1. Work and Energy
SPH3U

2. Work
Work is the energy transferred to an object when a force F moves it through a distance Dd:

3. Work
Work is the energy transferred to an object when a force F moves it through a distance Dd:

4. Work
Work is the energy transferred to an object when a force F moves it through a distance Dd:
Fcosq is the component of the force in the same direction as the motion.

5. Work
Work is the energy transferred to an object when a force F moves it through a distance Dd:
Note that work is a scalar quantity.

6. Work: Example
A book of mass 1.5 kg sliding across a table is brought to a stop in 1.0 m. The coefficient of kinetic friction between the book and the table is What is the work done by friction on the book?

7. Work: Example
A book of mass 1.5 kg sliding across a table is brought to a stop in 1.0 m. The coefficient of kinetic friction between the book and the table is What is the work done by friction on the book?

8. Work: Example
A book of mass 1.5 kg sliding across a table is brought to a stop in 1.0 m. The coefficient of kinetic friction between the book and the table is What is the work done by friction on the book?

9. Work: Example
A book of mass 1.5 kg sliding across a table is brought to a stop in 1.0 m. The coefficient of kinetic friction between the book and the table is What is the work done by friction on the book?
Note that friction is opposite the direction of motion.

10. Work: Example
A book of mass 1.5 kg sliding across a table is brought to a stop in 1.0 m. The coefficient of kinetic friction between the book and the table is What is the work done by friction on the book?

11. Work: Example
A book of mass 1.5 kg sliding across a table is brought to a stop in 1.0 m. The coefficient of kinetic friction between the book and the table is What is the work done by friction on the book?
1 Newton x metre = 1 Joule

12. Work: Example
A book of mass 1.5 kg sliding across a table is brought to a stop in 1.0 m. The coefficient of kinetic friction between the book and the table is What is the work done by friction on the book?
Note also that the work done is negative: friction reduces the energy of the book.

13. Kinetic Energy
Friction reduced the kinetic energy of the box:

14. Work Energy Theorem W=Fd W=mad W=m(v2-v1)d W= m (v2-v1) (v2+v1)t
t w= ½ mΔ v2

15. Kinetic Energy
Friction reduced the kinetic energy of the box:

16. Kinetic Energy: Example
What was the initial speed of the book?

17. Kinetic Energy: Example
What was the initial speed of the book?

18. Kinetic Energy: Example
What was the initial speed of the book?
zero

19. Kinetic Energy: Example
What was the initial speed of the book?
zero
so the initial kinetic energy must be equal to –W.

20. Kinetic Energy: Example
What was the initial speed of the book?

21. Kinetic Energy: Example
What was the initial speed of the book?

22. Kinetic Energy: Example
What was the initial speed of the book?

23. Kinetic Energy: Example
What was the initial speed of the book?

24. Gravitational Potential Energy
Work can also be done to increase the gravitational potential energy of an object:

25. Gravitational Potential Energy
Work can also be done to increase the gravitational potential energy of an object:
The force required to lift the object against gravity must have a vertical component equal to the weight Fg.

26. Gravitational Potential Energy
Work can also be done to increase the gravitational potential energy of an object:
the vertical displacement

27. Gravitational Potential Energy: Example
You climb a flight of stairs 12 steps high, each step is 20 cm in height. What is your increase in gravitational potential energy?

28. Gravitational Potential Energy: Example
m= 65 kg
h= 0.20m x 12 = m
g= 9.80 m/s
(65 kg) (9.80 m/s)(2.40 m)
= 1.53 x 103 J

29. Work & Potential Energy