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10.1 Machines and Mechanical Advantage10.2 Work10.3 Energy and Conservation of Energy
Work and Energy
Chapter 10 ObjectivesChapter 10 Objectives
1. Calculate the mechanical advantage for a lever or rope and pulleys.
2. Calculate the work done in joules for situations involving force and distance.
3. Give examples of energy and transformation of energy from one form to another.
4. Calculate potential and kinetic energy.5. Apply the law of energy conservation to systems
involving potential and kinetic energy.
Chapter 10 Vocabulary TermsChapter 10 Vocabulary Terms machine energy input force output force thermal energy ramp gear screw rope and
pulleys closed system work
lever friction mechanical
system simple machine potential energy kinetic energy radiant energy nuclear energy chemical energy mechanical
energy
mechanical advantage
joule pressure energy conservation of
energy electrical
energy input output input arm
output arm fulcrum
10.1 Machines and Mechanical 10.1 Machines and Mechanical AdvantageAdvantage
Key Question:
How do simple machines work?
10.1 Machines10.1 Machines
The ability of humans to build buildings and move mountains began with our invention of machines.
In physics the term “simple machine” means a machine that uses only the forces directly applied and accomplishes its task with a single motion.
10.1 Machines10.1 Machines
The best way to analyze what a machine does is to think about the machine in terms of input and output.
10.1 Mechanical Advantage10.1 Mechanical Advantage
Mechanical advantage is the ratio of output force to input force.
For a typical automotive jack the mechanical advantage is 30 or more.
A force of 100 newtons (22.5 pounds) applied to the input arm of the jack produces an output force of 3,000 newtons (675 pounds)— enough to lift one corner of an automobile.
10.1 Mechanical Advantage10.1 Mechanical Advantage
MA = Fo
Fi
Output force (N)
Input force (N)
Mechanicaladvantage
10.1 Mechanical Advantage of 10.1 Mechanical Advantage of a Levera Lever
MAlever = Li
Lo
Length of input arm (m)
Length of output arm(m)
Mechanicaladvantage
10.1 Calculate position10.1 Calculate position
Where should the fulcrum of a lever be placed so one person weighing 700 N can lift the edge of a stone block with a mass of 500 kg?
The lever is a steel bar three meters long. Assume a person can produce an input force
equal to their own weight. Assume that the output force of the lever
must equal half the weight of the block to lift one edge.
10.1 Wheels, gears, and rotating 10.1 Wheels, gears, and rotating machinesmachines
Axles and wheels provide advantages. Friction occurs where the wheel and axle touch or where the wheel
touches a surface. Rolling motion creates less wearing away of material compared with
two surfaces sliding over each other.
With gears the trade-off is made between torque and rotation speed.
An output gear will turn with more torque when it rotates slower than the input gear.
10.1 Ramps and Screws10.1 Ramps and Screws
Ramps reduce input force by increasing the distance over which the input force needs to act.
A screw is a simple machine that turns rotating motion into linear motion.
A thread wraps around a screw at an angle, like the angle of a ramp.
10.2 Work10.2 Work
Key Question:
What are the consequences of multiplying forces in machines?
10.2 Work10.2 Work
In physics, work has a very specific meaning.
In physics, work represents a measurable change in a system, caused by a force.
10.2 Work10.2 Work
If you push a box with a force of one newton for a distance of one meter, you have done exactly one joule of work.
10.2 Work (force is parallel to 10.2 Work (force is parallel to distance)distance)
W = F x dDistance (m)
Force (N)
Work (joules)
10.2 Work (force at angle to 10.2 Work (force at angle to distance)distance)
W = Fd cos ()
Distance (m)
Force (N)
Work (joules) Angle
10.2 Work done against gravity10.2 Work done against gravity
W = mgh
Height object raised (m)
Gravity (m/sec2)
Work (joules)
Mass (g)
10.3 Why the path doesn't matter10.3 Why the path doesn't matter
10.3 Calculate work10.3 Calculate work
A crane lifts a steel beam with a mass of 1,500 kg.
Calculate how much work is done against gravity if the beam is lifted 50 meters in the air.
How much time does it take to lift the beam if the motor of the crane can do 10,000 joules of work per second?
10.3 Energy and Conservation of Energy10.3 Energy and Conservation of Energy
Energy is the ability to make things change.A system that has energy has the ability to do
work.Energy is measured in the same units as work
because energy is transferred during the action of work.
10.3 Forms of Energy10.3 Forms of Energy
Mechanical energy is the energy possessed by an object due to its motion or its position.
Radiant energy includes light, microwaves, radio waves, x-rays, and other forms of electromagnetic waves.
Nuclear energy is released when heavy atoms in matter are split up or light atoms are put together.
The electrical energy we use is derived from other sources of energy.
10.3 Potential Energy10.3 Potential Energy
Ep = mgh Height (m)
Mass (kg)
Potential Energy (joules)
Accelerationof gravity (m/sec2)
10.3 Potential Energy10.3 Potential Energy
A cart with a mass of 102 kg is pushed up a ramp.
The top of the ramp is 4 meters higher than the bottom.
How much potential energy is gained by the cart?
If an average student can do 50 joules of work each second, how much time does it take to get up the ramp?
10.3 10.3 KineticKinetic Energy Energy
Energy of motion is called kinetic energy.
The kinetic energy of a moving object depends on two things: mass and speed.
Kinetic energy is proportional to mass.
10.3 10.3 KineticKinetic Energy Energy
Mathematically, kinetic energy increases as the square of speed.
If the speed of an object doubles, its kinetic energy increases four times. (mass is constant)
10.3 10.3 KineticKinetic Energy Energy
Ek = 1 mv2
2
Speed (m/sec)
Mass (kg)
Kinetic Energy (joules)
10.3 Kinetic Energy10.3 Kinetic Energy
Kinetic energy becomes important in calculating braking distance.
10.3 Calculate Kinetic Energy10.3 Calculate Kinetic Energy
A car with a mass of 1,300 kg is going straight ahead at a speed of 30 m/sec (67 mph).
The brakes can supply a force of 9,500 N.
Calculate:a) The kinetic energy of the
car.b) The distance it takes to
stop.
10.3 Law of Conservation of 10.3 Law of Conservation of EnergyEnergy
As energy takes different forms and changes things by doing work, nature keeps perfect track of the total. No new energy is created and no existing energy is destroyed.
10.3 Energy and Conservation of 10.3 Energy and Conservation of EnergyEnergy
Key Question:
How is motion on a track related to energy?
Application: Hydroelectric PowerApplication: Hydroelectric Power
