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Physical Science Chapter 12
What is Motion?

12-1              Frames   of   Reference

The background or object used for comparison to movement is called the frame of reference.

The most common frame of reference is the Earth.

12-2            Measuring   Motion

Motion is a change of position in a certain amount of time.

Distance is the length between two places.

Speed is the rate at which an object moves.
          
             Speed   =     Distance
                                  Time

Speed is usually measured in meters per second (m/s)  or kilometers per hour (kph).

Constant speed is speed that does not change.

Constant speed is total distance divided by
total time.   (Graph on p305)

When speed varies - dividing total distance by total time gives you "average speed."  
           (Graph on p306)

Velocity is  speed  and  direction.

12-3            Changes   in   Velocity

Acceleration is the rate of change in velocity.

Acceleration = final velocity  - initial velocity
                                        time

Acceleration is measured is measured in
meters per second per second (m/s/s )
or kilometers per hour per hour (km/hr/hr)

Decreasing speed or slowing down (negative acceleration) is called "deceleration."

A change in direction is also acceleration.  

 A car driving around a curve in the road at constant speed is undergoing acceleration.


Circular motion always involves acceleration because direction (and therefore velocity) is always changing.

12-4                Momentum

       All moving objects have momentum!

Momentum depends on velocity and mass.

         Momentum = Mass x Velocity

                 p = m x v

Momentum is measue kilogram-meters per second.           kg-m/s

The total momentum of any group of objects remains constant unless outside forces act on the objects.

Physical Science Chapter 13


13-1    The  Nature  of  Forces
A Force is a push or a pull.

A force will give energy to an object - often causing a change in movement or direction.

Forces acting in the same direction add together.

Forces acting in opposite directions subtract from each other.

When the total force is in one direction it is an "unbalanced force."

An unbalanced force changes an object's motion.

Forces in opposite directions that are equal are called "balanced forces."

13-2         Friction: A Force Opposing Motion

Friction:   A force that acts in a direction opposite to the motion of a moving object.

sliding friction:

rolling friction:

fluid friction:   

Lubricants reduce friction because fluid friction is usually less than sliding friction.

13-3         Newton's   Laws   of Motion

Inertia is "laziness."  It is the tendency of objects to keep on doing what they are already doing.

Newton's 1st Law:      
An object at rest will remain at rest and an object in motion will remain in motion unless an unbalanced force acts on it.

Newton's 2nd Law:
Force =   mass x acceleration       (F=ma)

Force is therefore measured in kg x m/sec/sec
which are units called "newtons."

Newton's 3rd Law:
For every action, there is an equal and opposite reaction.

13-4                 Gravity

Gravity is a force that all matter exerts on all other matter.  The force of gravity is directly proportional to mass (more mass means more gravity) and inversely proportional to distance (the force of gravity decreases as the distance between two objects increases).

All falling objects accelerate at the same rate.

The acceleration of a falling object is due to the force of gravity between the object and the earth.

Acceleration due to gravity on Earth is
            9.8 m/s/s

Air resistance is the force the air exerts against any object moving through it (fluid friction).

Any falling object encounters air resistance.

When the force of air resistance equals the force of gravity a falling object will no longer accelerate, but will fall at a constant velocity; "terminal velocity."

Weight is a measure of the force of gravity on an object.
On Earth,
  weight = mass x acceleration due to gravity

                      w = mg        (g = 9.8 m/s/s)