Transcript 2-1 Notes * Combining Forces

2-1 Notes – Combining Forces
Chapter 2, Lesson 1
Force
• When more than one force acts on an object,
the combined effect is caused by the sum of
all applied forces.
• A push or a pull on an object is a force.
Contact and Noncontact Forces
• A contact force is exerted only when two
objects are touching.
• A noncontact force is exerted when two
objects are not touching.
• Magnets exert a noncontact force on each
other.
contact force
noncontact
force
Force is a Vector
•
•
•
•
Vectors have magnitude and direction.
Force has a direction and a magnitude.
Force is measured in Newtons (N).
Arrows can be used to describe forces:
• The width of the arrow indicates the magnitude
• The head of each arrow points in the direction of
the force.
large force
small force
Combining Forces
• When more than one force acts on an object,
the forces combine.
• The combination of all the forces acting on an
object is called the net force.
+
=
+
=
0
Combining Forces in the Same Direction
• Forces are vectors and require a reference
direction.
• When forces in the same direction combine,
the net force is in the same direction and the
size of the force is the sum of the forces.
Combining Forces in the Same Direction
• If two people push the dresser to the right, the
forces are added together.
Combining Forces in Opposite Directions
• When forces in opposite directions combine,
the net force is in the direction of the larger
force.
• The size of the net force is the difference
between the two forces.
Combining Forces in Opposite Directions
• Two people exerting a force in opposite
directions show how vectors combine to
create net force.
Unbalanced and Balanced Forces
• If the net force on an object is not zero, it is an
unbalanced force.
• If the net force on an object does equal zero, it
is a balanced force.
Balanced Forces
Balanced Forces and Motion
• If two people push with the same force, the
dresser does not move.
• The forces have the same magnitude.
• The forces are in opposite directions.
• The net force on the dresser is zero and the
forces are balanced.
• The motion of the dresser does not change.
Unbalanced Forces and Motion
• Unbalanced forces change motion.
• If two people push with different force, the
dresser moves.
• The forces have a different magnitude and are
in opposite directions.
• The net force on the dresser is not zero and
the forces are unbalanced.
• The dresser moves in the direction of the
larger force.
Newton’s First Law of Motion
• If the net force on an object is zero,
the velocity of the object does not
change.
• If the net force is zero and the object
is at rest, it remains at rest.
• If the net force is zero and the object
is moving, it continues to move in a
straight line with constant speed.
Newton’s First Law of Motion
• Also known as the “Law of Inertia”
• Inertia is the tendency of an object to resist
change in its motion unless acted on by a
force.
• The “Lazy Law”
• The “Seatbelt Law”
Newton’s First Law of Motion
Galileo noticed what happened when
you rolled a ball down a track which
went down and then up again. He
realized that if the nature of matter
was to be stopped, it would just stop
when it got to the bottom part of the
track. Instead, it kept going up again,
even against the force of gravity. He
realized that a rolling ball, once it
was moving, tended to keep moving.
Inertia and Mass
• Mass is the amount of matter in an object.
• The more mass an object has, the more inertia it
has and the harder it is to change its motion.
Inertia and Mass
• A bicycle is easier to push than a car because it
has less mass.
Inertia and Mass
• Donkey Kong has more mass than Yoshi, and
thus has greater inertia – harder to get him
started moving from rest, but also harder to stop
him.
2.1 Combining Forces
Which of the following changes
occur when an unbalanced force
acts on an object?
A
motion
B
mass
C
inertia
D
weight
2.1 Combining Forces
What is the net force acting on a
door if one person pushes to the
right with a force of 2000 N and a
second person pushes with a force
of 1500 N to the right?
A
3500 N to the right
B
3500 N to the left
C
500 N to the left
D
500 N to the right
2.1 Combining Forces
You are riding a bike. In which of the
following situations are the forces
acting on the bike balanced?
A
You pedal to speed up.
B
You turn at constant speed.
C
You coast to slow down.
D
You pedal at constant speed.
What causes a change in motion on
an object?
A
zero net force
B
unbalanced force
C
weight
D
balanced force
SCI 2.a
Which of the following best describes
why force is a vector?
A
speed and size
B
magnitude
C
direction and friction
D
magnitude and direction
SCI 2.b
Two people are pushing in opposite
directions on a box. One person is
pushing to the left with a force of
250 N, the other is pushing to the
right with a force of 400 N. What is
the resulting force vector?
A
150 N to the right
B
650 N to the right
C
150 N to the left
D
400 N to the right
A force is acting on each of the objects. What
can be concluded about these forces?
A
B
C
same because they point toward objects
same because they have same magnitudes
different because they have different
magnitudes
D
different because they have different
directions
Four forces are acting on a box, as shown.
This box will increase in speed...
A
B
C
D
downward and to the left
downward and to the right
upward and to the left
upward and to the right
Two students are pushing on a cart. The cart
will move as if it were acted on by a single
force with a magnitude of...
A
B
C
D
50 N
150 N
200 N
350 N