Chapter 11 Motion and Forces
Section 11.1: Measuring Motion
What is Motion?
 Motion is an object's change in position over time when compared with a reference
point.
 Frame of Reference - A frame of reference is a system for specifying the precise
location of objects in space and time. Objects that make up a frame of reference
are treated as if they are not moving. A stationary object used to measure a
distance and time of an object moving away.
Distance – a measurement of how far an object moves
Displacement – a measure of how far you are from the starting point
Speed vs Velocity
 Speed is the rate at which an object moves. Average speed the total distance
traveled divided by the total time interval during which the motion occurred.
 Velocity describes both how fast an object is moving AND what direction it is
moving. Average velocity is calculated by dividing the total distance by the time
interval during which the distance occurred. And a given direction
 Instantaneous Speed is the speed of an object at a given instant, or point in time.
 Constant Speed is speed that doesn’t change, represented by a straight line on a
distant(position)-time graph. Objects’ distance traveled would be the same each
interval of time that passed.
Distance(position)-Time graph.
 The slope of the line connecting two points on a graph of position versus time
equals the change in distance over change in time. Change in the Rise over change
in the Run.
Combining Velocities
5 m/s 
5m/s 
3 m/s 
Assign the two velocities positive and negative based on direction of travel.
For example, Object traveling North positive and an opposing force a negative
number.
Practice – Showing Motion on a Graph
0
Time
1
2
3
4
5
6
1
Distance
10
20
30
40
50
60
2
3
4
Time
1
2
3
4
5
6
5
6
Distance
5
10
15
20
25
30
1. Which car had the fast rate of motion? How could you tell?
2. Which car did not maintain a constant speed? How could tell from the graph?
Section 11.2: Acceleration
How is Acceleration related to Velocity?
 Acceleration is the rate at which velocity changes over time. An object accelerates
if it changes speed, direction, or both.
 An object that changes direction is accelerating, even if its speed is constant.
 Centripetal acceleration is the acceleration that occurs when an object moves in a
circular path. Every object that orbits another object is experiencing centripetal
acceleration.
Velocity-Time Graphs
 Acceleration measures the rate of change in velocity. The average acceleration
equals the change in velocity divided by the time required to make the change. The
acceleration at any moment can be calculated from a velocity - time graph.
Equation for Average Acceleration
 Average acceleration is the average rate at which an object's velocity changes over
some time interval. This rate includes both magnitude and direction.
Acceleration (a) = ____________________________________________
Vfinal = _____________
Vinitial = _____________
Delta Time = _____________
a = _____________
Graphing Acceleration
 Constant Acceleration is acceleration that does not change with time. The slope of
a straight line on a graph of velocity vs. time is equal to an objects’ acceleration. A
line with a positive slope is speeding up, and a negative slope indicates an object is
slowing down.
Label the parts of the graph where the object is slowing down, speeding up, and
has a constant speed.
Determine the acceleration of;
Slope A
Slope B
Slope C
Comparing Distance-Time graphs with Velocity-Time Graphs
Speed vs. Time
Distance vs. Time
1. What does a straight line mean on a speed-time graph? _________________________
2. What does a straight line mean on a distance time graph? _________________________
3. What does a curve line mean on a speed-time graph? _________________________
4. What does a curve line mean on a distance time graph? _________________________
11.3 Forces and Motion Notes
Force is a push or pull that causes a change in motion of an Object
Represented by a capital F
Formula to determine net Force is F = ma (Newtons 2nd Law)
The unit for Force is the Newton represented by a capital N
Four Fundamental Forces in nature
Fundamental Force
Short or Far distances
Strong nuclear Force
Short distances
Relative Strengths (1
strongest)
1
Weak Nuclear Force
Short distances
1/10
Electromagnetic Force
Far distances
1/100
Gravity
Far distances
1/1040
Forces can act through contact or at a distance
Contact Forces – require contact between the surfaces – ex. Friction
Field Forces – act over a distance ex. Gravity or Magnetism
Balanced and Unbalanced Forces
Balances Forces do not change motion.
Unbalanced Forces causes changes in motion, (does not cancel)
The Force of Friction
The force of Friction always opposes motion, and is in the opposite direction of
motion.
Friction occurs because the surface of any surface is not smooth. Friction causes
kinetic energy to be converted to heat energy.
Types of Friction
Static
Kinetic Friction
Sliding
Rolling
Fluid Friction
meaning
b/t stationary surfaces
b/t moving objects
Sliding past each other
Rolling over surface
Contact with Liquid or
gas
examples
Car parked on hill
Car skidding on road
Skateboard, bicycle
Boat or airplane
Friction and Motion
Without friction, many everyday tasks would not be possible.
Getting an object in motion and then stopping are possible due to the force of
friction. Other everyday tasks include writing, cleaning, cooking, cleaning and
personal care.
Unwanted Friction can be lowered
 The use of low friction materials. Ex. Non-stick cookware
 The use of Lubricants – Silicon, Graphite, Oil, Butter, Waxes
Helpful friction can be increased
 Making surfaces rougher increases friction (heat is byproduct)
Ex. Sand on icy roads, baseball gloves
 Increase force between objects that are touching
Ex. Spring on cars and bikes
Ex. Weight on stack of papers on windy day
Cars could not move without friction
Tires push on the road (the cause) and the road pushes back on the car (the
effect)
Because of friction, a force must be applied to keep the car in motion at all
times.