Buoyant Force, Work and Power

1. Buoyant Force, Work and Power 2.24-2.28

2. Do Now 2.25.14 If an object is putting a pressure of 45 kPa on an area that is 9 m2, what is the force of the object?

3. Buoyancy • Buoyancy is the apparent loss of weight in a fluid. • How things float

4. Archimedes Principle The buoyant force on an object is equal to the weight of the fluid displaced by the object. What does displaced mean?

5. Density and Buoyancy If an object is less dense than the fluid it is in it will float

6. Density and Buoyancy When the buoyant force is equal to the weight, the object is suspended.

7. Density and Buoyancy When the buoyant force is less than the weight the object will sink When the object is more dense than the fluid it will sink.

8. CFU A 350 N block of wood is thrown into a lake where it floats. What is the buoyant force acting on it?

9. Previous Knowledge What else besides a liquid is considered a fluid?

10. Mythbusters • Our scientific inquiry is low. • Buoyancy is a pretty easy concept to grasps • Things float and sink • Work on inquiry through a mythbusters activity.

11. Mythbusters Looking at buoyancy in air. Evaluate and assess how they are testing buoyancy.

12. Review What are the steps to a scientific investigation?

13. 10 minutes To finish filling in your assessment of their experiment.

14. Discussion

15. Do Now 2.27.14 What do you think of when you think of work? How do you think work applies to physics?

16. What is Work? Moves to the right Objects only move when they have an unbalanced force. Work is when a force acts on an object in the direction it moves

17. Work Requires Motion • For a force to do work on an object • The force must act in the same direction as motion • What would it be called if there is a force acting in the opposite direction? • If there is no movement, no work is done.

18. Work Depends on Direction Any part of a force that does not act in the direction of the motion does not do work.

19. Calculating Work Work= Force x Distance W=Fd

20. Units of Work Work is measured in Joules. 1 joul = 1 Nm Force is measured in ___________ Distance is measured in __________ Thus, Nm makes sense.

21. What is Power? Power is the rate of doing work. Doing work at a faster rate requires _________ power. To increase power increase the amount of work done in a given time.

22. Calculating Power • Power= work/time • Power is measured in watts (W) • Joules/second • Where have you heard of watts before?

23. Light Bulb A 40 watt light bulb requires 40 joules of power per second.

24. Individual Challenge A desk exerts an upward force to support a computer resting on it. Does this force do work? Explain Two cars have the same weight, but one car has an engine that provides twice the power of the other. Which car can make it to the top of the mountain first? Which car does more work?

25. Individual Challenges How much work does a 25 newton force do to lift a potted plant from floor to shelf at a distance of 1.5 meters. You lift a large bag of flour from the floor to a 1 meter high counter, doing 100 joules of work in 2 seconds. How much power did you use to lift the bag?

26. Do Now A person pushes on a box with a force of 85N and it traveled a distance of 5 meters. What was the work on the box? How much work is required to power a 90W light bulb for 2 seconds?

27. Work and Machines A Machine is a device that changes a force. Machines make work easier to do. They change the size of a force needed. Think of a wrench.

28. Machines can… Increase a force Increase a distance Change direction Because of friction, the work done BY a machine is always less than the work done ON the machine.

29. Machines Forces Input Force: The force YOU exert on a machine Input Distance: The distance the input force acts through Work Input: Work done by the input force

30. Machine Forces Output Force:Force exerted by a machine Output Distance: The distance caused by an output force Work Output: Output force multiples by the output distance

31. Row Boat