WORK and POWER discussion for grade 8 students

WORK AND POWER

1. WORK

2. What comes to your mind when you hear the word ‘work’? https:// www.youtube.com/watch?v=gv4cshx8Khg

3. Learning targets After today’s lesson, the students will be able to: Define what is work and power Understand what is mechanical energy Determine the factors affecting kinetic and potential energy.

4. What is work? Work is done when a constant force is applied on an object and causes an object to move in the same direction as the force applied. Example: A little box pulled by a string towards the right with 50N. WORK = FORCE X DISPLACMENT (Joules)= (Newton) (meters)

5. When we will know that we do work? When the object is moved by the force applied when the direction force and motion is the same Example: Situation A: A person pushing a push cart towards the right. Situation B: A man lifting a box and moving forward.

6. What is considered as work done in physics: You push a heavy shopping trolley for 10m You lift your school bags upwards by 1m.

7. What is not considered as work done: You push against a wall Jumping continuously on the same spot Holding a chair and walking around the classroom.

8. Example Suppose you push the table eastward with a force equal to 50 N. Then, it moves 10 m east. How much work did you exerted? Suppose the floor is rough. As you push the table to the east, the frictional force would oppose the movement of the table. The frictional force is equal to 15 N and the table moves 5 m east. What will be the value of work done on the table?

9. When the direction of the applied force is the same as the direction of the movement of the object, work done is positive. When the direction of the applied force and the direction of the movement of the object are opposite each other, work done is negative. When the direction of the applied force and the distance covered by it are perpendicular to each other, work is equal to zero. Remember...

10. Work is done if the object you push moves a distance in the direction towards which you are pushing it.

11. No work is done if the force you exert does not make the object move.

12. No work is done if the force you exert does not make the object move in the same direction as the force you exerted.

13. Exercise # 1 Identify the one doing the work and on which object the work is done.

14. Identify the one doing the work and on which object the work is done.

15. Sample problem A librarian pushes the book cart eastward with a force of 20 N. The book cart moves 15 m east. How much work done on the book cart? Suppose a woman is pushing a grocery cart with a 500 Newton force along the 7 meters aisle, how much work is done in pushing the cart from one end of the aisle to the other? A certain car covered a distance of 5 m, south. The road is rough and a frictional force equal to 1000 N opposes the movement of the car. Calculate the work done on the car.

16. Sample problem How much work do you do by pushing a sack of rice with a force of 100 N across a distance of 5 meters? How much work is done by a student who lifts a 5 kg box to a vertical height of 1.5 m?

17. POWER

18. POWER It is the quantity of work done per unit of time. It measures the rate of the work done in a certain period. It involves two quantities: Work Time

19. Mathematically, power can be described as: Power can also be expressed either in terms of: Force and velocity Force, distance and time

20. The unit of power is Joule per second or J/s or equivalent to Watt (W). Another unit of measurement is the horsepower (hp). It is also used in measuring the power output of some electrical appliances. 1 hp = 746 W

21. example Suppose you have done work equal to 1000 J from either walking up stairs or running down the stairs. Walking up stairs, you reached the top in 20 s. On the other hand running down the stairs you reached the bottom of the stairs in 10 s. What is the amount of power exerted in running and walking? A gym instructor does work equal to 500 J on a barbell in 5 s. What is the amount of power exerted on it? A force of 150 N was applied to a table. It moved 8 m in 16 s. How much power was exerted on it?

22. energy

23. What is energy? In physics, energy is the quantitative property that must be transferred to an object in order to perform work. OR It is the capacity/ability to do work. It is transferred by a force moving an object through a distance. Work is a method of transferring energy.

24. Different forms of energy

25. MECHANICAL ENERGY Mechanical energy is the energy that is possessed by an object due to its motion or due to its position. In physics, mechanical energy is the sum of potential energy and kinetic energy. Mechanical Energy = Kinetic Energy + Potential Energy

26. KINETIC ENERGY Kinetic Energy is a form of mechanical energy that is present in moving objects. Kinetic energy is the energy an object has because of its motion. Example: Running Man

27. what are the factors affecting kinetic energy? The mass of an object. The speed of an object. K.E. = 1/2 mv 2 K.E = mv 2 2

28. POTENTIAL ENERGY Potential energy is a form of mechanical energy that is possessed by an object at rest. It is stored energy by an object due to an object’s position above the ground.

29. What are the factors affecting the potential energy? The mass of an object The height of an object from the ground. P.E. = mgh M= mass G=acceleration due to earth’s gravity(9.8m/s 2 ) H= the height of an object from the ground