Download Work and Energy - Lecture Slides - General Physics | PHYS 110 and more Study notes Physics in PDF only on Docsity!
Chapter 6
Work and Energy
Units of Chapter 6
- Work Done by a Constant Force
- Kinetic Energy, and the Work-Energy Principle
- Potential Energy
- Conservative and Nonconservative Forces
- Mechanical Energy and Its Conservation
- Problem Solving Using Conservation of Mechanical Energy
- Other Forms of Energy; Energy Transformations and the Law of Conservation of Energy
- Energy Conservation with Dissipative Forces: Solving Problems
- Power
6-1 Work Done by a Constant Force
The work done by a constant force is defined as the distance moved multiplied by the component of the force in the direction of displacement: (6-1)
6-1 Work Done by a Constant Force
In the SI system, the units of work are joules: As long as this person does not lift or lower the bag of groceries, he is doing no work on it. The force he exerts has no component in the direction of motion.
6-1 Work Done by a Constant Force
Solving work problems:
- Draw a free-body diagram.
- Choose a coordinate system.
- Apply Newton’s laws to determine any unknown forces.
- Find the work done by a specific force.
- To find the net work, either find the net force and then find the work it does, or find the work done by each force and add.
6-1 Work Done by a Constant Force
Work done by forces that oppose the direction of motion, such as friction, will be negative. Centripetal forces do no work, as they are always perpendicular to the direction of motion. Think-Pair-Share
- Question 3: Can the normal force on an
object ever do work? Explain.
Think-Pair-Share
- Problem 3: A 1300-N crate rests on the
floor. How much work is required to move
it at constant speed (a) 4.0 m along the
floor against a friction force of 230 N, and
(b) 4.0 m vertically?
6-3 Kinetic Energy, and the Work-Energy
Principle
This means that the work done is equal to the change in the kinetic energy:
- If the net work is positive, the kinetic energy increases.
- If the net work is negative, the kinetic energy decreases. (6-4)
6-3 Kinetic Energy, and the Work-Energy
Principle
Because work and kinetic energy can be equated, they must have the same units: kinetic energy is measured in joules. Think-Pair-Share
- Problem16: (a) If the kinetic energy of an
arrow is doubled, by what factor has the
speed increased? (b) If its speed is
doubled, by what factor is its KE inreased?
6-4 Potential Energy
An object can have potential energy by virtue of its surroundings. Familiar examples of potential energy:
- A wound-up spring
- A stretched elastic band
- An object at some height above the ground
6-4 Potential Energy
In raising a mass m to a height h , the work done by the external force is We therefore define the gravitational potential energy: (6-5a) (6-6)
6-4 Potential Energy
This potential energy can become kinetic energy if the object is dropped. Potential energy is a property of a system as a whole, not just of the object (because it depends on external forces). If , where do we measure y from? It turns out not to matter, as long as we are consistent about where we choose y = 0. Only changes in potential energy can be measured. Think-Pair-Share
- Problem 27: A 7.0-kg monkey swings
from one branch to another 1.2 m higher.
What is the change in PE?
6-4 Potential Energy
Potential energy can also be stored in a spring when it is compressed; the figure below shows potential energy yielding kinetic energy.
