Standard 8.2
Unbalanced forces cause changes In
velocity.
Anticipatory Set
How do you win at tug-o-war? What does it take?
Standard 8.2 (a-c)
A. Students know a force has both direction and
magnitude.
B. Students know when an object is subject to two
or more forces at once, the result is the
cumulative effect of all the forces.
C. Students know when the forces on an object are
balanced, the motion of the object does not
change.
LANGUAGE OF THE
DISCIPLINE
• Force: is a push or pull. Every force has a
magnitude and a direction.
• Newton: (N) is the unit of measurement for the
magnitude of a force
• Balanced Forces: will not change the motion of
the object
• Unbalanced Forces: causes a change in the
motion of an object.
• Net Force: the sum of all of the forces acting on
the object
Forces
• What causes MOTION
• Forces have a magnitude and direction. (Just
like velocity)
• Magnitude for Force= NEWTONS (N)
• Use ARROWS to indicate direction of forces
and length of arrow to indicate magnitude
– Example:
Net Forces
• NET means total
– You combine all forces on
an object to get NET
• It determines if an object
moves, which direction it
moves, and how far.
Unbalanced Forces
• If you have a net force (not = to 0) then it is
unbalanced forces.
• It will cause a change in velocity (direction or
speed)
Balanced Forces
• There will be NO change in velocity of object
(won’t move)
Check for Understanding
• If two girls push a box the same direction, both
with 5 N of force. What is the net force?
10 N
• If two boys push a wagon with 2 N each and in
opposite directions what is the net force?
0N
• In a tug of war game if team A pulls the rope with
7 N and team B moves the rope with 3 N. What is
the net force?
4 N (towards team A)
Practice & HW
• Guided Practice:
– Read through Guided instruction on page 41-42 and
highlight important information
– Complete the Guided questions in Measuring up on
page 41-42
– Raise hand for a stamp
• Independent Practice:
– Complete questions 1-4 on page 42
• HW:
– complete questions 1-4 on page 43.
Anticipatory Set
How would these surfaces affect an obejct’s motion?
Standard 8.2 (d)
D. Students know how to identify separately the
two or more forces that are acting on a single
static object, including gravity, elastic forces due
to tension or compression in matter, and friction.
LANGUAGE OF THE
DISCIPLINE
• Gravity: force that pulls objects together and
pulls toward earth.
• Compression: is a force that develops when
the molecules of an object are squeezed
together.
• Tension: force that develops when molecules
are stretched apart.
• Friction: force between objects that works
against their movement past each other
Gravity
• Mass: amount of matter in an object
• Weight: amount of gravitational force on an
object
– So mass never changes, but weight depends on
where you are in space
This is why when you
step on a scale it tells
your weight, because
it is how much gravity
is pulling down on you
on to the scale
Gravity and Mass
• Gravity= Mass x 10 m/s2
• Gravity amount doesn’t change if you put
another force on it.
• the more mass the move gravity pulls
• Gravity pulls down and the surface an object is
on pushes up
Elastic Forces
• Matter is elastic if it returns to its original shape after
being squeezed or stretches.
• Compression- SQUEEZING
• Tension- PULLING
• These are the forces that work against gravity to keep
forces BALANCED
• Gravity, Compression, and Tension are forces that act
“up and down”
Friction
• Friction depends on the types of surfaces and
how hard they push together.
• Example: lightly put your hands together and rub
them. Now push your hands together firmly and
try to rub them. Notice the Difference?
• Friction is a SIDE to SIDE force
– So if you place a sideways force on an object it is and
equal force of friction that will keep it stationary
Friction Problem
• Ex: a boy is pulling a box along the floor with
40 N. The force of friction is 10 N between
the floor and the box. What is the net force?
– 40 N (from boy) MINUS (-) 10 N (of friction)
• Net force: 30 N
• ALWAYS SUBTRACT THE FRICTION FORCE
Check for Understanding
• What forces are up and down forces?
–Compression, tension, gravity
• What is a force that deals with surfaces?
