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Chapter 4:
Work and Machines
- Work is different in science than in everyday life
- Force applied through a distance
- No movement=no work
- Direction of force can affect how much work you are doing
- When force and motion are parallel, work is equal to force times distance
W=Fd
- Work is measured in Joules
- Remember force is Newtons and distance is meters
Example Problem 1 page 107
Homework: Read 4.2, write down main ideas and define vocab words
Practice Problems 1-4
- Force perpendicular to motion, net force is zero
- For force in other directions, work is more than zero but less than when force and motion are parallel
- Figure 2 page 108
Share your vocab word definitions with a partner.
Section 1 Review Page 112 10-15
Energy Transformations
- You only do work while you are touching an object
What are some everyday machines you use?
- Mechanical Energy: Sum of kinetic energy and potential energy of the objects in a system
+ Does not include nuclear, thermal, or chemical potential energy
- Falling objects: When an object falls, it gets closer to earth, the GPE of the system decreases, but total energy does not change
Machines
How are machines useful?
- Device that changes the force or increases motion from work
- Bicycle is a machine
- Simple machine: does work with only one movement
- Compound machine: Combination of two or more simple machines
- Projectile Motion: When an object moves in a curved path
- Figure 12 page 122
- Swings: Summarize mechanical energy transformation energy for a swing, page 122
- Explain the energies in the picture on page 123
Efficiency
- Increase speed
- Change direction of force
- Increase force
-Mechanical advantage pg 111
- Example Problem 3 page 112
- Practice 8-9
Homework: read lab on pages 128-129, be prepared to make a hypothesis, procedure, data table
Test Monday, Kahoot EC Due Friday
- ratio of output work to input work
- Efficiency = output/input * 100
- Page 110
-Example problem 2
- Practice 5-7
Energy
Forms of Energy
- Change requires energy
- Energy is the ability to cause change
- Work transfers energy
- Energy, like work, measured in joules
- Energy can be transferred between systems
Homework: Read 4.3
- Mechanical, electrical, chemical, radiant
- Kinetic and Potential energies
Other Energy transformations
What is the Law of Conservation of Energy?
Section 3 Review
- it might appear sometimes as if energy is being destroyed, but there are other forces at work
- Friction and air resistance convert some mechanical energy into thermal energy
- Electric energy: stoves transform electric energy into thermal energy, televisions transform electric energy into radiant energy
Kinetic Energy
Section 2 Review
Mini Lab Page 126
- Energy due to motion
- Objects in motion can collide with other objects and cause change
- Kinetic Energy (Joules) = 1/2 (mass) * (speed)^2
- KE = 1/2mv^2
- Example problem 4
- Practice Problems 16-17 Page 116
- Chemical potential energy can be transformed by a car into mechanical and thermal energy
- Some energy transformations are not obvious
Power
- rate at which energy is converted
Power (watts) = Energy (J)/Time (s)
P=E/T
- Example Problem 6
- Practice 26-27 Page 126
Mini Lab Page 117
What are two different types of energy?
Potential Energy
- Energy that is stored due to the interactions between objects
- For example energy is stored between an apple and a tree because of the gravitational force between the two
Everyday use of different forms of energy (mechanical, electrical, chemical, radiant, kinetic, potential)
Elastic Potential Energy
- energy that is stored by compressing or stretching an object
- Like a rubber band
Gravitational Potential Energy
Example Problem 5
Practice Problems 18-19 Page 119
Chemical Potential Energy
- Energy that is due to chemical bonds
- For example, when you burn a substance
- Energy due to the gravitational forces between objects
- Apple and earth have gravitational potential energy
- GPE = mgh
- Gravitational Potential Energy (J) = Mass (kg) * gravity (N/Kg) * Height (m)
- height must be measured from a reference level
- gravitational energy may be positive or negative depending on reference level