Transcript Simple Machines, Work and Power

Work
• Work: using a force
for a distance
• W=Fxd
• The work done by forces on an object
= changes in energy for that object.
• Work and Energy are measured in
Joules
• 1 Joule=1 Newton • meter
42 How much work is performed when a 50 kg crate is pushed 15
m with a force of 20 N?
F 300 J
Use the formula Work = Force x distance
G 750 J
H 1,000 J
Force of 20 N x 15 meters = 300 Joules
J 15,000 J
Answer:
6 Types of simple machines
• Some Simple
Machines:
• Inclined planes
• Screws
• Pulleys
• Wheel and axle
• Levers
• Wedge
Why use a machine?
• In an ideal (perfect)
machine the work put
into the machine (Win) =
the work put out by that
machine (Wout)
Machines make work easier
• The ideal mechanical advantage
of a machine (IMA) of a machine
is the number of times the output
force is larger than the input force
IMA=Fout/Fin
• A machine can only make this
happen by moving the input force
through a farther distance than
the output force
• Fin • din=Fout • dout
48 The diagram shows an
electric motor lifting a 6 N
block a distance of 3 m. The
total amount of electrical
energy used by the motor is
30 J. How much energy
does the motor convert to
heat?
F 9J
G 12 J
H 18 J
J 21 J
Work
Input =
30J done
by the
motor
Work Output = Resistance
Force x Resistance Distance
Workout = 18J = 6N x 3m
The difference is lost as heat
due to friction, which is 30J
– 18J = 12J
Answer G
Real Machines use Energy
• No real machine is
100 % efficient. i.e.
none put out more
work than is put in
• Efficiency of a
machine is work
output/work input X
100 %
• Eff = Wout X 100%
W
in
Machines use power
• Power: the rate at which
energy is used (work is
done)
• P=Work/time
• Power is measured in H.P.
or watts
• 1 watt = 1 Joule
1 sec
45 If a force of 100 newtons was
exerted on an object and no work was
done, the object must have —
A accelerated
rapidly
B remained
motionless
C decreased its
velocity
D gained
momentum
Work = Force x Distance
Work = 0 Force = 100 N so
0 J = 100 N x d
distance must be 0
It did not move!