–friction
• What is the net force of a 6 N weight hanging
from a spring and then is pulled down 4 N?
–6+4= 10
Practice & HW
• Guided Practice:
– Read through Guided instruction on page 45-47 and
highlight important information
– Complete the Guided questions in Measuring up on
page 45-47
– Raise hand for a stamp
• Independent Practice:
– Complete questions 1-6 on page 47
• HW:
– complete questions 1-4 on page 48.
Standard 8.2 (e-g)
E. Students know that when the forces on an
object are unbalanced, the object will change its
velocity (that is, it will speed up, slow down, or
change direction).
F. Students know the greater the mass of an
object, the more force is needed to achieve the
same rate of change in motion.
G. Students know the role of gravity in forming and
maintaining the shapes of planets, stars, and the
solar system.
LANGUAGE OF THE
DISCIPLINE
• Newton’s 2nd law of motion: motion of an
object will change when unbalanced forces
work upon it
• Inversely proportion: as one quantity
increases, the other quantity will decrease.
• Universal force: force that works on all objects
• Unbalanced forces: (see slide)
Newton’s Second Law
• Acceleration depends on the net force acting on the object
and the object’s mass
Acceleration= Net force
Mass
• So to speed up acceleration you need to either increase
force or decrease mass
• To slow down acceleration you need to decrease force or
increase mass
• SO MASS IS INVERSERLY PROPORTIONAL To
ACCELERATION
Newton’s 2nd Law
Calculating Force
Acceleration= Net force
Mass
So
Force= Mass x Acceleration
(1 N is = to 1kg x m/s2)
Calculating Acceleration
• ex: a wagon has a mass of 10 kg. Sarah
pushes it with 3 N and Brandon pushes it with
8 N in the same direction. There is a force of 9
N of friction acting on it. What is the
acceleration?
– ADD UP FORCES SUBTRACT FRICTION (3+8-9)
– Divide it by mass (10)
– Acceleration = .2 m/s2
Calculating Force
• Example: What force is needed to accelerate
a box has a mass of 40 kg at 5 m/s2?
• F= M x A
• 40 x 5 = 200 N
UNBALANCED FORCES
• Unbalanced forces will cause an object to either
– Accelerate
– Slow down
– Or stop
• Other forces that will change acceleration
– Gravity- will pull objects down (especially if in the air)
– Friction- will slow objects down if touching surface
Reading Informational Text
• Read pages 55-56- ANSWER QUESTIONS GUIDED QUESTIONS AS YOU
READ
• Why is gravity a universal force?
– It exists between all objects in the universe
• Why don’t you float away when you jump or skip?
– Force of gravity pulls you to earth
• What influences gravitational attraction between
objects
– Mass and distance
• What is weight
– Measure of gravity on an object
Gravity between OBJECTS
• The force of gravity between two objects
– increases with greater mass
– decreases with greater distance.
• Gravitational force= Mass of object A X Mass of object b
Distance Between objects
• Example: Which pair of objects has the greatest gravitational
force on each other?
• The 2nd and 3rd
Calculating weight
• On the moon acceleration due to gravity is 1.6
m/s2. On the earth it is 9.8 m/s2. If something
weighs 784 kg on earth, how much will it weigh
on the moon?
• USE CROSS MULTIPLICATION!
1.6 = 9.8
X 784
1.6 (784)= 9.8 x Solve for X
X= 128
Check for Understanding
• What is the equation for force
– F= M x A
• What is the equation for acceleration
– A= F/M
• Three tennis balls are sitting on a table, ball A
and B are 3 feet apart and ball B and C are 10
feet part. Which has a greater gravitational
force between them?
– Ball A and B
Practice & HW
• Guided Practice:
– Read through Guided instruction on page 50-52 and
highlight important information
– Complete the Guided questions in Measuring up on
page 50-52
– Raise hand for a stamp
• Independent Practice:
– Complete questions 1-6 on page 52
• HW:
– complete questions on page 53,56,57.