on the body when you are in the push-up position.

1. Draw a free body diagram and label all of the forces on your body in the push-up position. Remember the forces on your body are at different places on your body.

2. What do you choose to be a axis of rotation (pivot) when you do a push-up? Explain how your choice helps solve the question of where your center of mass is. Think about what points are moving when you do a push-up (head, shoulders, hips, legs, or feet).

Part 3 - Position Measurements 3. For each team member measure the position that the forces acting on their body occur when doing a push-up. You do NOT have to do an actual push-up to make these measurements. Measure each teammate’s height as well. Make sure you keep track of which teammate is which number. Don’t mix up teammate and measurements!! Teammate Position of Feet (m) 1 2 3 4 5 6

Position of Hands (m)

Height (m)

Part 4 - Force Measurements Now you are going to use the force plate to measure the forces on your body.

4. Each teammate measure their weight in Newtons by standing on the force plate and click Record in Capstone. Record weight for a few seconds, then click Stop.

5. Now based on your choice of the axis of rotation, measure the forces on your body while in the static push-up position; i.e., when your arms are full extend. Remember the force acting at the axis of rotation does NOT exert a torque (r = 0). Don’t mix up teammate and measurements!!

Teammate Weight (N) Normal Force at Hands (N) 1 2 3 4 5 6

Part 5 - Center of Mass You now have all the information you need to calculate the center of mass for each teammate. Remember when anything is static calculations!!

and

. Don’t mix up teammate and

6. Calculate the center of mass for each teammate.

7. Calculate the ratio of the center of mass to height for each teammate.

Teammate Center of Mass (m) Center of Mass/Height

1 2 3 4 5 6

Part 6 - Class Distribution We now want to aggregate the the Center of Mass/Height result for the whole class and see if there is any patterns in distributions of center of mass. A common idea is men and women have different centers of mass.

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Access the spreadsheet “Center of Mass” on the Google Drive. You can edit this spreadsheet and enter the ratio of center of mass/height for each teammate. Make sure you select the correct tab at the bottom spreadsheet for your section of Physics 131....