Physics 151 Week 10 Day 2

Topics: Apparent Weight & Conservation of Energy Apparent Weight Conservation of Energy

Scales and Elevators (Apparent Weight)

Slide 4-19

Consider a person with a mass of 60 kg is in an elevator standing on a scale. The elevator is accelerating upward.

• Draw a system schema and 2 force diagrams: One for the person and one for the scale

• What does the scale read?

• Use Newton’s 2nd law to determine what the scale reads (This is apparent weight)

Scales and Elevators (Apparent Weight)

Slide 4-19

Suppose a person with a mass of 60 kg is in an elevator standing on a scale.

Use the system schema and force diagrams of the scale and the person to determine what the scale would read for the following situations:

A. If the elevator is descending at 4.9 m/s.

B. If the elevator has a downward acceleration of 4.9 m/s/s.

C. If the elevator has an upward acceleration of 4.9 m/s/s.

Apparent Weight

Slide 5-24

A 50 kg student gets in a 1000 kg elevator at rest. As the elevator begins to move, she has an apparent weight of 600 N for the first 3 s. How far has the elevator moved, and in which direction, at the end of 3 s?

Example Problem

Slide 5-25

Clicker QuestionThe apparent weight of an object is

A. the pull of gravity on the object.B. the object’s mass times the acceleration of gravity.C. the magnitude of the contact force that supports the

object.D. the pull of gravity on an object that is accelerating

upward.

Slide 5-7

Answer2. The apparent weight of an object is

A. the pull of gravity on the object.B. the object’s mass times the acceleration of gravity.C. the magnitude of the contact force that supports the

object.D. the pull of gravity on an object that is accelerating

upward.

Slide 5-8

Energy Model

Types of EnergyKinetic Energy KE = 1/2 mv2

Potential Energy:Gravitational Potential Energy PEg = mgySpring Potential Energy Pes = 1/2 kL2

Conservation of Energy (Closed System)

Visualizations:• Energy Bar Charts

Before After

KEi + PEgi + PEsi + ΔEsys =KE f + PEgf + PEsf + ΔEth

The Basic EquationKf Uf ΔEth Ki Ui

A few things to note:• Work can be positive (work in) or negative (work out)• We are, for now, ignoring heat.• Thermal energy is…special. When energy changes to

thermal energy, this change is irreversible.

Slide 10-24

Conceptual Example Problem A car sits at rest at the top of a hill. A small push sends it rolling down a hill. After its height has dropped by 5.0 m, it is moving at a good clip. Write down the equation for conservation of energy, noting the choice of system, the initial and final states, and what energy transformation has taken place.

Slide 10-25

Checking Understanding Three balls are thrown off a cliff with the same speed, but in different directions. Which ball has the greatest speed just before it hits the ground?

A. Ball AB. Ball BC. Ball CD. All balls have

the same speed

Slide 10-26

Answer Three balls are thrown off a cliff with the same speed, but in different directions. Which ball has the greatest speed just before it hits the ground?

A. Ball AB. Ball BC. Ball CD. All balls have

the same speed

Slide 10-27

Trucks with the noted masses moving at the noted speeds crash into barriers that bring them to rest with a constant force. Which truck compresses the barrier by the largest distance?

Additional Questions

Slide 10-54

Trucks with the noted masses moving at the noted speeds crash into barriers that bring them to rest with a constant force. Which truck compresses the barrier by the largest distance?

Answer

E.

Slide 10-55