CHAPTER TWO NOTES - NEWTON'S FIRST LAW OF MOTION
CONCEPTUAL PHYSICAL SCIENCE (EXPLORATIONS HEWITT)
I. SPEED - distance traveled divided by the time it took to get there.
A. CONSTANT SPEED- when the speed stays the same; an object travels equal distances in the same amount of time.
B. AVERAGE SPEED - the result of dividing distance by time; we generally figure the average speed rather than the speed at a particular moment in time.
C. INSTANTANEOUS SPEED - how fast an object is going at amoment in time.
Note: Students need to draw and interpret distance-time graphs.
II. VELOCITY - quantity describing both speed and direction. Example: 45 mph South on Route 27.
A. CHANGES IN VELOCITY - If either the speed or direction changes, the velocity changes.
B. FORMULA - velocity = distance divided by time plus the direction. Example: 12 km/h North is a velocity.
Students should be able to create and interpret distance-time and velocity-time graphs. In a velocity-time graph, anything in the negative sector means heading toward the motion detector while anything in the positive sector means moving away.
Try to graph the following: Stand still 2m from the origin for 8 seconds. Move toward at a slow constant speed for 4s. Stand still for 4 s. Walk away at a fast constant pace.
Practice: Draw a velocity-time graph to match the distance-time graph on the left. You should have the answer to check. This was in your velocity-time graph packet.
Velocity can be represented by vectors which are lines with arrows. The longer a vector, the faster the velocity. Arrow indicates direction.
1 cm = 10 m/s is an example of a scale
Note: Students need to draw and interpret and velocity vectors.
VECTOR QUANTITY - can be represented by a vector (line with arrow at end) and shows both magnitude and direction. Examples of vector quantities are velocity and force.
SCALAR QUANTITY - shows the magnitude only (no direction) so cannot be drawn and isn't as valuable as information as a vector quantity. Examples of scalar quantities are speed, mass, volume.
ACCELERATION
ACCELERATION - Change in velocity divided by the time interval in which the change took place. Any change in velocity is called an acceleration.
CHANGE IN ACCELERATION HAPPENS- when the speed changes (speeding up or slowing down) or the direction changes.
BALANCED FORCES - When forces cancel out so the net force is zero. The object with balanced forces will maintain it's current velocity.
UNBALANCED FORCES - When the net force doesn't equal zero. The object will accelerate.
NET FORCE - the sum of all forces acting on an object. Consider a 30N force to the right and a 10 N force to the left
The net force would equal 20N. The smaller force is considered negative
INERTIA - the tendency of an object to resist a change in motion. The more mass an object has, the more inertia it has. They are directly proportional.
NEWTON'S FIRST LAW OF MOTION - An object at rest remains at rest unless acted upon by an unbalanced force. An object in motion remains in motion at a constant velocity unless acted upon by an unbalanced force.
2.4 Motion Is Relative
Whether or not an object is considered to be in motion depends upon the observer. While you may not be moving relative to a car if you are sitting in the car, you are actually moving the same speed as the car (according to someone standing on the sidewalk).
2.6 Net Force – The Combination of All Forces That Act on an Object
Net force- the change in an object’s state of motion
Newton (N) - the scientific unit of force
Force is a vector quantity
Vectors are added in a free body diagram to determine the net force.
When forces are applied in opposite directions, one force is considered positive (+) and the other is considered to be negative (-) when adding vectors.
2.7 Equilibrium for Objects at Rest
Equilibrium rule- when the net force of an object is zero (mechanical equilibrium)
∑F=0
∑- “vector sum of” F- “forces”
2.8 The Support Force – Why Don’t We Fall Through the Floor
Support force (normal force)- force that pushes in the equal and opposite direction of gravity, canceling it out
2.9 Equilibrium for Moving Objects
An object moving at a constant velocity is in equilibrium. The net force is zero.
So an object at rest is in equilibrium (net force 0) but objects in motion can also be in equilibrium if they are not accelerating.
2.10 The Earth Moves Around the Sun
The Earth moves very rapidly through space in its orbit around the sun. Although objects on earth are moving at the same rate as the Earth, it isn’t noticeable nor does it have an effect because relative to the Earth, we aren’t moving at all.