Recognizing Forces

The purpose of this activity is to utilize an understanding of the various types of forces in order to analyze a given situation and to identify the presence (or absence) of a variety of forces.

There are several situations described below. For each situation, decide which forces are present and absent; identify what features of the situation you used to determine the presence (P) or absence (A) of each force. Then use the "pop-up menus" to view the answers.

1. A block hangs at rest from the ceiling by a piece of rope. Consider the forces acting on the block.

Gravity: P or A?
Depress mouse to view answer. Gravity is Present. Since the block has mass, there is a gravitational force on it.
Spring: P or A?
Depress mouse to view answer. Spring force is Absent. Since no spring is touching the box, there is not a spring force on it.
Tension: P or A?
Depress mouse to view answer. Tension is Present. Since an unrelaxed string is attached to the block, there is a tension force on it.
Normal: P or A?
Depress mouse to view answer. Normal is Absent. Since the block is not supported by any surface, there is not a normal force on it.
Friction: P or A?
Depress mouse to view answer. Friction is Absent. Since the block is not in contact with another surface, there is not a friction force on it.
Air Resistance: P or A?
Depress mouse to view answer. Air Resistance is Absent. Since the object is not moving relative to the surrounding air, there is not an air resistance force on it.

2. A block hangs from the ceiling by a spring. Consider the forces acting on the block when it is at rest (at its equilibrium position).

Gravity: P or A?
Depress mouse to view answer. Gravity is Present. Since the block has mass, there is a gravitational force on it.
Spring: P or A?
Depress mouse to view answer. Spring force is Present. Since an elongated spring is touching the block, there is a spring force on it.
Tension: P or A?
Depress mouse to view answer. Tension is Absent. Since an unrelaxed string is not attached to the block, there is not a tension force on it.
Normal: P or A?
Depress mouse to view answer. Normal is Absent. Since the block is not supported by any surface, there is not a normal force on it.
Friction: P or A?
Depress mouse to view answer. Friction is Absent. Since the block is not in contact with another surface, there is not a friction force on it.
Air Resistance: P or A?
Depress mouse to view answer. Air Resistance is Absent. Since the object is not moving relative to the surrounding air, there is not an air resistance force on it.

3. A ball is shot into the air with a spring-loaded cannon. Consider the forces acting on the ball while it is in the air.

Gravity: P or A?
Depress mouse to view answer. Gravity is Present. Since the ball has mass, there is a gravitational force on it.
Spring: P or A?
Depress mouse to view answer. Spring force is Absent. Since no spring is touching the ball, there is not a spring force on it.
Tension: P or A?
Depress mouse to view answer. Tension is Absent. Since an unrelaxed string is not attached to the ball, there is not a tension force on it.
Normal: P or A?
Depress mouse to view answer. Normal is Absent. Since the ball is not supported by any surface, there is not a normal force on it.
Friction: P or A?
Depress mouse to view answer. Friction is Absent. Since the ball is not in contact with another surface, there is not a friction force on it.
Air Resistance: P or A?
Depress mouse to view answer. Air Resistance is Present. Since the ball is moving relative to the surrounding air, there is an air resistance force on it.

4. A skydiver (who hasn't opened his parachute yet) falls at terminal velocity. Consider the forces acting on the skydiver.

Gravity: P or A?
Depress mouse to view answer. Gravity is Present. Since the skydiver has mass, there is a gravitational force on it.
Spring: P or A?
Depress mouse to view answer. Spring force is Absent. Since no spring is touching the skydiver, there is not a spring force on it.
Tension: P or A?
Depress mouse to view answer. Tension is Absent. Since an unrelaxed string is not attached to the skydiver, there is not a tension force on the skydiver.
Normal: P or A?
Depress mouse to view answer. Normal is Absent. Since the skydiver is not supported by any surface, there is not a normal force on the skydiver.
Friction: P or A?
Depress mouse to view answer. Friction is Absent. Since the skydiver is not in contact with another surface, there is not a friction force on the skydiver.
Air Resistance: P or A?
Depress mouse to view answer. Air Resistance is Present. Since the skydiver is moving relative to the surrounding air, there is an air resistance force on the skydiver.

