# 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 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? Click to show or hide the answer! Gravity is Present. Since the block has mass, there is a gravitational force on it. Spring: P or A? Click to show or hide the answer! Spring force is Absent. Since no spring is touching the box, there is not a spring force on it. Tension: P or A? Click to show or hide the answer! Tension is Present. Since an unrelaxed string is attached to the block, there is a tension force on it. Normal: P or A? Click to show or hide the 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? Click to show or hide the 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? Click to show or hide the 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? Click to show or hide the answer! Gravity is Present. Since the block has mass, there is a gravitational force on it. Spring: P or A? Click to show or hide the answer! Spring force is Present. Since an elongated spring is touching the block, there is a spring force on it. Tension: P or A? Click to show or hide the 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? Click to show or hide the 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? Click to show or hide the 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? Click to show or hide the 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? Click to show or hide the answer! Gravity is Present. Since the ball has mass, there is a gravitational force on it. Spring: P or A? Click to show or hide the answer! Spring force is Absent. Since no spring is touching the ball, there is not a spring force on it. Tension: P or A? Click to show or hide the 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? Click to show or hide the 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? Click to show or hide the 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? Click to show or hide the 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? Click to show or hide the answer! Gravity is Present. Since the skydiver has mass, there is a gravitational force on it. Spring: P or A? Click to show or hide the answer! Spring force is Absent. Since no spring is touching the skydiver, there is not a spring force on it. Tension: P or A? Click to show or hide the 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? Click to show or hide the 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? Click to show or hide the 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? Click to show or hide the 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? Click to show or hide the answer! Gravity is Present. Since the block has mass, there is a gravitational force on it. Spring: P or A? Click to show or hide the answer! Spring force is Absent. Since no spring is touching the block, there is not a spring force on it. Tension: P or A? Click to show or hide the 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? Click to show or hide the 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? Click to show or hide the 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? Click to show or hide the 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? Click to show or hide the answer! Gravity is Present. Since the block has mass, there is a gravitational force on it. Spring: P or A? Click to show or hide the answer! Spring force is Absent. Since no spring is touching the block, there is not a spring force on it. Tension: P or A? Click to show or hide the 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? Click to show or hide the 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? Click to show or hide the 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? Click to show or hide the 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? Click to show or hide the answer! Gravity is Present. Since the block has mass, there is a gravitational force on it. Spring: P or A? Click to show or hide the answer! Spring force is Absent. Since no spring is touching the block, there is not a spring force on it. Tension: P or A? Click to show or hide the 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? Click to show or hide the answer! Normal is Present. Since the ramp is touching the block; there is a normal force on it. Friction: P or A? Click to show or hide the 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? Click to show or hide the 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? Click to show or hide the answer! Gravity is Present. Since the table has mass, there is a gravitational force on it. Spring: P or A? Click to show or hide the answer! Spring force is Absent. Since no spring is touching the table, there is not a spring force on it. Tension: P or A? Click to show or hide the 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? Click to show or hide the answer! Normal is Present. Since the table is supported by a surface, there is a normal force on it. Friction: P or A? Click to show or hide the 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? Click to show or hide the 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.

9. The wheels of a car are locked as it skids to a stop while moving across a level highway.

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

10. A car is attached by a cable to a moving truck and is being pulled along a level highway. Consider only the forces acting on the car.

 Gravity: P or A? Click to show or hide the answer! Gravity is Present. Since the car has mass, there is a gravitational force on it. Spring: P or A? Click to show or hide the answer! Spring force is Absent. Since no spring is touching the car, there is not a spring force on it. Tension: P or A? Click to show or hide the answer! Tension is Present. Since an unrelaxed string is attached to the car, there is a tension force on it. Normal: P or A? Click to show or hide the answer! Normal is Present. Since the car is supported by a surface, there is a normal force on it. Friction: P or A? Click to show or hide the answer! Friction is Present. Rolling wheels on a car are unlike locked wheels. Locked wheels encounter kinetic (or sliding) friction. Rolling wheels encounter a static friction force which simply causes the tire to grip the road surface. This is sometimes referred to as traction - it keeps the wheels rolling. Air Resistance: P or A? Click to show or hide the answer! Air Resistance is Present. Since the car is moving relative to the surrounding air, there is an air resistance force on it.

11. A person is sitting on a sled and gliding across loosely-packed snow along a horizontal surface. Consider only the forces acting on the person.

 Gravity: P or A? Click to show or hide the answer! Gravity is Present. Since the person has mass, there is a gravitational force on it. Spring: P or A? Click to show or hide the answer! Spring force is Absent. Since no spring is touching the person, there is not a spring force on it. Tension: P or A? Click to show or hide the answer! Tension is Absent. Since an unrelaxed string is not attached to the person, there is not a tension force on it. Normal: P or A? Click to show or hide the answer! Normal is Present. Since the person is supported by a surface, there is a normal force on it. Friction: P or A? Click to show or hide the answer! Friction is Present. The person is moving along with the sled; static friction between the sled's top surface and the person's seat prevents the person from sliding off. Air Resistance: P or A? Click to show or hide the answer! Air Resistance is Present. Since the person is moving relative to the surrounding air, there is an air resistance force on him.