Transcript Force - TeacherWeb

Forces and Motion
2
Force
• A force is a push or a pull that one object
exerts on another object.
• Objects like floors, chairs, and Earth also
exert forces on other objects, including
people.
Forces and Motion
2
Force Has Direction and Size
• Just like velocity and acceleration, a force
has both size and direction.
• The direction of a force is the direction of the
push or pull.
• Pushing or pulling harder increases the size
of the force you exert. The size of a force is
measured in newtons (N).
Forces and Motion
2
How Forces Combine
• When more than one force acts on an object,
the forces combine.
• The combination of all the forces acting on
an object is the net force.
Forces and Motion
2
Combining Forces in the Same
Direction
• When two forces act in the same direction on
an object, like a box, the net force is equal to
the sum of the two forces.
Forces and Motion
2
Combining Forces in Opposite
Directions
• If two forces of equal strength act on the box
in opposite directions, the forces will cancel,
resulting in a net force of zero.
Forces and Motion
2
Combining Forces in Opposite
Directions
• When two unequal forces act in opposite
directions on the box, the net force is the
difference of the two forces.
Forces and Motion
2
Balanced and Unbalanced Forces
• When the net force on an object is zero, the
motion of the object doesn’t change.
• The forces acting on an object are balanced
forces if the net force is zero.
• The forces acting on an object are unbalanced
forces if the net force is not zero.
Forces and Motion
2
Contact and Non-contact Forces
• A force is exerted when
one object pushes or
pulls on another.
• A force that is exerted
only when two
objects are touching is
a contact force.
• Non-contact forces are forces that can be
exerted by one object on another even when
the objects aren’t touching.
Forces and Motion
2
Gravity
• Gravity is a non-contact force that every
object exerts on every other object due to
their masses.
• The gravitational force between two objects is
an attractive force that tends to pull the two
objects closer together.
Forces and Motion
2
Gravity
• The size of the gravitational force between
two objects depends on the masses of the
objects and the distance between them.
• The gravitational force between two objects
increases if the mass of one or both of the
objects increases.
• If two objects move closer together, the
gravitational force between them increases.
Forces and Motion
2
Mass and Weight
• Because weight is a force, it is measured in
newtons.
• Weight is not the same as mass. Mass is the
amount of matter an object contains, and is
measured in kilograms.
Forces and Motion
2
Friction
• Friction is a contact force that resists the
sliding motion of two surfaces that are
touching.
• Friction causes a sliding object to slow down
and stop.
• Friction also can prevent surfaces from sliding
past each other.
Forces and Motion
2
Static Friction
• Static friction
is the force
between two
surfaces in
contact that
keeps them
from sliding
when a force
is applied.
Forces and Motion
2
Sliding Friction
• Sliding friction is the force that opposes the
motion of two
sliding surfaces
in contact.
• Sliding friction
exists between
all sliding
surfaces that
are touching.
Forces and Motion
2
What causes friction?
• When two surfaces are in contact, the
surfaces stick to each other where the dips
and bumps on one surface touch the dips and
bumps on the other surface.
• Friction is caused by the sticking of the two
surfaces at these bumps and dips.
Forces and Motion
2
The Buoyant Force
• The buoyant force is a force exerted by a
fluid on an object that is in the fluid.
• The buoyant force is always upward.
• If you are
floating in
water, the
buoyant force
is large enough
to balance your
weight.
Forces and Motion
2
Air Resistance
• Air resistance is a contact force that opposes
the motion of objects moving in air.
• Just like friction, air resistance acts in the
direction opposite to an object’s motion.
• Air resistance is less for a narrow, pointed
object than for a wide, flat object.
Section Check
2
Question 1
A force is a _______ or a _______.
Answer
Force is a push or a pull.
FL: SC.C.2.3.2
Section Check
2
Question 2
A force exerted between two objects that
are not touching is known as _______.
Answer
The answer is a long-range force. Gravity is a
long-range force; although we are not touching
the Sun, our planet is being acted on by the Sun
to keep it in orbit.
