FORCES LAB: HUMAN TABLE
PHYSICS
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BACKGROUND
Balanced Forces: To be in equilibrium simply implies that all forces acting on an object are balanced (Fnet = 0). When all
forces acting on an object are balanced, that object maintains its current state of motion (remains at rest or maintains a
constant velocity). Unbalanced force(s) acting on an object cause an acceleration and as a result, the object’s velocity
and/or direction changes.
Free-Body Diagrams: Free-body diagrams are diagrams used to show the relative magnitude and direction of all forces
acting upon an object in a given situation. A free-body diagram is a special example of the vector diagrams you learned
about previously (Flight Lesson Vectors, Advanced Vector Worksheet, and Chapter 3). Free-body diagrams will be used
throughout our study of physics. The size of the arrow in a free-body diagram reflects the magnitude of the force. The
direction of the arrow shows the direction that the force is acting. Each force arrow in the diagram is labeled to indicate
the exact type of force.
INSTRUCTIONS
You are going to attempt to create a human table. The secret to this table-esque trick is a system of balanced forces.
Even though gravity is pulling you and your friends down, for the human table to work, each person must be supported
by the legs of the person behind them. After attempting to create your human table, you will draw a free-body diagram
representing the force of gravity (Fg) and the normal force (Fn) acting on the entire group and acting on you (you will
draw two diagrams).
Prediction: If each of your friends has a different mass, is it possible to create a human table in equilibrium? Write your
prediction below and make sure to explain your reasoning using the language of physics.
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Procedure:
1. Set up four chairs in a square. The chairs need to be close enough that a person can lay backwards and have
their shoulders rest on another chair.
2. Have four friends sit in the chairs. The friends need to have their feet on the ground. Each friend needs to lay
backwards, resting their shoulders on the legs of the person behind them.
3. One by one, remove the chairs from under your friends.
4. Much to their amazement, your friends remain in their reclined position, despite lacking the support of the chairs.
5. Before your friends collapse, replace the chairs and let them sit-up. Ta-da!
Data Collection: Record the force of gravity (N) acting on each person below:
(Example for 150lb person: 150lb x 1kg/2.2lb x -9.81m/s^2 = -669 N)
Friend 1) _________________N
Friend 2) _________________N
Friend 3) _________________N
Friend 4) _________________N
FORCES LAB: HUMAN TABLE
PHYSICS
Free-Body Diagram #1: As unit, draw in the vector arrows to represent the force of gravity and the normal force
(upward force of the floor) acting on the group as a whole. Include your data and all the forces must be balanced.
Free-Body Diagram #2: As an individual, draw all of all the forces acting on you. Make sure to use the force data you
collected.
Key
You
Friend
Friend
Conclusion: In your conclusion make sure to restate your prediction and report whether or not your results
supported or refuted your claim. Discuss how your free-body diagram force vectors represent the human table
scenario.
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