CHAPTER 1Matter in Motion
SECTION 1: MEASURING MOTION A Reference Point is: an object that
appears to stay in place
Motion: an object’s change in position overtime relative to a reference point
Motion can be north, south, east, west, up and down.
Common reference points are: the Earth’s surface, trees, buildings, and sometimes other moving objects
During the interval between the times that these pictures were taken, the
hot-air balloon changed position relative to a reference point—the
mountain.
Speed: the distance traveled divided by the time interval during which the motion occurred
Example: Time = 10s and Distance=50m
Speed = distance ÷ time = 50m ÷ 10s = 5 m/s
Standard units (SI) for speed is meters per second (m/s).
Other commonly used units are:
Kilometers per hour (km/hr)Feet per second (ft/s)Miles per hour (mi/h)
Not everything moves at a constant rate though so you need to be able to calculate average speed.
Average Speed =total distance total time
SPEED ON A GRAPH: SHOWN ON A GRAPH OF DISTANCE VERSUS TIME
PRACTICE PROBLEM: An athlete swims a distance from one
end of a 50 m pool to the other end in a time of 25 s
What is the athlete’s average speed?
Step 1: Write the equation for average speed:
Average Speed =total distance total time
Step 2: Replace the total distance and total time with the values given, and solve
NOW YOU TRY!!!1. Kira jogs to a store 72 m away in a
time of 36 s. What is Kira’s average speed?
2. If you travel 7.5 km and walk for 1.5 hr, what is your average speed?
3. An airplane traveling from San Francisco to Chicago travels 1,260 km in 3.5 hr. What is the airplane’s average speed?
Velocity: the speed of an object in a particular direction
Is like an object’s rate of change of that objects position
If speed and direction don’t change then velocity is constant, so constant velocity is motion along a straight line
The speeds of these cars may be similar, but the
velocities of the cars differ because the cars are
going in different directions.
Resultant Velocity: is a combination of 2 or more velocities
Acceleration: the rate at which velocity changes over time
An object accelerates if its speed, direction or both change
Positive acceleration means an increase in velocity
Negative Acceleration means a decrease in velocity or what is called deceleration
faster the velocity changes, the greater the acceleration is
CALCULATING AVERAGE ACCELERATION
Acceleration is measured in meters per second per second, or m/s2
EXAMPLE:A plane passes over point A a velocity of 240 m/s
north. Forty seconds later, it passes over point B at a velocity of 260 m/s north. What is the plane’s average acceleration?
Starting velocity = 240 m/sFinal velocity = 260 m/sTime = 40 s
Plug in your numbers:
260-240 = 20m/s = 0.5m/s2 north 40 s 40s
What is the average acceleration of a subway train that speeds up from 9.6 m/s to 12 m/s in 0.8 s on a straight section of track?
CIRCULAR MOTION: An object traveling in a circular motion
is always changing its direction so its velocity is always changing, so it’s accelerating.
This is called centripetal acceleration
The blades of these windmills are constantly changing direction. Thus, centripetal acceleration is occurring
SECTION 2: WHAT IS A FORCE?
Force: a push or pull exerted on an object in order to change the motion of the object
Force has size and direction
Newton: the SI unit for force, symbol is N
All forces act on objects
Forces do not always cause motion
Example: you are exerting a force on your chair, however the chair doesn’t move. That is because the floor is exerting a force on the chair
You cannot see all forces! For example the air around you is held near Earth’s surface by the force of gravity.
Something that you cannot see exerts a
force that makes this cat’s fur stand up.
The bulldozer is exerting a force on the pile of soil. But the pile of soil also exerts a force by just sitting
on the ground!
Net Force: the combination of all of the forces acting on an object
When forces are moving in the same direction, you add them together
When forces are moving in opposite direction you subtract the smaller force from the larger one.
When the forces on an object produce a net force of 0N, then those forces are balanced
Balanced forces do not cause a change in motion of an object
When the Forces of an object are not 0N, the forces are unbalanced and a change in motion occurs.
When forces act in the same direction, you add the forces to determine the net force. The net force will be in the same
direction as the individual forces.
