Transcript what is the force

TAKS
Objective 5
Motion , Forces
and Energy
Motion can be described as
a
change in an
object’s position
 Average velocity
(speed) is the
change of position
of an object over
time
Velocity Graphs
V = distance
time
Velocity
(v) is
the slope (rise
over run) of a
position (d) vs.
time (t) graph
Distance (m)
 Velocity
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Series1
20
Series2
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1 3 5 7 9 11 13 15
Time (sec)
40 The diagram represents the total travel of a
teacher on a Saturday. Which part of the trip is
made at the greatest average speed?
FQ
How do we work this one?
GR
Calculate v = d/t for each segment.
HS
J T
Acceleration Graphs
Acceleration (a) is
the slope of a
velocity (v) vs. time
(t) graph
 Plotted on a
distance vs. time
graph, acceleration
is an exponential
curve

Velocity ((m/s)(m)
Acceleration
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20
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1
3
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Time (sec)
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Acceleration is a change in an
objects velocity (speed or
direction)
When an object’s
speed changes over
time it is accelerating
(or decelerating)
 A = vfinal – vinitial
time
 Units for acceleration
m/s/s or m/s2

Definition of a Force

A Force is a
push or a pull
Balanced Force

A force that
produces no
change in an
object’s motion
because it is
balanced by an
equal, opposite
force.
4 The picture shows the position of a ball
every 0.25 second on a photogram.
Using a ruler, determine the velocity of
the ball.
F 3.5 cm/s
G 10.5 cm/s
H 14.0 cm/s
J 28.0 cm/s
Use the ruler on the side of the
chart and the equation for velocity.
The answer was H.
Measure from the center of ball 1 to the
center of ball 2 and multiply by 4.
Unbalanced
Forces
Are forces
that results
in an
object’s
motion
being
changed.
+
Friction
A force that acts in a
direction opposite to the
motion of two surfaces in
contact with each other.
Friction
Friction causes an
object to slow
down and stop.
Since the amount of
energy stays
constant, the
energy becomes
heat.
Newton’s 1st Law of
Motion
 Object
in
motion
stays in
motion
Newton’s 1st Law of
Motion
And
Objects
at rest
stay at
rest
Newton’s 1st Law of
Motion
 Until
they are acted upon
by unbalanced forces.
Inertia or Newtons 1st Law
Tendency for an
object to stay at
rest or moving in
a straight line at
a constant speed.
 The mass (m
measured in kg)
of an object
determines its
inertia

Newton’s 2nd
Law of Motion
Force = Mass X
Acceleration
F=ma
Weight (pull of gravity) is a
commonly measured force,
calculated by F=mg, g is the
acceleration due to gravity 9.8
m/s2
Newton’s 2nd Law of Motion
The greater the
mass of an
object, the
greater the
force required
to change its
motion.
Newton’s 2nd Law of Motion
 The
greater the
acceleration of
an object, the
greater the
force required
to change its
motion.
11 The frog leaps from its resting position at the lake’s
bank onto a lily pad. If the frog has a mass of 0.5 kg
and the acceleration of the leap is 3 m/s2, what is the
force the frog exerts on the lake’s bank when
leaping?




A 0.2 N
B 0.8 N
C 1.5 N
D 6.0 N
Formula chart says F=ma, m is mass
in kg, a is acceleration in m/s2.
So, .5 kg x 3 m/s2= 1.5 N
Newton’s 3rd Law of Motion
 For
every
action force
there is an
equal and
opposite
reaction
force.
Newton’s 3rd Law of Motion
All forces come
in actionreaction pairs
Ex: feet push
backward on floor,
the floor pushes
forward on feet
27 A ball moving at 30 m/s has a
momentum of 15 kg·m/s. The mass of
the ball is —
A 45 kg
B 15 kg
C 2.0 kg
Formula Page says that
Momentum = Mass x Velocity
D 0.5 kg
So 15 kg.m/s = M x 30 m/s
solving for M it is:
Work
Work: using a force
for a distance
W= F xd
 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:
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
so
Force = 100 N
0 J = 100 N x d
distance must be 0
It did not move!
6 Types of simple
machines
 Some
Simple
Machines:
 Inclined planes
 Screws
 Pulleys
 Wheel and axle
 Levers
 Wedge
Universal Law of Gravitation
All objects in
the universe
attract each
other by the
force of
gravity
Universal Law of
Gravitation
1) the mass of the object
doing the pulling, and
Gravity varies depending
on two factors:
2) the distance from the center
of that object
On Earth gravity = 9.8
m/s/s
For
every
second that an
object falls its
speed increases
by 9.8 m/s
Weight= Mass (m) X gravity (g)
 Weight
Unit of mass = kg
 Unit of acceleration =
m/s/s
 Unit of weight = Newton
 1 Newton= about ¼
pound
USE THE FORMULA PAGE
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Some of the
problems
require you to
grid in an
answer. Make
sure you pay
attention to the
decimal point in
the square in
the middle.