1.20 Understand types of energy, conservation of energy and energy transfer. -- Explain work in terms of the relationship among the applied force to an object, the resulting displacement of the object, and the energy transferred to an object. -- Explain the relationship among work, and power The SI unit of work is the joule (J), which is defined as the work expended by a force of one newton through a distance of one meter.

(Careful! This is a very difficult question) A waiter carries a tray full of meals above his head by one arm straight across the room at constant speed. The tray weighs 100g and the distance across the room is 200 meters. How much work was done by the waiter carrying the tray? (Careful! This is a very difficult question)

When a force acts upon an object to cause a displacement of the object, it is said that work was done upon the object. 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.

Work Done = Force x Distance moved in the direction of the force

A waiter carries a tray full of meals above his head by one arm straight across the room at constant speed. This is not an example of work. There is a force (the waiter pushes up on the tray) and there is a displacement (the tray is moved horizontally across the room). Yet the force does not cause the displacement. To cause a displacement, there must be a component of force in the direction of the displacement.

A book falls off a shelf and free falls to the ground. This is an example of work. There is a force (gravity) which acts on the book which causes it to be displaced in a downward direction (i.e., "fall").

force to a wall and becomes exhausted. A teacher applies a force to a wall and becomes exhausted. This is not an example of work. The wall is not displaced. A force must cause a displacement in order for work to be done.

Practice Question How much work is done in pushing an object 7.0 m across a floor with a force of 50 N and then pushing it back to its original position? Work = 7 m X 50 N X 2 = 700 N/m or J

Practice Question A weightlifter lifts a 1600 Newton barbell over his head. The barbell is lifted to a height of 1.0 meters. How much work does he do? Work = 1600N x 1.0m = 1600 N/m or 1600 J

It is said that knowledge is power It is said that knowledge is power. Find the definition of POWER and use it to either support or refute the validity of this claim.

Power is the rate at which work is done. 1.20 Understand types of energy, conservation of energy and energy transfer. -- Explain work in terms of the relationship among the applied force to an object, the resulting displacement of the object, and the energy transferred to an object. -- Explain the relationship among work, and power Power is the rate at which work is done.

Power = Work / time or P = W / t The quantity work has to do with a force causing a displacement. Work has nothing to do with the amount of time that this force acts to cause the displacement. The quantity that has to do with the rate (time) at which a certain amount of work is done is known as the power. Power is the rate at which work is done.

Power is a measure of how fast the work is done.

Check for Understanding Two physics students, Ben and Bonnie, are in the weightlifting room. Bonnie lifts the 50 kg barbell over her head (approximately .60 m) 10 times in one minute; Ben lifts the 50 kg barbell the same distance over his head 10 times in 10 seconds. - - Which student does the most work? - - Which student delivers the most power? Explain your answers. Ben and Will do the same amount of work. They apply the same force to lift the same barbell the same distance above their heads. Yet, Ben is the most "power-full" since he does the same work in less time. Power and time are inversely proportional.

The standard metric unit of power is the Watt The standard metric unit of power is the Watt. As is implied by the equation for power, a unit of power is equivalent to a unit of work divided by a unit of time. Thus, a Watt is equivalent to a Joule/second. For historical reasons, the horsepower is occasionally used to describe the power delivered by a machine. One horsepower is equivalent to approximately 750 Watts

Another Formula for Power

Check Your Understanding During a physics lab, Jack and Jill ran up a hill. Jack is twice as massive as Jill; yet Jill ascends the same distance in half the time. - - - Who did the most work? ______________ - - - Who delivered the most power? _________ Explain your answers. Jack does more work than Jill. Jack must apply twice the force to lift his twice-as-massive body up the same flight of stairs. Yet, Jill is just as "power-full" as Jack. Jill does one-half the work yet does it one-half the time. The reduction in work done is compensated for by the reduction in time.

Force=Mass x Acceleration How much power will it take to move a 10 kg mass at an acceleration of 2 m/s/s a distance of 10 meters in 5 seconds? This problem requires you to use the formulas for force, work, and power all in the correct order. Force=Mass x Acceleration Work=Force x Distance Power = Work/Time

Force=Mass x Acceleration How much power will it take to move a 10 kg mass at an acceleration of 2 m/s/s a distance of 10 meters in 5 seconds? This problem requires you to use the formulas for force, work, and power all in the correct order. Force=Mass x Acceleration Force=10 x 2 Force=20 N Work=Force x Distance Work = 20 x 10 Work = 200 Joules Power = Work/Time Power = 200/5 Power = 40 watts

Practice Questions Work = 7 m X 50 N X 2 = 700 N-m J; How much work is done in pushing an object 7.0 m across a floor with a force of 50 N and then pushing it back to its original position? How much power is used if this work is done in 20 sec? Work = 7 m X 50 N X 2 = 700 N-m J; Power = 700 N-m/20 sec = 35 W or

Check for Understanding 1.Two physics students, Ben and Bonnie, are in the weightlifting room. Bonnie lifts the 50 kg barbell over her head 10 times in one minute; Ben lifts the 50 kg barbell the same distance over his head 10 times in 10 seconds. Which student does the most work? Which student delivers the most power? Explain your answers.

Power = Work / time or P = W / t The quantity work has to do with a force causing a displacement. Work has nothing to do with the amount of time that this force acts to cause the displacement. Sometimes, the work is done very quickly and other times the work is done rather slowly. The work is the same for both students. They apply the same force to lift the same barbell the same distance above their heads. Yet, Ben is the most "power-full" since he does the same work in less time. Power and time are inversely proportional. The quantity that has to do with the rate at which a certain amount of work is done is known as the power. Power is the rate at which work is done. It is the work/time ratio. Mathematically, it is computed using the following equation. Power = Work / time or  P = W / t