FL: SC.C.2.3.1
Section Check
2
Question 3
You try to push a desk across the floor one
direction but a friend is trying to push it the
opposite direction. As a result, the desk
doesn’t move. This is an example of what
kind of forces?
A. balanced forces
B. continuing forces
C. extended forces
D. unbalanced forces
FL: SC.C.2.3.6
Section Check
2
Answer
The answer is A. When forces are balanced, the
net force is zero.
FL: SC.C.2.3.6
The Laws of Motion
3
The First Law of Motion
• According to the first law of motion, if the
forces acting on an object are balanced, then
an object at rest remains at rest and an object
in motion keeps moving in a straight line
with constant speed.
• When the forces on an object are balanced,
the motion of the object doesn’t change.
The Laws of Motion
3
The First Law of Motion
• The forces on an object are unbalanced when
the net force on the object is not zero.
• According to the first law of motion, for an
object to change speed or direction, the net
force acting on the object must not be zero.
The Laws of Motion
3
Changing Direction
• A moving object changes direction when the
net force acting on the object is not in the
same direction as the
object’s motion.
• Then the direction of
motion curves toward
the direction of the
unbalanced net force.
The Laws of Motion
3
The Second Law of Motion
• Newton’s second law of motion describes
how the net force on an object, the mass of
the object, and the acceleration of the object
are related.
• According to the second law of motion, the
acceleration of an object depends on the net
force acting on the object and the object’s
mass.
The Laws of Motion
3
Acceleration and Mass
• Acceleration is larger when the speed of an
object changes more quickly.
• If the net force is the same, the acceleration
of an object decreases as the mass of an
object increases.
The Laws of Motion
3
Acceleration and Direction of the
Net Force
• According to the second law of motion, when
a force acts on an object, its acceleration is in
the same direction as the force.
• When an object slows down, the direction of
its acceleration is opposite to its direction of
motion.
The Laws of Motion
3
Acceleration and Net Force
• When you lift your backpack, its speed
depends not only on its mass, but also on the
force you exert.
• According to the second law of motion, the
acceleration of an object increases when the
net force on the object increases.
The Laws of Motion
3
The Third Law of Motion
• The third law of motion, states that when
one object exerts a force on a second object,
the second
object exerts
an equal force
in the opposite
direction on
the first
object.
The Laws of Motion
3
Force Pairs
• The forces two objects exert on each other
are called force pairs.
• The forces in a force pair act in opposite
directions and are always equal in size.
The Laws of Motion
3
Action and Reaction
• One force of the force pair is called the action
force, and the other force is the reaction
force.
• For every action force, there is a reaction
force that is equal in size, but in the opposite
direction.
The Laws of Motion
3
Action and Reaction
• The swimmer’s hands exert a backward
action force on the water.
• The water then exerts a forward reaction
force on the
swimmer.
This is the
force that
propels the
swimmer
forward.
Section Check
3
Question 1
Newton’s second law of motion states that
_________ of an object is in the same direction
as the net force on the object.
A. acceleration
B. momentum
C. speed
D. velocity
FL: SC.C.2.3.6
Section Check
3
Answer
The answer is A. Acceleration can be
calculated by dividing the net force in newtons
by the mass in kilograms.
FL: SC.C.2.3.6
Section Check
3
Question 2
The forces two objects exert on each other are
called _______.
A. action pairs
B. force pairs
C. gravity
D. reaction pairs
FL: SC.C.2.3.3
Section Check
3
Answer
The answer is B. The forces in a force pair act
in opposite directions and are always equal in
size.
FL: SC.C.2.3.3
Section Check
3
Question 3
When an object slows down, the direction of
its acceleration is _______ its direction of
motion.
A. the same as
B. opposite
C. behind
D. in front of
FL: SC.C.2.3.5; SC.C.2.3.6
Section Check
3
Answer
The answer is B. When you stop a ball from
rolling, the force of your foot is opposite the
direction of motion of the ball.
FL: SC.C.2.3.5; SC.C.2.3.6
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