Unbalanced Forces ProbSolv

Questions to Consider:

1. Have you ever felt heavier or lighter when riding in an elevator?  Is this sensation caused by the music or by the motion of the elevator?  Does everyone feel heavier or lighter at the same times?

2. Describe the times in the elevator when you feel your “normal” weight.

3. Describe the times in the elevator when you feel heavier than your “normal” weight.

4. Describe the times in the elevator when you feel lighter than your “normal” weight.

5. Do these “feelings” occur when you are moving at a constant speed or when you are accelerating?

6. You discover in the last activities that unbalanced forces always produce accelerations.

7. What seems to be the relation between the direction of the FNet and the acceleration?  

Quick Experimentation:

1. Hang a 0.50 kg or 1.0 kg mass on the end of a spring balance and record the force reading on the balance.

   1. 1. 1. Fg =  ___ N

2. Start the mass just above the floor and try to lift the mass to simulate an elevator ride.

3. Describe the force reading when the mass was accelerating upward, then moving at a steady speed, then slowing down (accelerating downward).

4. Next, start the mass at about 2.0 meters above the floor and perform the downward ride, stopping before the kg hits the floor.

5. Describe the force reading when the kg was accelerating downward, then moving at a steady speed, then slowing down (accelerating upward).

Problem Solving:

An elevator ride to the top of a tall building has three phases: 

1. beginning the ascent by accelerating upwards, 

2. traveling at a constant velocity, and 

3. slowing to a stop by accelerating downwards. 

We will look at all three for an 85 kg passenger.

1. After pressing the button to travel upwards to the penthouse apartment, The elevator now accelerates upward at 2.0 m/s2.

   1. Construct a force diagram for the passenger.

   2. Write an equation for the vertical forces on the passenger. 

   3. Calculate the force the floor exerts on the passenger.

2. An elevator is moving up at a constant velocity of 2.5 m/s, as illustrated in the diagram below: 

   1. Construct a force diagram for the passenger. 

   2. Calculate the force the floor exerts on the passenger. 

3. Upon reaching the top floor of the building the elevator accelerates downward at 3.0 m/s2.

   1. Construct a force diagram for the passenger.  

   2. Write an equation for the vertical forces on the passenger.

   3. Calculate the force the floor exerts on the passenger.