Work and Machines
If you exert a force of 20 newtons to push a desk 10 meters, how much work do you do on the desk?
200 joules
wedge
A device that is thick at one end and tapers to a thin edge at the other end.
machine
A device that makes work easier
watt
A measure of power equal to one joule of work per second.
lever
A rigid bar that is free to move around a fixed point
pulley
A simple machine that consists of a rope that fits into a groove in a wheel
wheel and axle
A simple machine that consists of two attached circular or cylindrical objects that rotate about a common axis, each one with a different radius.
screw
A simple machine that is an inclined plane wrapped around a central cylinder to form a spiral
joule
A unit of work equal to one newton-meter
The ideal mechanical advantage of a wheel and axle is the radius of the wheel times the radius of the axle.
False, radius of wheel divided by radius of the axle
A wheel and axle is a compound machine.
False, simple
In machine 2, which letter represents the input force?
In machine 2, G represents the input force.
In what class of lever is the direction of the input force opposite to the direction of the output force?
In the first class lever
Why is it more difficult to steer a bike when your hands are close together on the handlebars than when they are far apart?
It is more difficult because of where the fulcrum is located.
Explain how the ideal mechanical advantage and efficiency of a machine determine the machine's actual mechanical advantage.
It is often related to the measurements of a machine. What have you calculated so far (output force divided by input force) is actual mechanical advantage. If you know input work and the output work for a machine you can calculate a machine's efficiency.
How can a hockey stick be considered a machine?
It multiplies distance
Is machine 1 classified as a simple or a compound machine?
Machine 1 is classified as a simple machine.
Which machines multiply the input force?
Machines 4 and 5.
To which class of lever does each of the following machines belong: (i) fishing pole; (ii) wheelbarrow; (iii) bottle opener; (iv) pliers?
The i belongs to third class levers, the ii belongs to the second class lever, the iii belongs to second class levers, the iv belongs to first class levers.
The output force of a lever with an ideal mechanical advantage of 3 is used as the input force of a pulley system with an ideal mechanical advantage of 2. What is the ideal mechanical advantage of the compound machine? Explain.
The ideal mechanical advantage is 6. You take the input force and multiply it by the output force.
efficiency
The percentage of the input work that is converted to output work
Explain why wedges and screws are actually types of inclined planes.
The threads on a screw act like an inclined plane to increase the distance over which you exert the input force. When you use a wedge, instead of moving an object, you move the inclined plane itself.
A bricklayer lifts a stack of bricks onto his shoulder, carries it across a room, and then lifts the bricks onto a ledge above his head.Explain if work is being done in each of these three situations.
Work is being done to the bricks.
inclined plane
a flat, sloped surface
compound machine
a machine made of more than one simple machine
work
any time you exert a force on an object that causes the object to move some distance
One of the simple machines that turns is the wheel and
axle
One example of a compound machine is a
bicycle
A single fixed pulley changes the of a force.
direction
A machine makes work easier by multiplying force or , or by changing direction.
distance
Machines are devices that allow you to in an easier way.
do work
Divide output work by input work and multiply by 100% to find a machine's
efficiency
A knife is an example of a lever.
false
A machine's mechanical advantage is the output force multiplied by the input force.
false
If a compound machine is made of a pulley with a mechanical advantage of 3 and a wheel and axle with a mechanical advantage of 5, the mechanical advantage of the compound machine is 8.
false
If a machine changes the direction, but not the amount of the input force, the mechanical advantage is greater than 1.
false
If the machine increases distance, the output force is greater than the input force.
false
If you lift a box from the floor to a height of 1 m and then carry the box for 10 m, you do work only when you carry the box.
false
If you pull at an angle instead of in the direction in which the object moves, more of your force does work.
false
The ideal mechanical advantage of an inclined plane is the length of the plane divided by its force.
false
The lower arm acts like a first-class lever when it bends at the elbow.
false
The more power you use to move an object, the more work you do.
false
The part of a wheel and axle with the smaller radius is the wheel.
false
The thicker the wedge, the greater its mechanical advantage.
false
To find the mechanical advantage of a wheel and axle, multiply the radius of the wheel by the radius of the axle.
false
Joule is the rate at which work is done.
false, the amount of work done
fulcrum
fixed point on a lever
If tight scissors have an efficiency of 50 percent, half of your work is wasted due to
friction
The fixed point that a lever pivots around is the
fulcrum
The ideal mechanical advantage for an inclined plane is equal to the length of the incline divided by the
height of the incline
A flat, sloped surface is called a(n)
inclined plane
A ramp in a parking garage is an example of a simple machine called a(n)
inclined plane
The threads of a screw act like a(n)
inclined plane
The force you exert when you use a machine is the
input force
The unit of work is the
joule
A system of fixed and pulleys is called a block and tackle.
movable
simple machine
one of the six basic types of machines, which are the basis for all other forms of machines
In a compound machine, the of one simple machine becomes the input force of another simple machine.
output force
The force exerted by the machine is the
output force
Work divided by time equals
power
The wedge, screw, and lever are all
simple machines
power
the amount of work done per unit of time
output force
the force exerted on an object by a machine
The mechanical advantage of a machine is the number of times a machine increases
the force exerted on the machine.
input force
the force you exert on a machine
mechanical advantage
the number of times a machine increases a force exerted on it
For work to be done on an object,
the object must move some distance as a result of a force.
A bicycle is a compound machine because it contains more than one simple machine.
true
An ideal machine has no friction.
true
If the output force is greater than the input force, the mechanical advantage of the machine is greater than one.
true
In a first-class lever, the fulcrum is between the input force and the output force.
true
Power is the rate at which work is done.
true
The farther you move an object, the more work you do.
true
The force exerted by a machine is called the output force.
true
To find the mechanical advantage of a pulley, count the sections of rope that support the object.
true
When you use a wedge, instead of moving an object along the inclined plane, you move the inclined plane itself.
true
Work is done on an object when the object moves in the same direction in which the force is applied.
true
You do work on an object when you lift it off the floor onto a shelf.
true
A device that is twice as powerful as another can do the amount of work in the same amount of time.
twice
The unit of power is the
watt
Power is measured in units called
watts
A device that is thick at one end and thin at the other end is a
wedge
A simple machine that might be thought of as an inclined plane that moves is a
wedge
A screwdriver and a doorknob are examples of a(n)
wheel and axle
A screwdriver is a simple machine called a(n)
wheel and axle
Force multiplied by distance equals
work
Power is equal to divided by time.
work
The force you exert on an object multiplied by the distance the object moves is equal to , which is expressed in joules.
work