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Forces and Motion
Chapter 12
A force is a push or pull that acts on an object.
* A force can cause a resting object to move
* A force can accelerate an object by changing its speed or
direction.
* Force is measured in Newtons. A Newton is the force that
causes a 1-kilogram mass to accelerate at a rate of 1 m/s2.
1 N = 1kg*m/s2
* You can use an arrow to represent the direction and strength of
a force.
* The direction of the arrow represents the direction of the
force.
* The length of the arrow represents the magnitude of the
force.
* Forces can be combined. Forces in the same direction can be
added. Forces in opposite direction are subtracted.
* The net force is the overall force acting on an object after all
the forces are combined.
* When the forces on an object are balanced, the net force is zero
and there is no change in the object’s motion.
* Balanced forces have a net force of zero.
* When an unbalanced force acts on an object, the object
accelerates
* Unbalanced forces have a net force NOT equal to zero.
Friction is a force that opposes the motion of objects that touch as
they move past each other.
* There are four main types of friction:
1) Static Friction – the friction force that acts on objects
that are not moving.
* Always acts in the opposite direction to that of the
applied force.
2) Sliding Friction – a force that opposes the direction of
motion of an object as it slides over an object.
* Sliding friction is less than static friction. This means
that less force is needed to keep an object moving
than is needed to start the movement.
3) Rolling Friction – friction force that acts on rolling objects.
* The force of rolling friction is about 100 to 1000 times
less than the force of static or sliding friction.
4) Fluid Friction – the force that opposes motion through a
fluid (liquid or gas).
* Fluid resistance increases as the speed of the object
moving through the fluid increases.
* Air resistance – fluid friction acting on an object
moving through the air.
Gravity is a force that acts between any two masses
* Gravity can act over long distances, unlike friction.
* Earth’s gravity acts downward toward the center of Earth. An
upward force usually balances the downward force.
Ex. Boulder
* Gravity causes objects to accelerate downward, whereas air
resistance acts in the direction opposite to the motion and
reduces acceleration.
* Terminal velocity – the constant velocity of a falling object when
the force of air resistance equals the force of gravity.
Projectile Motion is the motion of a falling object (projectile) after it
is given an initial forward velocity.
* Air resistance and gravity are the only forces acting on a
projectile.
* 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
Developing an understanding of forces and motion took about 2000 yrs.
* Aristotle – incorrectly proposed that force is required to keep an
object moving at a constant speed.
* Galileo – concluded that moving objects not subjected to friction
or any other force would continue to move indefinitely.
* Newton – Defined mass and force and laid out his 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.
Or
An object moving at a constant velocity keeps moving at that
velocity unless a net force acts on it.
* Inertia – the tendency of a object to resist a change in its
motion. Newton’s first law is sometimes called Law of Inertia.
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.
Or
A net force acting on an object causes the object to accelerate in
the direction of the force
* The amount of acceleration depends on the size of the force and
the mass of the object.
* Mass is a measure of the inertia of an object and depends on the
amount of matter the object contains.
*
Acceleration = Net force
or a = F or F = ma
Mass
m
Weight is the force of gravity acting on an object. (Also measured in
Newtons)
* An object’s weight is a product of the object’s mass and
acceleration due to gravity.
* Weight formula:
Weight = Mass X Acceleration due to gravity
Fg = m X g
* Unit for weight: Newton
* The weight formula is basically the same as the formula for
Newton’s second law. Weight (W) has been substituted for Force (F),
and acceleration due to gravity (g) has been substituted for
acceleration (a).
12.3 Newton’s Third Law of Motion and Momentum
Newton’s Third Law of Motion
Whenever one object exerts a force on a second object, the
second object exerts an equal and opposite force on the first object.
Or
For every action there is an opposite and equal reaction.
* Example: Bumper cars.
Action force – the force your car exerts on the other car.
Reaction force – the force the other car exerts back on
your car.
* Other examples: Hammer and nail, swimmer, person jumping off
a boat
* Action and Reaction forces do not cancel because they are acting
on different objects.
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.
* Momentum formula:
Momentum = Mass X Velocity
* Unit for momentum: Kilogram*meter/per second (kg*m/s)
Law of Conservation of Momentum – if no net force acts on a system,
then the total momentum of the system does not change.
* In a closed system, the loss of momentum of one object equals
the gain in momentum for another object – momentum is conserved.
12.4 Universal Forces
Electromagnetic force is a force associated with charged particles,
which has two aspects, electric force and magnetic force.
* Electric force and magnetic force are the only forces that can
both attract and repel.
* Electric forces act between charge objects or particles such as
electrons and protons. Opposite charges attract, like charges repel.
* Magnetic forces act on certain metals, on the poles of magnets,
and on moving charges. Magnets have two poles. Opposite poles
attract, like poles repel.
Nuclear Forces are forces in the nucleus of an atom that hold the
particles together.
* The strong nuclear force is a powerful force of attraction that
acts only on neutrons and protons in the nucleus, holding them together.
* Acts only over small distances, but is 100 times stronger
than the electric force of repulsion at the same distance.
* The weak nuclear force is an attractive force that acts only
over a short range, shorter than the range of the strong nuclear force.
* Involved in certain radioactive processes.
Gravitational Force is an attractive force that acts between any two
masses.
* Newton’s Law of Universal Gravitation states that every object
in the universe attracts every other object.
* The amount of gravitational force depends on distance and mass.
* The greater the mass, the greater the gravitational force.
* The greater the distance, the less the gravitational force.
* Gravity is the weakest universal force, but is the most effective
over long distances.
* A centripetal force is a center-directed force that continuously
changes the direction of an object to make it move in a circle. Ex. Moon
orbits earth, artificial satellites