Work and Power. Work: In physics we say that work is done on an object if a force is applied to it and that force causes it to move a certain distance.

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Presentation transcript:

Work and Power

Work: In physics we say that work is done on an object if a force is applied to it and that force causes it to move a certain distance. Work = W = Fd Nm Nm = J = Joule WORK IS ENERGY!!!

What is a Joule? N m kg m m s 2 kg m 2 s 2 The energy required to lift a small apple one meter straight up.

Here’s the important thing about work…work is only done if the force has a component in the same direction as the displacement. m = 10 kg F = 25N d = 2m Is this guy doing work on the box? Yes. The force vector is in the same direction as the displacement. How much work is being done? W = Fd W =25N(2m) W = 50 Nm = 50J

Is work done when pulling this dog? Yes. The force vector has some component in the same direction as the displacement. How much work is being done? d = 10m 30° F FxFx FyFy = 70N F x = cosΘ = A H F x = FcosΘ F x = 70Ncos30° F x = 61N W = Fd W =61N(10m) W =610J

Is work being done by this waiter? No. He could carry around that tray all day and according to physics he wouldn’t be doing any 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.

How much work is needed to lift at a constant speed a 15kg book 3m? W = Fd W = mgd W = (15kg)(10m/s 2 )(3m) W = 450 J

Which path (incline vs. ladder) requires more work to get the box to the top? 10 m 5 m 30° m box = 10 kg W = Fd W = (50N)(10m) W = 500 J W = Fd W = (100N)(5m) W = 500 J Same amount of work!

A particular task may require a certain amount of work but it might be done over different lengths of time… This is known as Power (P). It measures the rate at which work is done. P = W t P = Fd t d = v t P = Fv P = Fd t J = watt = W s

Who has more power? Dan Parker and Brad Bowen are in the weightlifting room. Dan lifts the 50 kg barbell over his head 10 times in one minute; Brad lifts the 50 kg barbell over his head 10 times in 10 seconds. Which student does the most work? Which student delivers the most power? Brad is more "power-full" since he does the same work in less time. Power and time are inversely proportional.

Try this… A crane lifts a load with a mass of 1000kg a vertical distance of 25m in 9s at a constant velocity. How powerful is the crane? P = W = Fd t t = F g d t = mgh t = (1000kg)(10m/s 2 )(25m) 9s = W

Try this… A 45 kg bicyclist climbs a hill at a constant speed of 3 m/s by applying an average force of 80 N. How much power does the bicyclist develop? P = Fv P = (80 N)(3 m/s) P = 240 W

Power is a rate (ENERGY PER SECOND). Your electric bill (power bill) is based on your rate of energy use. A lightbulb with a 60 Watt power rating means that the bulb uses 60 joules of energy per second. W = ΔE T Work = a change in total energy

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