Objects that move with a constant velocity (same speed ... same direction) experience a balance of forces. Oppositely-directed forces are of the same strength. On the other hand, objects that accelerate experience an unbalanced force that is directed in the direction of the object's acceleration.
An elevator rider is at rest on the ground floor. The Force Diagram for this state of motion is shown on the left. What will the Force Diagram look like when this rider is in a state of moving upward with an increasing speed? Select a force diagram from the choices provided on the right.

The Force Diagram provided for the elevator rider shows a balance of forces: the up force and the down force are of equal strength. This will always be the case for an at-rest object. But you need to determine what the Force Diagram looks like if the elevator rider moves upwards and is speeding up. To do so, you will need to determine the direction of the acceleration. If the acceleration is upwards, then there must be more up force than down force. But if the acceleration is downwards, then there would be more down force than up force. Read on ... 

The Direction of the Acceleration

The direction of an object's acceleration depends on two important features of the object's motion - the direction that the object is moving and whether the object is speeding up or slowing down. The general rule is: If an object is slowing down, then the direction of the object's acceleration is the opposite of the direction that the object is moving. On the other hand, if the object is speeding up, then the direction of the object's acceleration is the same as the direction that the object is moving.

Now apply this information to the elevator rider and determine the direction of its acceleration.


Respect the Weight of the Situation

The down force is the force of gravity. This force depends on the elevator rider's mass. Since the rider's mass is not changing, the force of gravity does not change.  If you need a Force Diagram with more up force than down force, then make sure you accomplish this by increasing the up force since the down force (gravity) cannot be changed.

Inspect your answer options carefully to insure that the force of gravity in your selected answer is the same size as the force of gravity in the given Force Diagram. You should be able to rule out a few options just by inspecting this detail.​

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