Unit 5 Review Flashcards
What is work?
force applied through a distance.
What two elements are required for work to be done?
- the applied force must make the object move
- the movement must be in the same direction as the applied force
True or False: If you push against a desk and nothing moves, you have done work.
False
Fill in the blank: Work requires both _______ and motion.
force
What is the formula for calculating work?
W = Fd
work (in joules) = applied force (in newtons) x distance (in meters)
W = Fd
True or false: Both work and energy are measured in joules.
True
During the ascent phase of a rep of the bench press, the lifter exerts an average vertical force of 1000N against a barbell while the barbell moves 0.8 m upward.
How much work did the lifter do to the barbell?
800 J
The rate at which energy is transferred is also known as what?
power
How do you calculate power?
you divide the amount of work done by the time it takes to do that work
P = W / t
What unit is power measured in?
watts
True or false: One watt is equal to one joule per second.
True
This means that joules per second and watts can be used interchangeably.
Provide the definition of machine.
a device that changes the force or increases the motion from work
A simple machine is a machine that does work with only ____ movement of the machine.
one
List the six types of simple machines.
- Lever
- Pulley
- Wheel and axle
- Inclined plane
- Screw
- Wedge
True or false: The pulley and the wheel and axle are modified levers.
True
What two simple machines are modified inclined planes?
- Screw
- Wedge
Provide the definition of efficiency.
a measure of how much of the work put into a machine is changed into useful output work by the machine
Simplified: how much input work is changed to output work
True or false: Some machines are 100% efficient.
False
True or false: Ideal machines1 are impossible to build.
1 | A machine that has 100% mechanical efficiency
True
Why are ideal machines impossible to build?
Every machine has moving parts, and some use input work to overcome friction
Provide the equation for efficiency.
e = Wout / Win * 100
Efficiency is usually expressed as a percentage
List one way that machines can be made more efficient.
adding a lubricant
Some examples of lubricants are oil and grease.
List the two forces involved when a machine is used to do work.
- Fin
- Fout
Fin - stands for the effort force
Fout - the force applied by the machine
The ratio of the output force to the input force is also known as what?
mechanical advantage
MA = Fout / Fin
True or false: Every change that occurs involves energy.
True
The ability to do work is also known as ______.
energy
Provide the definition of a system.
Anything that you can imagine a boundary around.
A system can be a single object, such as a baseball, or a group of objects, such as the solar system.
Provide the six different forms of energy.
- Electrical
- Chemical
- Radiant (light)
- Thermal
- Nuclear
- Sound
Any form of energy can be converted to another.
The energy a moving object has because of its motion is also known as …
kinetic energy.
The kinetic energy of a moving object depends on the object’s _____ and its ________.
mass, velocity (speed)
Provide the kinetic energy equation.
KE = 1/2 mv2
Provide the definition of potential energy.
the stored energy of position possessed by an object due to the interactions between another object
Ex.) Gravitational Potential Energy - An object at a height relative to the Earth has potential energy because of the gravitational pull between the object and the Earth. The higher the object, the more energy it has stored.
Provide the definition of chemical potential energy.
Energy stored due to chemical bonds
Gasoline, food, and some other substances all have what in common?
They all have chemical potential energy
Chemical potential energy is stored due to the bonds that hold the ______ together and is released when the _____ is burned.
atoms, gas
Explanation: In food, your body breaks the bonds in the molecules and uses the released energy to power you. Think of it like a battery–it stores energy that can be released when you “use it” (break bonds).
What is going on below?
chemical potential energy is released
Energy due to the gravitational forces between objects is also known as what?
Gravitational Potential Energy (GPE)
Provide the equation to calculate GPE.
GPE = mgh
gravitational potential energy (J) =
mass (kg) x gravity (N/kg) x height (m)
All forms of energy can be measured in ________.
joules
What does the law of conservation of energy state?
energy cannot be created nor destroyed
True or false: The total amount of energy in the universe is changing.
False
The law of conservation of energy states that energy never can be created or destroyed, therefore the total amount of energy in the universe is constant.
When energy conversion takes place, does the total amount of energy change?
no
Mechanical energy is the sum of the _________ and _________ energies of objects in a system.
kinetic, potential
True or false: The mechanical energy of a system remains constant or nearly constant.
True
This happens because as potential energy decreases, kinetic energy increases, and vice versa.
If you were swinging or riding a rollercoaster, the potential energy is at its greatest at the ___ of your path.
top
If energy is conserved, why does kinetic energy not stay constant so that you could bike, swing, or rollerblade forever?
Friction and air resistance cause some of the mechanical energy to change to other forms of energy.
Fuel stores ________ potential energy.
chemical
The engine transforms the chemical potential energy of gasoline molecules into the kinetic energy of a moving car or bus.
A construction worker is lifting a load of bricks onto a truck. He does 200 J of work in 40 seconds.
What is his power output?
5 watts (W)
In which situation is a person doing work on an object?
A. A man exerts 350 N force on a rope attached to a house.
B. A worker holds a box 1 m off the floor.
C. A school crossing guard raises a stop sign that weighs 10 N.
D. A student walks while wearing a backpack that weighs 15 N.
C. A school crossing guard raises a stop sign that weighs 10 N.
Work is only done when a force is applied, and it causes an object to move in the direction of the force.
A roller coaster is at the top of a 72 m hill and weighs 134 kg. The coaster has _________ energy. Calculate it.
- potential
- 94,550.4 J
