Conceptual Physics Work & Power

When a force acts upon an object to cause a displacement of the object, it is said that work was done upon the object. No movement = no work! work done = force x distance

There are three key ingredients to work - force, displacement, and cause. In order for a force to qualify as having done work on an object, there must be a displacement and the force must cause the displacement.

Force and direction

In physics, work represents a measurable change in a system, caused by a force. Work (J) = force used (N) × distance moved in the direction of the force (m)

Work is measured in units of joules (J), where 1 J = 1 N · m = 1 kg · m2/s2. If you push a box with a force of one newton for a distance of one meter, you have done exactly one joule of work. the joule is the unit for both work and energy.

Displacement Are Parallel Work When Force and Displacement Are Parallel If force and displacement are both in the same direction, the work done is the product of the magnitudes of force and displacement.

Work When Force and Displacement Are Not Parallel The amount of work done by a force on any object is given by the equation Work = F * d * cosine(Theta) where F is the force, d is the displacement and Theta is the angle between the force and the displacement vector.

A hammer does work on the nail it strikes

A rocket accelerates through space. This is an example of work. There is a force (the expelled gases push on the rocket) which causes the rocket to be displaced through space.

Work is done by gravity Example: How much work is done by gravity on a water balloon of mass m is dropped from a height h.

How much work is done by gravity if a 5g balloon is thrown horizontally from a height 10m with an initial velocity of v 0?

The work done by the force of gravity is the same if the object falls straight down or if it makes a wide parabola and lands 100 m to the east. This is because the force of gravity does no work when an object is transported horizontally, because the force of gravity is perpendicular to the horizontal component of displacement. W=mgh

W = mgh Work done against gravity When we lift things up we do work against gravity. Because of gravity, anything with mass has weight. Weight is a force and is measured in newtons. W = mgh

How much work does the crane do? Work = mgh W = (1500)(9.8)(50) W = 735,000 Joules

If force and displacement are perpendicular, no work is done. holding a bag of groceries, you do no work – but you get tired!! The upward force is perpendicular to the displacement Thus… W=0.

The force supplied by the waiter on the tray is an upward force and the displacement of the tray is a horizontal displacement

Total displacement must be positive for work to be done if a weightlifter raises the barbells 1 m, then lowers it to its original position, the weightlifter has not done any work on the barbell.

Quick review Only Fapp does work. Fgrav and Fnorm do not do work since a vertical force cannot cause a horizontal displacement.  Wapp= (10 N) * (5 m) *cos (0 degrees) = +50 Joules Only Ffrict does work. Fgrav and Fnorm do not do work since a vertical force cannot cause a horizontal displacement. Wfrict =(10 N) * (5 m) * cos (180 degrees) = -50 Joules   Fapp and Ffrict do work. Fgrav and Fnorm do not do work since a vertical force cannot cause a horizontal displacement. Wapp = (10 N) * (5 m) * cos (0 deg) = +50 Joules Wfrict = (10 N) * (5 m) * cos (180 deg) = -50 Joules

On occasion, a force acts in the direction opposite the objects motion in order to slow it down. A force that acts opposite to the direction of motion of an object does negative work.

If you push against the direction of motion, you are doing negative work

Energy Work Energy is the capacity for doing work. You must have energy to accomplish work work is done on an object when you transfer energy to that object

When a car or truck brakes, the kinetic energy lost by the vehicle is the same as the work done by the brakes braking distance = work done by the brakes ÷ braking force

Energy Work The net work done on an object is equal to the change in its kinetic energy. If the work is positive work, then the object will gain energy. If the work is negative work, then the object will lose energy. The gain or loss in energy can be in the form of potential energy, kinetic energy, or both.