When two forces act in opposite directions, you subtract the smaller force from the larger force to determine the
net force. The net force will be in the same direction as the larger force.
Because all the forces on this house of cards are balanced, none of the cards move.
The soccer ball moves because the players exert an unbalanced force on
the ball each time they kick it
A boy pulls a wagon with a force of 6 N east as another boy pushes it with a force of 4 N east. What is the net force?
A dog pulls on his chew toy with a force of 10 N towards the east. His owner pulls on the other end f the toy with a force of 15 N to the west. What is the net force?
SECTION 3: FRICTION: A FORCE THAT
OPPOSES MOTION
Friction: a force that opposes motion between two surfaces that are in contact.
Can cause a moving object to slow down or eventually stop
Occurs because the surface of any object is rough
When the hills and valleys of one surface stick to the hills and valleys of another surface, friction is created
Two factors that affect the amount of friction are:
1) the force pushing the surfaces
together and
2) the roughness of the surfaces.
If the force pushing objects together increases then the hills and valleys come in closer contact and the friction increases
The rougher the surface, the greater the friction is
TWO TYPES OF FRICTION:
1. Kinetic Friction: friction between moving surfaces
Mount depends on how the surfaces move
Sliding objects experience more friction than rolling objects
2. Static Friction: when a force is applies to an object but does not cause the object to move
The force of the friction is balanced with the force applied
Can be overcome with a large enough force
WAYS TO REDUCE FRICTION: Lubricants: substances
applied to reduce friction between objects (motor oil, wax, grease, air on a hockey table, etc)
Switching from sliding to rolling friction (using ball bearings in wheels)
Make surfaces that rub together smoother (sanding wood surfaces)
When you work on a bicycle, watch out for the chain! You
might get dirty from the grease or oil that keeps the chain moving freely. Without
this lubricant, friction between the sections of the chain would quickly wear the
chain out.
WAYS TO INCREASE FRICTION:
Make surfaces rougher (sand on ice, textured batting gloves, etc)
Increase the force pushing the surfaces together (pressing harder on sandpaper)
No one likes cleaning dirty pans. To get this chore done quickly, press
down with the scrubber to increase friction.
SECTION 4: GRAVITY: A FORCE OF ATTRACTION
http://www.youtube.com/watch?v=efzYblYVUFk&feature=fvwrel
Gravity: a force of attraction between two objects that is due to their masses
Gravity is a result of mass, so matter is affected by gravity
Objects experience a pull toward all other objects, called gravitational force
Because Earth has such a large mass, it also has a very large gravitational force, pulling everything toward its center
SIR ISAAC NEWTON – A BRITISH SCIENTIST Proposed the idea of gravity
He said that an unbalanced force on apples caused them to fall from trees, and that an unbalanced force on the moon kept it circling the Earth; and that both of these forces were really the same, a force of attraction called gravity
Wrote the Law of Universal Gravitation, which describes the relationships between gravitational force, mass, and distance
Sir Isaac Newton realized that the same unbalanced force affected the
motions of the apple and the
moon
http://www.youtube.com/watch?NR=1&v=jwPc0kK9VHU&feature=fvwp
Law of Universal Gravitation: All objects in the universe attract each other through gravitational force; the size of the force depends on the masses of the objects and the distance between the objects.
http://www.youtube.com/watch?v=Jk5E-CrE1zg
TWO PARTS:
1. Gravitational Force Increases as Mass Increases
2. Gravitational Force Decreases as Distance Increases
Venus and Earth have approximately the same
mass. But because Venus is closer to the sun, the
gravitational force between Venus and the sun is
greater than the gravitational force between
Earth and the sun
The gravitational force between objects decreases as the distance
between the objects increases. The length of the arrows indicates the strength of the gravitational force
between two objects
Weight: is a measure of the gravitational force on an object; SI unit is Newtons (N)
Mass: is the amount of matter in an object; SI unit is a kilogram (kg) but is also measured in grams (g) or milligrams (mg)
http://www.youtube.com/watch?v=SegMt7sa42E
The Vomit Comet: http://www.youtube.com/watch?v=BTkFIE_-kL8