1. General Physics 101 PHYS Dr. Zyad Ahmed Tawfik
Website : zyadinaya.wordpress.com

2. Review and resolve quizzes on work and energy
Lecture No. 7
Review and resolve quizzes on work and energy

3. Calculate work done on an object

4. 1- With out angle
In this case it means that F and x MUST be parallel. To ensure that they are parallel we add the cosine on the end.
Apply FORCE
Displacement

5. 1- With out angle
In this case it means that F and x MUST be parallel. To ensure that they are parallel we add the cosine on the end.
Apply FORCE
Displacement
So the equation used to calculate the work(W) in this case it:
W= F . d

6. Also with friction force
The equation used to calculate the work(W) in this case it:
W= Ff . d
This signal(-) Because in this case the friction force in the opposite direction for displacement
Friction FORCE
Displacement

7. With angle
So in this case, the work done given by
W = F . d cos q

8. 3- Kinetic Energy
and
Work
Vectors

9. K = ½ mv2 (1) What is Kinetic Energy?
Kinetic Energy is the energy of a particle due to its motion
K = ½ mv (1)
K is the kinetic energy
m is the mass of the particle
v is the speed of the particle

10. Work and Kinetic Energy
Also from equation (1) we can Wright
W = Kf - K0 _______(2)
The equation (3) say
(total work done= The final kinetic energy of an object - initial kinetic energy)
_____________________________________________
But the equation 2 can be written in this format
W =½ mvf2 - ½ m v (3)
where k= ½ m v2
But DK = ½ mvf2 - ½ mvo2
so w= DK (5)
The equation (5) say
“the net WORK done on an object is equal to the change in kinetic energy of the object."

11. 4- Potential
Energy
Vectors

12. Potential Energy potential energy is energy stored in an object.
This energy has the potential to do work.
Gravity gives potential energy to an object.
This potential energy is a result of gravity pulling downwards.
Potential Energy means the work done by gravity on the object.
The formula for potential energy (U) due to gravity is U = m.g.h
P.E. = mass x height x gravity

13. Potential Energy and Kinetic energy
Total mechanical energy = kinetic energy + potential energy
E = K + u
Potential Energy and Kinetic energy

14. 6- Power
The Laws of Motion

15. Power Power is the time rate of energy transfer. The Power is given by
One useful application of Energy is to determine the RATE at which we store or use it. We call this application POWER!
Power is the time rate of energy transfer.
The Power is given by
W F . d
P = =
t t
Units of Power
Where the unit of work(W) is joule and unit of time(t) is second. So The unit of power is a Watt
where 1 watt = 1 joule / second

16. also
since, = = =
h =d and F = mg
W F . h m.g.h P
t t t

17. Quiz
Find the potential energy of 20 Kg mass child sitting on a roof 10m above the ground?
A truck is pulling a box of 20 Kg mass on a horizontal surface, a distance of 10 m with a constant speed. The force of friction between the box and the surface is 20 N.
- Find the work it did against the force of friction.
3. A ball of 3 Kg mass was dropped from rest the top of tower
50 m high.
- Find the speed of the ball 20 m above the base of the tower.
4. A car of 800 Kg mass is travelling at 20 m/s speed coasts to a stop in 400 m on a rough horizontal road.
- Find the energy loss.

18. 5. A woman of 50 Kg mass climb s a mountain 4000 m high.
a) . Find the work she did against gravitational forces
b). A Kilogram of fat supplies energy of 3.7x10^7 J. If
she converts fat to energy with efficiency rate of 25%.
How much fat he consumed in the climb.
6. A boy of 50 Kg mass climb’s a wall 500 m high.
a) Find the work he did against gravitational forces.
b) A Kilogram of fat supplies energy of 3.7x10^7 J. If he
converts fat to energy with efficiency rate of 25%. How
much fat he consumed in the climb.
7. A car of 800 Kg mass is travelling at 20 m/s speed
coasts to a stop in 400 m on a rough horizontal road.
- Find the force of friction.

19. 8. A car of 800 Kg mass is travelling at 20 m/s speed hits a
concrete wall and comes to rest after smashing 1.5 meter
of the front of the car.
- Find the reactive force acting on the car body during the
crash.
A man raises a 10 Kg mass vertically upwards a distance of 0.5 m. He practices that 1000 times.
a) Find the work he did against gravitational forces.
b) A Kilogram of fat supplies energy of 3.7x10^7 J. If the
man converts fat to energy with efficiency rate of
25%. How much fat he consumed in the exercise

20. 10. A child of 30kg mass is running with speed 5m/s on a
rough horizontal floor skids a distance 3 m till stopped .
-Find the force of friction?
11. A child 0f 25 kg mass climbs a tower 50m height above
the ground.
Find his potential energy at the top of the tower ?
12. A car of 100 Kg mass is travelling at 15 m/s, speed hits a
concrete wall and comes to rest after smashing 1.5 meter
of the front of the car.
a) Find the kinetic energy of the car
b) Find the reactive force acting on the car body during the
crash.

21. Thank You for your Attention