3. WORK
Work is defined as the product of the net
force acting on a body and the distance
moved in the direction of the force
Work = force x parallel distance
W = F x d
SI unit: J - joules
(1 joules = 1Nm = 1kgm2/s2)
5. Only the component of the applied force, F, in
the direction of the motion of the lawn mower,
FHorizontal, is used to do work on the lawn
mower.
WORK
6. Practice!
1. An object is moved with a force of 15 N
across a horizontal surface. How much work
is done if the object is moved 50 m?
2. 650 J of work is done in moving a desk a
horizontal distance of 5 m. How much force is
used to move the desk?
3. How much work is done in lifting a 10 kg box
1.5 m off the fl oor?
7. How much work is done in lifting a 10 kg
box 1.5 m off the fl oor?
W=Fd
W = (9.81 N)(1.5 m)
= ?
Practice!
10. POWER
Power is the rate at which work is done
SI unit: watts (1 watt=1 joule/s)
11.
12. Which person below does more work?
Which person has greater power? Explain.
13. 1. A force of 150 N is used to push a motorcycle
10 m along a road in 20 s. Calculate the
power in watts.
2. An 80 kg man runs up a flight of stairs 5.0 m
high in 10 seconds. What is the man’s power
output in watts?
3. A pump lifts 30 kg of water a vertical distance
of 20 m and 50 seconds. What is the power
output?
Practice!
14. Problem #1. How much power is required to do 200
joules of work in 4 seconds?What if you do the same
work in 2 seconds? Did you notice anything? Say what
you noticed.
Problem #2.
Which person is more powerful?
It takes John 2 seconds to lift a 100-kg barbell a
distance of 20 centimeters.
It takes Peter 3 seconds to lift a 200-kg barbell a
distance of 15 centimeters.
Problem #3.
The power of a machine is 4 kilowatts. How much
work can this machine do in 1 second?
20. EXAMPLE:
A 200 kg roller coaster car that is moving with a
speed of 12.0 m/s. What is the value of its
kinetic energy?
Answer: 14,400 J
21. Practice!
1. What is the kinetic energy of a 60 kg girl on
traveling at 20 m/s?
2. A sports car is moving at 4.0 m/s. If the mass
of the car is 800 kg, how much kinetic energy
does it have?
22. Potential Energy
• PE is stored energy
The compressed spring has
potential energy because
when released it can do work
on the mass, m
• Gravitational PE is energy
of position
24. EXAMPLE:
A 1.0 kg brick placed on top of a 2.0 kg toy car is
carried along an inclined plane to a height of a
tabletop that is 0.50 m high. Determine the
gravitational potential energy of the toy car at the
table top.
Answer: 14.7 Nm or 14. 7 J
25. Practice!
1. A mass of 100 kg is lifted a distance of 50 m.
How much potential energy does it possess?
2. A 70kg diver standing on a diving platform
possesses 35000 J of PE. How high is the
platform?
26. Conservation of Energy
• The sum of kinetic energy and potential
energy in a system is constant, in absence of
friction.
• Energy cannot be created or destroyed; it
may be transformed from one form to
another, but the total amount of energy in a
system remains constant.
28. Practice!
1. A 10kg boulder rests at the edge of a 100m
cliff.
a. How much potential energy does the rock
possess
b. The rock rolls off the cliff and falls to the
bottom. How much kinetic energy does the
rock possess at the bottom of the cliff?
