Work , Energy, and Power

2. WORK
Work is the amount of energy by force
through a distance in the direction of
the force. Like energy, it is a scalar
quantity, with SI units of joules.

3. In order for work to be done, the
following conditions must be met.
 There must be a force acting on the object.
 The object has to move a certain distance called
displacement.
 There must be a movement parallel in the direction of the
force.
*The work is calculated by multiplying the force by the amount of movement of an
object (W = F * d).

5. Sample Problem
Suppose a woman is pushing a grocery cart with a 500-newton force along a 7-
meter aisle; how much work is done in pushing the cart from one end of the aisle to
the other?
 Given: Find= _____
F= 500 N
d= 7m
 Solution:
W=Fd
W= (500 N) (7m)
W= 3500 Nm or 3500 J

6. POWER
 Power is the amount of work done per unit of time. Time is
an important factor in calculating power. The shorter the
time required to do the work, the greater the power.

7. Sample Problem
A 60 kg man runs up a staircase 3m high in 2.5 seconds what power did he
developed?
 Given: Find: ______
m= 60 kg
h= 3 m
t= 2.5 s
 Solution:
P=
𝑚𝑔ℎ
𝑡
=
60 𝑘𝑔 (9.8
𝑚
𝑠2
)(3𝑚)
2.5 𝑠
=
1764 𝐽
2.5 𝑠
=705.6 watts

8. ENERGY
 Energy is the capacity to do work. Energy is measured in
terms of work it can do. Energy-like work is a scalar quantity.
Energy is a quantity that can be transformed from one form
to another.
 Work is not energy but a method of transmitting energy.
 The body doing the work loses the energy while the body on
which the work is done gains energy.

9. Kinetic energy
 Kinetic energy is the energy an object has because of its
motion. If we want to accelerate an object, then we must
apply force.
 K=
1
2
mv

10. Sample Problem
Determine the kinetic energy of a 625-kg roller coaster car that is moving with a
speed of 18.3 m/s.
 Given: m= 625 kg Find: __________
V= 18.3 m/s
 Solution:
K=
1
2
mv2
K=
1
2
(625 kg)( 18.3 m/s)2
K=
1
2
(209306.25 kg.m/s)
K=104 653.125 J

11. Potential Energy
 An object can store energy as a result of its position. For
example, the heavy ball of a demolition machine is storing
energy when it is held at an elevated position. This stored
energy of position is referred to as potential energy.
 Potential energy is the stored energy of position possessed
by an object.
 Use these formula: PE = m*g*h

12. Sample Problem
A cart is loaded with a brick and pulled at constant speed along an inclined plane to
the height of a seat-top. If the mass of the loaded cart is 3.0 kg and the height of
the seat top is 0.45 meters, then what is the potential energy of the loaded cart at
the height of the seat-top?
 Given: m= 3.0 kg Find: ___________
g= 9.8 m/s2
h=0.45 m
 Solution:
PE = m*g*h
PE= (3.0 kg) (9.8 m/s2) (0.45 m)
PE= 13.2 J

13. Gravitational Potential Energy

14. Sample Problem
A crane lifts a 75kg mass at the height of 8 m. Calculate the gravitational
potential energy gained by the mass (g = 9.8 N/kg).
 Given: m=75kg Find: ______
g= 9.8 m/s2
h= 8m
 Solution: GPE= mgh
=(75kg)(9.8m/s2)(8m)
=5880J

15. ELASTIC POTENTIAL ENERGY

16. SAMPLE PROBLEM
A truck spring has a spring constant of 5 x 104 N/m. When unloaded, the
truck sits 0.8 m above the road. When loaded with goods, it lowers to 0.7 m
above the ground. How much potential energy is stored in the four springs?
Note: The difference in the height of the truck is 0.1 m, (0.8 m – 0.7 m). This
tells us the compression of the springs Δx. Substituting into the equation for
the potential energy in a spring:

17. SOLUTION
 Given: Find: Elastic Potential Energy
k= 5 x 104 N/m
e= 0.1 m
 Solution: Ee=
1
2
ke2
=
1
2 (5 x 104 N/m)(0.1m)2
=250 J x 4 springs
=There are 1000 J stored in the four springs.

18. CENTER OF MASS AND GEOMETRIC
CENTER
 Every object has some mass, and due to gravitational force, the object weighs a
certain amount
 . The center of mass is a position defined relative to an object or system of
objects. It is the average position of all the parts of the system, weighted
according to their masses. For simple rigid objects with uniform density, the
center of mass is located at the centroid.

19. CENTER OF GRAVITY AND CENTROID

20. MOMENTUM AND IMPULSE
 Momentum is a physics term; it refers to the quantity of motion that an
object has
 Momentum can be defined as "mass in motion." All objects have mass,
so if an object is moving, then it has momentum - it has its mass in
motion

21. Sample Problem
 Rich Gossage set a fastball record by hurling a 0.14 kg
baseball at a speed of 46.3 m/s. What is the magnitude of
the ball’s momentum as it left his hand?
 Given: m= 0.14 kg v= 46.3 m/s Find: p
 Solution: p=mv
p= (0.14 kg) (0.14 kg)
p= 6.5 kg.m/s