Simple Machines, Chp. 13 Work & Energy
What is the amount of power needed to do one joule of work in one second?
1 watt
What is a perpetual motion machine?
A machine designed to keep going forever without any input of energy. Since a perpetual motion machine could work only in the absence of friction, these machines cannot exist in our world.
What happens if a machine has a mechanical advantage greater than 1?
A machine that has a mechanical advantage greater than 1 multiplies the input force.
What happens if a machine has a mechanical advantage less than 1?
A machine that has a mechanical advantage less than 1 does not multiply the force but increases distance and speed
How are scissors an example of a compound machine?
A pair of scissors uses two first-class levers joined at a common fulcrum; each lever arm has a wedge that cuts into paper.
Describe how a ramp can make lifting a box easy without changing the amount of work being done.
A ramp allows the use of a smaller force exerted over a longer distance, so that work is unchanged.
How does a ramp make doing work easier?
A ramp makes doing work easier by multiplying the force that is applied.
What is the Law of Conservation of Energy?
Energy cannot be created or destroyed.
What is the relationship between energy and work?
Energy is the ability to do work.
True or False: Energy always stays in the same form.
FALSE. Energy readily changes from one form to another.
True or false: Work is measured in calories.
FALSE. Work is measured in joules.
work
Force exerted on an object that causes it to move
Using moving pulleys or more than one pulley at a time will increase or decrease the mechanical advantage?
INCREASE
Define kinetic energy.
Kinetic energy is the energy an object has due to its motion.
How do machines help do work?
Machines help do work by changing the size of an input force, the direction of the force, or both.
Mechanical energy is the sum of _______ ______ and _________ ______.
Mechanical energy is the sum of kinetic energy and potential energy.
What is the process of multiplying the force?
Multiplying the force happens when machines make the force greater by decreasing the distance over which the force is applied.
What is useful work?
Only a portion of the work done by any machine is useful work, the work that the machine is designed or intended to do.
What is the word that means 'a quantity that measures the rate at which work is done or energy is transformed'?
POWER
Define potential energy.
Potential energy (PE) is sometimes called energy of position because it results from the relative positions of objects in a system.
How are work and power related?
Power is the rate at which work is done.
What is a third-class lever?
Since third-class levers multiply the difference rather than the force, they have a mechanical advantage of less than one. Example: a person's forearm. The biceps muscle, which is attached to the bone near the elbow, contracts a short distance to to move the hand a long distance.
True or false: All levers have a rigid arm that turns around a point called the fulcrum.
TRUE
True or false: Electrical energy results from the location of charged particles in an electric field.
TRUE
True or false: Gravitational potential energy depends on both mass and height.
TRUE
True or false: Gravitational potential energy is calculated by multiplying force x distance.
TRUE
True or false: Kinetic energy depends on both the mass and the speed of an object..
TRUE
True or false: Kinetic energy is expressed in kilograms times meters squared per second squared, which is equivalent to joules.
TRUE
True or false: Measurements of energy and work are expressed in the same units (joules).
TRUE
True or false: Pulleys are modified levers.
TRUE
True or false: Whenever work is done, energy is transformed or is transferred from one system to another system.
TRUE
True or false: Work is zero when an object is not moving.
TRUE
True or false: Some kinds of wedges, such as nails, are used as fasteners.
TRUE! Yes, nails are wedges.
True or false: A wheel and axle is a lever connected to a shaft.
TRUE. A wheel and axle is made of a lever or a pulley (the wheel) and connected to a shaft (the axle).
True or false: Different forces can do the same amount of work.
TRUE. For example, lifting a box directly onto a truck and pushing the box up a ramp require the same amount of work.
Define efficiency.
The efficiency of a machine is a measure of how much useful work a machine can do.
What is second-class lever?
The fulcrum is at one end of the arm, and the inout force is applied to the other end. Examples: wheelbarrows, nutcrackers, hinged doors
What is a first-class lever?
The fulcrum is between the points of application of the inout and output forces. Example: claw hammer
Simple machines are divided into 2 families. What are the two families?
The lever family and the pulley family
How is a pulley like a lever?
The point in the middle of a pulley is like the fulcrum of a lever. The rest of the pulley behaves like the rigid arm of a first-class lever.
How do machines make work easier?
They change the size of a force needed, the direction of a force, or the distance over which a force acts. Machines redistribute the work that we put into them and can change the direction of the input force.
What machines are used on bicycles?
Tire lever: A tire lever is used to pry the tire from its rim. Air pump: An air pump compresses air to increase the pressure in the tires. Wrench: A wrench changes the direction of an applied force.
True or false: A screw is an inclined plane wrapped around a cylinder.
True
True or false: A wedge is formed a modified inclined plane.
True. An inclined plane is formed of two inclined planes placed back to back. A wedge turns a single downward force into two forces directed out to the sides.
