Forces push or pull
The most
common forces are those that occur during contact between objects: A cyclist
pushing a pedal; a driver applying a brake. In these cases, the points at which
the forces are applied are easily identified. The forces that can act
by contact between material objects are Friction, Inertia, Pressure and Tension.
Other forces exist in Force Fields. Examples of these are: Gravity,
Magnetic Forces and Electric Forces. A Force Field is always located in a
particular (or defined) region in space. All force fields actually extend to the
farthest regions of the universe but their strengths beyond the defined regions
are so small that they are ignored. Inside the force field, the direction and
the strength of the force varies with distance from the source or pole.
Vectors
A vector is a
something that has size and direction. The direction in which it acts or moves
must be specified.
All
forces are vectors. They always act in a particular direction.
Forces can also be Active or Reactive. For every
action there is an equal and opposite reaction. If I use my hand to push against
a wall, an area of the wall exerts an equal and opposite pressure on my hand.
Inertia is a force that opposes motion. All material objects have
inertia. The mass of an object is the measure of its inertia. The inertia of an
object acts in a direction opposite to the direction of a force that is causing
movement. If an astronaut on a space walk throws a baseball, he (or she) will
accelerate in the opposite direction while he or she is pushing the ball. The
astronaut acts on the baseball and the inertia of the ball acts on the
astronaut. Both the baseball and the astronaut will then continue to move at a
constant speed until each is acted upon by a decelerating (or accelerating)
force.
The strength or size of a force is measured by its ability to
accelerate an object. The size of a force is equivalent to the mass of the
object multiplied by the rate at which the object is accelerated. Acceleration
is the rate of change of position.
The Speed of an object is the rate at
which it moves through space – irrespective of direction or change in direction.
It is the distance that it travels in a particular amount of time.
Displacement is the net distance that an object covers. (“As the crow
flies”) If an airplane flies from San Francisco to Los Angeles and then back to
San Francisco, its displacement is zero. If it flies from San Francisco to Los
Angeles and then to Chicago, its displacement is the distance from San Francisco
to Chicago.
The Velocity of an object is the net distance that it
travels in a particular amount of time – in a particular direction. Velocity is
the amount of displacement in a particular amount of time. Both speed and
velocity are measured in meters per second, feet per second, miles per hour,
etc. Velocities are vectors. A vector is a something that has size and
direction. The direction in which it acts or moves must be specified.
Acceleration is the rate of change of velocity.
acceleration =
(change in velocity) ÷ time.
Acceleration is a vector when it refers to
the rate of change of velocity. (Acceleration can sometimes be regarded as
a scalar when it refers to rate of change of speed.)
It is possible
for something traveling at a constant speed but to be accelerating if its
direction is changing. For example, something traveling in a
circular path at a constant speed - like a planet orbiting the Sun - is
constantly changing its direction towards the center of its orbit and is
accelerating.
Acceleration can be positive or
negative. A car slowing down to stop at a stop sign is
accelerating because its speed is changing. We might refer to this type of acceleration as deceleration or negative acceleration.