If a 1kg book were raised 1m in 1 second, Power is defined as the rate at which work is done or energy is converted power is measured in watts (W), representing the generation or absorption of energy at the rate of 1 Joule/sec If a 1kg book were raised 1m in 1 second, the power would be approx 10kg m per second = 10W

Both the pallet jack and forklift do the same amount of work However, the forklift is more powerful since it performed the work in only 2.5 sec compared to the 6 sec it took the pallet jack to finish the same amount of work

Light bulbs are rated by their power output Light bulbs are rated by their power output. A 75 W incandescent bulb emits 75 J of energy each second. Much of this is heat. Fluorescent bulbs are much more efficient and produce the same amount of light at a much lower wattage.

The power of an engine is typically measured in horsepower, a unit established by James Watt and based on the average power of a horse hauling coal. 1 hp = 33 000 foot pounds per minute = 746 W. A machine that applies 33,000 pounds of force over a distance of one foot over a time period of one minute is operating at 1 hp.

Electric companies charge customers based on how many kilowatt hours of energy used. It’s a unit of energy since it is power × time. 1 kW·h is the energy used by a 1000 W machine operating for one hour.

Connecting all three concepts

Quick Review Two physics students, Clevis and Clovis, are in the weightlifting room. Clevis lifts the 100-pound barbell over his head 10 times in one minute; Clovis lifts the 100-pound barbell over his head 10 times in 10 seconds. Which student does the most work? Which student delivers the most power?

Clevis and Clovis do the same amount of work Clevis and Clovis do the same amount of work. They apply the same force to lift the same barbell the same distance above their heads. Clovis is the most "power-full" since he does the same work in less time. Power and time are inversely proportional.

A machine is a tool used to make work easier A machine makes work easier to perform by accomplishing one or more of the following functions:

How do machines work? The best way to analyze what a machine does is to think about the machine in terms of input and output.

Machines are rated on their mechanical efficiency Mechanical efficiency measures the ratio of the useful work derived from a machine to the energy put into it No machine has 100 percent efficiency due to friction .

When a machine takes a small input force and increases the magnitude of the output force, a mechanical advantage has been produced. Mechanical advantage is the factor by which a machine multiplies the force put into it. If a machine increases an input force of 10 pounds to an output force of 100 pounds, the machine has a mechanical advantage (MA) of 10. .

a typical automotive jack has a mechanical advantage of 30 or more

Machines Work No machine can produce more work than the amount of work that is put into the machine. Thus we cannot invent a machine that decreases the actual amount of work needed to do a job

In physics the term “simple machine” means a machine that uses only the forces directly applied and accomplishes its task with a single motion.

Ordinary machines are typically complicated combinations of simple machines. There are six types of simple machines:

Lever A lever is a stiff bar that rests on a support called a fulcrum which lifts or moves loads. The closer the fulcrum is to the load the less force you have to use.

a screwdriver acts as a first class lever when using it to pry open a paint can

a second class lever is used when a person stands on tip-toe.

Pitchers use their arms as third-class levers.

Tweezers and tongs are pairs of third-class levers with the same fulcrum.

Identify the class of levers for Quick Review Identify the class of levers for The following items

wheel and axle can be thought of as simply a circular lever A wheel and axle is made from a wheel that turns on a center post (called the axle). The larger the wheel is, the less force will be needed to move the load.

A Ferris wheel is an example of a wheel and axle

the force pushes in the same direction that the skateboard is moving

Pulley Pulleys are simple machines that consist of a rope that slides around a disk, called a block. Their main function is to change the direction of the tension force in a rope.

Pulleys are used on flag poles

Sail boats use a pulley system known as “block and tackle” in their rigging

A block and tackle is two sets of pulleys linked by a single rope.

An inclined plane (ramp) decreases the force required to raise an object a given height by increasing the distance over which that force must be applied

Two inclined planes put back to back is known as a wedge When you use a wedge, instead of moving an object along the inclined plane, you move the inclined plane itself.

The latching mechanism, called the slide, is just a set of wedges.

Some people would rather wear the machine than use it !

A screw is really an inclined plane that is wrapped around a shaft.. A thread wraps around a screw at an angle, like the angle of a ramp.

the more the number of threads there is the less the effort is needed to move the load.