Work is calculated as force times distance. What units are used to express work? Hint: There are multiple equivalent units, which means work can be measured in different (but equal) units.
Work can be expressed as: newtons times meters (N x m), joules, and kg times meters squared per second squared. 1 N x m = 1 J = 1 kg x m2/s2 So, if a math problem uses 'N x m', you could substitute joules or 'kg x m2/s2- They all mean the same thing: the measure of how much 'work' is being done.
Can energy be present in an object if it is at rest?
YES
Can kinetic energy become potential energy?
YES
Can potential energy become kinetic energy?
YES
What is a factor?
a condition or event that brings about a result
What are some examples of a screw?
a drill bit, jar lids, spiral staircase
What is a compound machine?
a machine that combines two or more simple machines
What is it called when multiple pulleys are combined into a single unit?
block and tackle
How does an inclined plane reduce the force needed to lift an object?
by applying the force over a longer distance
What is the efficiency equation?
efficiency = useful work output divided by work input Efficiency is usually expressed as a percentage.
What are electromagnetic waves made of?
electric and magnetic fields
A 65kg rock climber ascends a cliff. What is the climber's gravitational potential energy at a point 35 m above the base of the cliff?
gravitational PE = mass x free-fall acceleration x height GIVEN: mass = 65 kg , height = 35 m free-fall acceleration = 9.8 m/s2 UNKNOWN: grav. PE grav. PE = 65 kg x 9.8 m/s2 x 35 m grav. PE = 2.2 x 10^4 J
What is the equation for gravitational potential energy?
gravitational PE = mass x free-fall acceleration x height PE = mgh
Which 3 simple machines belong to the inclined plane family?
inclined plane, wedge, screw
What is the kinetic energy of a 44 kg cheetah running at 31 m/s?
kinetic energy = 1/2 x mass x speed squared GIVEN: mass = 44kg , speed = 31 m/s UNKNOWN: kinetic energy KE = 1/2 x 44kg x 31 m/s KE = 2.1 x 10^4 J
When fireworks explode, potential energy is converted to many other forms of energy, such as what?
kinetic energy, sound energy, light energy, and heat energy
Pushing an object up an inclined plane requires more or less input force than lifting the same object does?
less input force
Which 3 simple machines belong to the lever family?
lever, pulley, wheel & axle
What are the six types of simple machines?
lever, pulley, wheel and axle, inclined plane, wedge, and screw
Both a short ramp and a long ramp reach a height of 1 m. Which ramp has a greater mechanical advantage?
long ramp
Calculate the mechanical advantage of a ramp that is 5.0 m long and 1.5 m high.
mechanical advantage = input distance divided by output distance GIVEN: input distance = 5.0 m , output distance = 1.5 m UNKNOWN: mechanical advantage mechanical adv. = 5.0 m divided by 1.5 m mechanical advantage = 3.3
What is the equation for mechanical advantage?
mechanical advantage = output force divided by input force = input distance divided by output distance
What is energy that lies at the level of the atom?
nonmechanical energy
Nuclear power plants use what process to release nuclear energy?
nuclear fission
The sun gets energy from what type of reactions?
nuclear reactions The sun's energy comes from nuclear fusion.
Chemical energy is which kind of energy?
potential energy
What is gravitational potential energy?
potential energy that depends on height
What are three forms of energy?
potential, kinetic, mechanical
Lifting an elevator 18 m takes 100 kJ. If doing so takes 20 s, what is the average power of the elevator during the process?
power = work divided by time GIVEN: work = 100 kJ (1 x 10^5 J) , time = 20 seconds UNKNOWN: power p = w / t p = 1 x 10^5 J divided by 20 s p = 5 x 10^3 Watts p = 5 kW
What is the equation for power?
power = work divided by time P= W / t
What are some examples of a wheel and axle?
screwdriver, steering wheel, and doorknob
What is mechanical energy?
the amount of work an object can do because of the object's kinetic and potential energies
What is elastic potential energy?
the potential energy of an object that is stretched or compressed
power
the rate at which work is done, or how much work is done in a given amount of time.
Define mechanical advantage.
the ratio between the output force and the input force Mechanical advantage is also equal to the ratio between the input distance and the output distance IF friction is ignored.
What is power measured in?
watts
The steering wheel of a car is what type of simple machine?
wheel and axe
What is the word that means 'the transfer of energy to an object by the application of a force that causes the object to move in the direction of the force'?
work
Imagine a father playing with his daughter by lifting her repeatedly in the air. How much work does he do with each lift if he lifts her 2.0 m and exerts an average force of 190 N? (pg. 432)
work = force x distance GIVEN: force = 190 N , distance = 2.0 m UNKNOWN: work W = 190 N x 2.0 m W = 380 N x m 380 N x m = 380 J = 380 kg x m2/s2 All of these units measure work. They are equal.
What is the equation for work?
work = force x distance W= Fd
