TOPIC: Forces & Motion

TOPIC: Forces & Motion
Learning Standard 4: The Physical Setting
Key Idea 5: Energy and matter interact through forces that result in changes in motion.
A. Frame of reference- background or object with which movement is compared
1. whenever you describe something that is moving, you are comparing it
with something that is assumed to be stationary, or not moving (frame
of reference)
2. all movement is described relative to frame of reference
A. Motion - change in position in a certain amount of time
B. SPEED - rate at which an object moves
1. Speed = distance/time
2. Constant speed - speed that does not change
3. Average speed = total distance/total time
C. VELOCITY - speed in a given direction
1. Example: 10 m/s east
D. ACCELERATION - rate of change in velocity, how fast velocity is changing
1. a = final velocity – original velocity
2. increasing velocity  positive acceleration object is accelerating
3. decreasing velocity  negative acceleration  object is decelerating
E. MOMENTUM - determines how difficult it is to stop an object’s motion
1. Momentum = mass x velocity
2. Conservation of momentum - the momentum lost by one object is
gained by another
a. the total momentum of any group remains the same unless
outside forces act on the objects
A. Force - a push or pull
1. gives energy to an object, sometimes causing it to start moving, stop
moving, or change direction
a. Example: to open a door, you exert a force on it (pull) to cause it to
B. forces acting in the same direction
1. total force is the sum of both individual forces
C. forces acting in opposite direction
1. total force is the difference between the individual forces (subtract them)
2. object moves in the direction of the greater force
D. unbalanced force -when the total force of an object is in one direction,
changes the motion of an object
E. balanced force - forces that move in opposite directions and are equal in size  no
change in motion
A. Friction - force that acts in a direction that opposes motion of the moving object
1. causes a moving object to slow down and finally stop
2. Example:
a. ice – little friction
b. Sticky surface – a lot of friction
B. Types of Friction
1. sliding friction - when solid objects slide over each other
2. rolling friction - friction produced by objects on wheels or ball bearings
a. tends to be less that sliding friction
3. fluid friction -force exerted by a liquid (water, oil, air)
a. air resistance slows down a falling object
b. Air resistance - opposes the downward motion of a falling object,
slows it down
A. FIRST LAW - an object at rest will remain at rest and an
object in motion will remain in motion at a constant velocity unless acted upon
by an unbalanced force (Law of Inertia)
1. Inertia - tendency of objects to remain in motion or to stay at rest
unless acted upon by an outside force
a. property of matter that tends to resist any change in motion
b. the greater the mass, the greater the inertia
1. shows how force, mass, and acceleration are related
2. a greater force is needed to accelerate an object that has a large mass
3. Force = mass x acceleration
(newtons) kg
4. Newton - unit of mass
a. 1 N = the force required to move 1 kg. At one meter/sec/sec
C. THIRD LAW - for every action, there is an equal and opposite
reaction (every force must have an equal and opposite force)
1. Example: as you walk, your feet push down on the floor, but at the
same time, the floor pushes with an equal and opposite force against
your feet
A. Acceleration due to gravity - the acceleration of a falling object is due to the
force of gravity between the object and the earth
1. A = 9.8 m/s^2
1. all objects in the universe attract each other by the force of gravity
a. size of the force depends on the masses of the object and the
distance between them
b. gravity increases as the masses of the objects increase
c. gravity decreases as the distance between the 2 objects increases
C. Weight - measure of the force of gravity on an object
1. Weight = mass x acceleration