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Physics Foundation:
The force that acts on an object has the ability to change its motion. This change is due to
the function of the force, and furthermore how fast this function lasts. The moving status
of an object changes as all motion changes. When you drive a vehicle from Rice
University to downtown Houston, the position, or location, of the car changes and it is
considered a motion change. In addition, when you stop at a stop sign and try to make a
right turn at Kirby Street, the direction of your vehicle changes, although you may
maintain the same speed, it is also considered as motion change. An object has the ability
and habit to maintain the original or previous status. For an object in rest, it maintains its
resting position to the next moment. This is the presence of its inertia. Furthermore, if an
unbalanced force acts upon it, the force will change the motion of this object from a
resting position. The force here is the necessary excitation to break the object from rest.
In the same logic, the object will maintain the same moving status during its motion since
this is the property of the object, which is called inertia. The object will move in the same
direction and the same speed in velocity, if no other forces exist. You may ask whether
the object will move slower and slower towards stopping. However, friction is the force
that prevents the object from keeping the same moving status, including direction and
speed. This is a famous experiment in history carried out by Sir Isaac Newton, and it is
concluded as Newton’s first law.
Speed is defined as a ruler in measuring how fast an object moves. It is the ratio between
the distance and time elapsed. Speed is the magnitude to express velocity. The concept of
average speed comes from an object which moves at a different speed. How to evaluate
the motion of such an object is a problem. Since velocity changes with time, it is obvious
that to select a local velocity in place of a global velocity is not accurate. The average
speed, however, utilizes the ratio between the total distance and total time that the
moving object travels. This philosophy is so important and widely used that you will see
it again and again during your science adventure. Time, measured in seconds, is graphed
on the x-axis, and distance, measured in meters, is graphed on the y-axis.
Speed is defined as the measuring of how fast an object moves. It is the ratio of the
distance that the object travels and the corresponding time elapsed. Speed is the
magnitude to express velocity. Velocity is a vector in physics. Both magnitude and
direction are required to define it. While speed expresses velocity in quantity, it does not
illustrate the direction of this quantity.
Acceleration is the rate of velocity change during a certain period of time. It is a
measurement to illustrate how fast the magnitude and the direction of velocity changes.
To calculate acceleration, first we need to know the change of velocity V2-V1, where V2
and V1 stands for the velocity at time point 2 and 1. The time (t) elapsed during this
velocity change is also required. Then the acceleration is a = (V2-V1)/t.
One single force may not determine the final movement status of an object since there
can be lots of different forces acting on it. These forces can be balanced or unbalanced.
The net force of all the forces on an object will determine the final motion. Because of
this, the net force can be equivalent to a single force, which is the simple case. This
equivalent single force, or the effect of these unbalanced forces, can change the direction
of the object, decrease the object’s speed by the function of acceleration, or increase its
speed in the opposite way. When an object is at rest on earth, it does not mean there is no
force acting on it. Instead, it means that all the acting forces on it cancel each other out
and remain balanced. For example, you park a vehicle in the parking lot. Is there is no
force on the car since it is not moving and at rest? No. All the forces are balanced and the
net force is zero.
When two forces act on an object, there are two possible results. First they can be
balanced, therefore the net force is zero. In this case the object will not move and stay at
rest. The other possible result is that the net force is not zero, and the object will move in
the direction of the stronger force, if these two forces act on the same line or
dimension.The net force therefore is determined by the difference of the two forces. The
net force direction is based on the stronger force. In general, if there are more than one
forces acting on an object, the best way to analyze is to find the net force. Forces in the
same direction can be equivalent, or a single net force, and can be concluded as the sum
of these forces. The net force will determine the final motion status of the object. The net
force on the object will push it in its direction and increase its speed. Thus, it will be an
acceleration motion since this net force keeps acting on the object.
Generally, motion is the object movement. To set a certain point as relative reference, the
motion can be observed. For example, a dog runs from back yard to the front yard. If the
back yard is set to be the reference, the motion of the dog can be described as movement
between the yards. Speed is the magnitude of the velocity. It is a scalar, which means it
gives numbers to illustrate how big or small it is, but it does not contain any direction
information. In detail, the speed is the measurement of how quickly or slowly an object
moves. But we do not know the direction of this motion due to the restriction of speed.
For instance, one drives 65 miles/hour on highway I-45. This explains that the car moves
at the value of 65 miles per hour. But can you get any information about where this car is
moving? Is it going to Dallas or to Galveston? No, we do not know.
The reason is the speed cannot give any information regarding the movement direction.
In addition, velocity covers the information for both speed and its direction. It is a vector.
In this way, we can say the car moves at 65 miles/hour on highway I-45 north. And this
expression illustrates what the velocity of the car is. From its velocity we know not only
how quickly the car moves, or 65 miles/hour, but also the specific direction of its
movement, which is the north to Dallas.
Furthermore, acceleration reveals the changing rate of the velocity. Since velocity is a
vector, the acceleration is also a vector. There are two parts for the vector. One is
quantity and the other one is the direction. Hence the velocity change in quantity or in
direction can be reflected in acceleration. If a truck moves faster or slower than the last
moment, there must be an acceleration change. The acceleration measures the rate of this
change. If a truck moves at the same speed, is there any change in acceleration? No. The
acceleration is zero, although the velocity is not zero in this case. If the truck makes a
turn with the same speed, is the acceleration zero? Yes, since the direction change is the
velocity change though it maintains at the same speed, and thus the acceleration must not
be zero
Foundation:
The inertia is the property of objects and substances. The law of inertia tells us due to the
inertia, an object will stay at rest if it was at rest. Also, the object will stay in the motion
with the same speed and maintain the same moving direction because of its inertia if
there are not any unbalanced forces to perform on it.
You have that experience when your car is moving forward and then slows down or stops
suddenly. What kind of motion is this? You will still continue to move with the same
speed in the same direction, or moving forward. The reason you stop or slow down is due
to the constraints of the seat belt. It gives the resisting force to prevent you from moving
in the same direction and speed.
Friction is another source of the resisting force. Newton’s law of force and acceleration
gives us the quantity and mathematical expression of how force can influence the object’s
motion. Force is equal to the mass multiplied by the acceleration, or F = ma. Under the
same force, if the object has more mass, the acceleration will be less. Thus more force is
necessary to act upon a heavier object than lighter ones. If the mass is a constant, the
bigger the force is, the higher the acceleration will be.
An earthquake happens when oceanic and continental tectonic plates collide. The forces
that act on each other are the same due to the collision. However, since the mass of
oceanic plate is bigger than that of the continental plate, the acceleration of the oceanic
plate is less than the continental plate’s acceleration. This law tells us that whenever the
force acts, there is an equal and opposite reaction happening together on both sides of the
objects.
Also, the process of rocket launch is making use of the action-reaction principle.
Scientists have discovered how to get enough force to launch the heavy space shuttle
against gravity into the atmosphere. They utilize the stream of fast, hot gases downward.
The force of the hot gases pushing the ground will yield enough force to push the rocket
upward. It is this opposite direction force that launches the rocket into space.