Download Chapter 12 Notes - Crestwood Local Schools

Chapter 12 General Physical Science Notes
Forces and Motion
Section 12.1 – Forces
A force is a push or a pull that acts on an object.
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A force can cause a resting object to move, or it can accelerate a moving object by
changing the object’s speed or direction.
Force is measured in Newtons, abbreviated at N.
1 N = 1 kg * m/s2
You can use an arrow to represent the direction and strength of a force.
Combining Forces
A net force is the overall force acting on an object after all the forces are combined.
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Try and determine the net force applied to a car that has run out of gas and 4
people are pushing it toward the gas station. (Draw a picture below.)
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Sometimes the net force acting on an object is zero.
When the forces on an object are balanced, the net force is zero and there is no
change in the object’s motion.
Examples of zero net force are sitting in a chair or leaning against a wall.
When an unbalanced force acts on an object, the object accelerates.
An example of an unbalanced force is when you push something and it moves.
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Friction
All moving objects are subject to friction, a force that opposes the motion of objects that
touch as they move past each other.
In a frictionless world, every surface would be more slippery than a sheet of ice.
Friction acts at the surface where objects are in contact.
There are four main types of friction: static friction, sliding friction, rolling friction, and
fluid friction.
Static friction is the friction force that acts on objects that are not moving. Static friction
always acts in the direction opposite to that of the applied force. A book being pushed
from rest is an example.
Sliding friction is a force that opposes the direction of motion of an object as it slides
over a surface. Because sliding friction is less than static friction, less force is needed to
keep an object moving than to start it moving.
Rolling friction is when a round object rolls across a flat floor, both the object and the
floor are bent slightly out of shape and the friction force that acts on the object is from
rolling objects.
Fluid friction is the force that opposes the motion of an object through a fluid. You feel
fluid friction when stirring thick cake batter.
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Fluid friction acting on an object moving through the air is known as air
resistance. At higher speeds, air resistance can become a significant force.
Gravity
Gravity is a force that acts between two masses.
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Gravity is an attractive force, that is, it pulls objects together.
Earth’s gravitational force exerts a force of attraction on every other object that is
near Earth.
Gravity can act over large distances.
Earth’s gravity acts downward toward the center of Earth.
Both gravity and air resistance affect the motion of a falling object. Gravity causes
objects to accelerate downward, whereas air resistance acts in the direction opposite to
the motion and reduces acceleration.
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As objects fall to the ground, they accelerate and gain speed. With increasing
speed comes increasing air resistance. If an object falls for a long time, the
upward force of air resistance becomes equal to the downward force of gravity. At
this point, the object has reached terminal velocity.
Terminal velocity is the constant velocity of a falling object when the force of air
resistance equals the force of gravity.
Projectile Motion
When you throw a ball forward, you’ll notice that it actually follows a curved path. This
curved path is an example of projectile motion, the motion of a falling object after it is
given an initial velocity.
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The combination of an initial forward velocity and the downward vertical force of
gravity causes the ball to follow a curved path.
12.2 – Newton’s First and Second Laws of Motion
Newton’s First Law of Motion
The state of motion of an object does not change as long as the net force acting on the
object is zero.
For example: a soccer ball resting on the grass remains motionless until a force is applied
to it in the form of a kick.
Newton’s first of motion is sometimes called the law of inertia. Inertia is the tendency of
an object to resist a change in its motion.
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In other words, an object at rest tends to remain at rest, and an object in motion
tends to remain in motion with the same direction and speed unless an outside
force acts on it.
Look at Figure 12 on page 364 and 365 to see how inertia works.
Newton’s Second Law of Motion
The acceleration of an object is equal to the net force acting on it divided by the object’s
mass.
An unbalanced force causes an object to move. In other words, the object accelerates. For
example, you apply a net force to a ball when you throw it. The harder you throw, the
more the ball accelerates. In fact, acceleration is directly proportional to the net force
acting on it.
Mass is a measure of the inertia of an object and depends on the amount of matter the
object contains.
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According to Newton’s second law of motion, the acceleration of an object is
equal to the force acting on it divided by the object’s mass.
The equation for Newton’s second law is as follows:
Acceleration = Force
Mass
OR…..
A=F
m
Acceleration = m/s2
Force = Newtons (N)
Mass = kg
Turn to page 367 in your book and complete the math practice box numbers 1-4.
1.
2.
3.
4.
On page 368, you will see a great example of how Newton’s second law works.
Weight and Mass
Are weight and mass the same thing? Do you have the same mass on Earth as you do on
the moon? Do you have the same weight on Earth as you do on the moon?
Weight is the force of gravity acting on an object. An object’s weight is the product of
the object’s mass and acceleration due to gravity acting on it.
The weight formula is as follows….
Weight = mass * acceleration due to gravity
Weight = Newtons (N)
Mass = kg
Acceleration due to gravity = 9.8 m/s2
Mass is a measure of the inertia of an object; weight is a measure of the force of gravity
acting on an object.
12.3 – Newton’s Third Law of Motion and Momentum
Newton’s Third Law
Whenever one object exerts a force on a second object, the second object exerts an equal
and opposite force on the first object.
These forces are called action and reaction forces.
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Pressing your hand against a wall produces a pair of forces. As you press against
the wall, your hand exerts a force on the wall. This is an action force. The wall
exerts an equal and opposite force against your hand. This is a reaction force.
Some action-reaction forces produce motion. An example would be swimming.
Some action-reaction forces do not produce motion. An example would be
pushing against the wall.
Action-reaction forces do not cancel out because they are not acting on the same
object.
Momentum
If a large truck and a small car are both moving at the same velocity and each has the
same braking force, which one is going to be harder to stop? Which one has a greater
momentum?
Momentum is the product of an object’s mass and its velocity.
An object has a large momentum if the product of its mass and velocity is large.
You can calculate momentum by using the following formula…
Momentum = mass * velocity
OR…
P=m*v
Momentum = p (kg * m/s)
Mass = kg
Velocity = m/s
Conservation of Momentum
According to the law of conservation of momentum, if no net force acts on a system,
then the total momentum of the system does not change.
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In a closed system, the loss of momentum of one object equals the gain in
momentum of another object = momentum is conserved. Look at Figure 17 on
page 376 to better understand how the law of conservation of momentum works.