Work & Power
If you exert a force of 20 newtons to push a desk 10 meters, how much work do you do on the desk?
20 X 10 = 20 JOULES
The mechanical advantage of a machine is the number of times a machine increases
the force exerted on the machine.
A simple machine that might be thought of as an inclined plane that moves is a
Wedge
Which of the following is a compound machine: door, pliers, bicycle, shovel?
bicycle
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 when the bricklayer lifts the stack of bricks to his shoulder and again when he raises the stack to the ledge. In both situations the bricklayer’s force on the stack and the motion of the stack are in the same direction. When the bricklayer carries the stack, no work is being done because his force on the stack (vertical) and the motion of the stack (horizontal) are not in the same direction.
If tight scissors have an efficiency of 50 percent, half of your work is wasted due to
friction
What is the formula for finding the ideal mechanical advantage for an inclined plane?
Length of the inclined plane divided by the height of the incline
A hockey stick is considered a machine because it multiplies what? Force, distance, direction or friction?
Distance
A screwdriver is a simple machine called a(n) ____________________.
wheel and axle
Explain how the ideal mechanical advantage and efficiency of a machine determine the machine’s actual mechanical advantage.
The actual mechanical advantage of a real machine is always less than the ideal mechanical advantage. The efficiency determines how close the actual mechanical advantage is to the ideal mechanical advantage. As friction is reduced and the efficiency approaches 100%, the actual mechanical advantage approaches the ideal mechanical advantage. If efficiency were 100%, the two would be equal.
Power is measured in units called
Watts
Pulling down on a rope to hoist a sail on a sailboat is an example of a machine that multiplies force, distance, or just changes the direction?
Changes the direction
The force exerted by a machine is called the _________force.
output
A ramp in a parking garage is an example of a simple machine called a(n) ____________________.
inclined plane
Explain why wedges and screws are actually types of inclined planes.
A wedge is shaped like a double-inclined plane. A screw is an inclined plane wrapped around a cylinder to form a spiral. Like an inclined plane, the wedge and the screw multiply the input force by having it move a greater distance along a sloping edge.
A device that is twice as powerful as another can do ____________________ the amount of work in the same amount of time.
twice (2x)
The wedge, screw, and lever are all examples of...
simple machines
A wheel and axle is a _______machine.
simple
You do ______on an object when you lift it off the floor onto a shelf.
work
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 of a compound machine is the product of the ideal mechanical advantages of the simple machines that make it up. So the ideal mechanical advantage of the compound machine is 2 x 3, or 6.
What is the equation to find Power?
Work divided by time
A machine makes work easier by multiplying force or ____________________, or by changing direction.
distance
The ideal mechanical advantage of a wheel and axle is the radius of the wheel _______the radius of the axle.
divided by
______ is the rate at which work is done.
power
Why is it more difficult to steer a bike when your hands are close together on the handlebars than when they are far apart?
The handlebars of a bike are an example of a wheel and axle. The center shaft of the handlebars is the axle. The part of the handlebars you use makes up the wheel. Using the handgrips at the ends of the handlebars gives the largest ideal mechanical advantage because the radius of the wheel is largest compared to the radius of the axle. When you move your hands close together on the handlebars, the radius of the wheel decreases and the ideal mechanical advantage decreases. A lower ideal mechanical advantage makes steering (turning the handlebars) more difficult.