5. A block rests on top of a table. Consider only the forces acting upon the block.

Gravity: P or A?
Depress mouse to view answer. Gravity is Present. Since the block has mass, there is a gravitational force on it.
Spring: P or A?
Depress mouse to view answer. Spring force is Absent. Since no spring is touching the block, there is not a spring force on it.
Tension: P or A?
Depress mouse to view answer. Tension is Absent. Since an unrelaxed string is not attached to the block, there is not a tension force on it.
Normal: P or A?
Depress mouse to view answer. Normal is Present. Since the block is supported by a surface (the table), there is a normal force on it.
Friction: P or A?
Depress mouse to view answer. Friction is Absent. Since there aren't any forces trying to slide the block along the table's surface, there is not a friction force.
Air Resistance: P or A?
Depress mouse to view answer. Air Resistance is Absent. Since the block is not moving relative to the surrounding air, there is not an air resistance force on it.

6. A block slides across the top of a table. Consider only the forces acting upon the block.

Gravity: P or A?
Depress mouse to view answer. Gravity is Present. Since the block has mass, there is a gravitational force on it.
Spring: P or A?
Depress mouse to view answer. Spring force is Absent. Since no spring is touching the block, there is not a spring force on it.
Tension: P or A?
Depress mouse to view answer. Tension is Absent. Since an unrelaxed string is not attached to the block, there is not a tension force on it.
Normal: P or A?
Depress mouse to view answer. Normal is Present. Since the block is supported by a surface (the table), there is a normal force on it.
Friction: P or A?
Depress mouse to view answer. Friction is Present. Since the block is being slowed down by the table's surface, there is a kinetic (or sliding) friction force on it.
Air Resistance: P or A?
Depress mouse to view answer. Air Resistance is Present. Since the block is moving relative to the surrounding air, there is an air resistance force on it.

7. A block rests on an incline plane without sliding. Consider the forces acting on the block.

Gravity: P or A?
Depress mouse to view answer. Gravity is Present. Since the block has mass, there is a gravitational force on it.
Spring: P or A?
Depress mouse to view answer. Spring force is Absent. Since no spring is touching the block, there is not a spring force on it.
Tension: P or A?
Depress mouse to view answer. Tension is Absent. Since a string under tension is not attached to the block, there is not a tension force on it.
Normal: P or A?
Depress mouse to view answer. Normal is Present. Since the ramp is touching the block; there is a normal force on it.
Friction: P or A?
Depress mouse to view answer. Friction is Present. Since the angled ramp is supporting the block and the block does not move, there is a friction force on the block.
Air Resistance: P or A?
Depress mouse to view answer. Air Resistance is Absent. Since the object is not moving relative to the surrounding air, there is not an air resistance force on it.

8. A block slides across the top of a table. Consider only the forces acting on the table.

Gravity: P or A?
Depress mouse to view answer. Gravity is Present. Since the table has mass, there is a gravitational force on it.
Spring: P or A?
Depress mouse to view answer. Spring force is Absent. Since no spring is touching the table, there is not a spring force on it.
Tension: P or A?
Depress mouse to view answer. Tension is Absent. Since an unrelaxed string is not attached to the table, there is not a tension force on it.
Normal: P or A?
Depress mouse to view answer. Normal is Present. Since the table is supported by a surface, there is a normal force on it.
Friction: P or A?
Depress mouse to view answer. Friction is Present. Since the table is stationary and the block is moving, there is a kinetic friction (with the block) and static friction (with the floor) force on the table.
Air Resistance: P or A?
Depress mouse to view answer. Air Resistance is Absent. Since the table is not moving relative to the surrounding air, there is not an air resistance force on it.

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