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70 Cards in this Set
- Front
- Back
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What is work?
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a quantity that measures the effects of a force acting over a distance
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Three children exhaust themselves trying to push a large rock that doesn't budge. Have they done any work?
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Work is defined as force times distance. If the rock has not moved any distance, no work has been done. Work is done only when force causes a change in motion.
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What are the units of work?
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J (joules)
N•m kg•m2/s2 |
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What are the units of power?
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watts
joules per second horsepower |
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What is a machine?
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a device that can multiply and change the direction of an input force
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How does a jack make it easier to lift a car?
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by multiplying the input force and spreading the work out over a large distance
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How do machines help us do work?
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Machines redistribute the work.
Machines can change the direction of the input force. Machines can increase the output force by changing the distance over which the force is applied. |
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What is the relationship between work input and work output?
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they are equal
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What does mechanical advantage tell you?
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Mechanical advantage tells how much a machine multiplies force or increases distance.
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Define mechanical advantage.
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a quantity that measures how much a machine multiplies force or distance
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How do you know if work is being done?
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Work is done only when force causes a change in the motion of an object.
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Write the equation for work.
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work = force x distance
(W = F x d) |
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When work is being done there is another factor that needs to be taken into consideration. What is this factor?
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The amount of time it takes to get the work done.
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Write the equation for power.
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power = work/time
P = W/t |
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What is a watt?
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A watt is the amount of power required to do 1 J of work in 1 s.
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What is the equation for mechanical advantage?
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mechanical advantage = output force/input force
mechanical advantage = input distance/output distance |
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What is the difference between work and power?
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Work is a quantity that measures the effects of a force acting over a distance.
Power is a quantity that measures the rate at which work is done. |
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What is the relationship between work and power?
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Power is the rate at which work is done.
Power equals the amount of work done per unit of time: P = W/t |
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Write the equation for power.
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power = work/time
P = W/t |
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Explain how you can exert a large force on an object without doing and work?
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If the object does not move, then no work is done because work is equal to force times distance.
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List the three types of simple machines in the lever family.
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a simple lever, a pulley, and a wheel and axle
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What are the three simple machines in the inclined plane family?
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a simple inclined plane, a wedge, and a screw
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Compare the mechanical advantage of a long, thin wedge with that of a short, wide wedge. Which is greater?
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A long, thin wedge has a greater mechanical advantage than a short, wide wedge because the ratio of length to height is greater for a long, thin wedge.
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Can an inclined plane have a mechanical advantage of less than 1?
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No.
If the mechanical advantage were 1, the plane would be as long as it is tall. You cannot get shorter than the actual height you need to lift. |
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What class of lever is a door?
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A door is normally a second-class lever, with a mechanical advantage greater than 1.
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How can a lever increase the force without changing the amount of work being done?
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A lever can increase the force without increasing the work done because the output foce will be exerted through a smaller distance.
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Why are pulleys in the lever family?
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The middle of the pulley is the fulcrum.
The wheel of the pulley is like a lever arm extended into a circle. |
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Describe a wheel and axle.
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a lever or a pulley (the wheel) connected to a shaft (the axle)
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Compare a wedge and a screw.
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A wedge is a modified inclined plane and a screw is an inclined plane wrapped around a cylinder
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How does an inclined plane make work easier?
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the ramp redirects the force to lift the object upward
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List the parts of a lever.
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the fulcrum and the arm
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Define a compound machine.
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a machine made of more than one simple machine
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How do levers multiply force or increase distance on the output side?
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Second-class levers always multiply force, third-class levers always increase distance, and first-class levers may either multiply force or increase distance.
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What is a block and tackle?
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an arrangement of multiple pulleys
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How does a wedge function?
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like two inclined planes back to back
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What is kinetic energy?
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the energy of a moving object due to its motion
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Kinetic energy depends on what two factors?
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speed and mass
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What happens to the kinetic energy of an apple as it falls to the ground?
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the apple is accelerating (speeding up), so the kinetic energy increases
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Why does kinetic energy depend on speed more than mass?
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In the kinetic energy equation, speed is squared so a small increase in speed produces a large increase in kinetic energy.
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Why is a car crash more dangerous at speeds above the speed limit?
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A car has much more kinetic energy at higher speeds, so it can do much more work -- more damage.
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What happens to energy whenever work is done?
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energy is transformed or tranferred
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What is the relationship between energy and work?
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one way to define energy is the ability to do work
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What units are used to express energy?
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joules
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Energy can be present in an object or system when nothing is happening. When then can energy be observed?
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only when it is transferred from one object or system to another
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How can you measure the amount of energy that is being transferred?
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by how much work is done
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Define potential energy.
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the stored energy resulting from the relative positions of objects in a system
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Why is potential energy sometimes called energy of position?
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potential energy results from the relative positions of objects in a system
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What is elastic potential energy?
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the energy stored in any type of stretched or compressed elastic material
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What is gravitational potential energy?
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the potential energy that results from the gravitational attraction between objects
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What are the two factors that affect gravitational potential energy?
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mass and height
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What is the equation for gravitational potential energy?
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PE = mgh
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How can you increase the average kinetic energy of particle in an object?
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heat the object
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What is the definition of nonmechanical energy?
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energy that lies at the level of atoms and does not affect motion on a large scale
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Define mechanical energy.
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the sum of the kinetic and potential energy of large-scale objects in a system
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Why is chemical energy considered a kind of potential energy?
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because chemical energy depends in part on the relative positions of the atoms it contains
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In almost every system there are hidden forms of energy that are related to the motion and arrangement of atoms and molecules. What is this energy called?
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nonmechanical energy
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What kind of energy do atoms and molecules have?
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kinetic energy (nonmechanical)
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How is kinetic energy transferred between particles?
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through collisions
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What happens to the average kinetic energy of particles as they get hotter?
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the average kinetic energy increases
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Explain the difference between potential energy and kinetic energy.
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Potential energy is energy due to position. Kinetic energy is the energy of motion.
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Describe the energy transformation when a ball rolls down a hill.
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potential energy becomes kinetic energy
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Describe the energy transformation when a pendulum is on the downswing.
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potential energy becomes kinetic energy
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Describe the energy transformation when you throw a ball up into the air.
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kinetic energy becomes potential energy
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Describe the energy transformation when a pendulum is on the upswing.
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kinetic energy becomes potential energy
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State the "Law of Conservation of Energy."
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Energy cannot be created or destroyed.
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What do we mean when we refer to the efficiency of a machine.
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The efficiency of a machine is the ratio of the useful work performed by the machine to the work required to operate the machine
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Explain why machines are not 100 percent efficient.
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Friction prevents machines from being 100 percent efficient.
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How do you determine the efficiency of a machine?
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efficiency = useful work output/work input
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How can mechanical energy change to nonmechanical energy?
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as a result of friction, air resistance, or other means
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Wht happens to the energy after a rock hits the beach?
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It is transferred into the kinetic energy of the sand, and sound energy (KE of air), and increased temperature (KE of the molecules of the rock and sand).
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