Chapter 2—Motion and ForceSection 1—Describing Motion

Motion and Speed

•Motion—occurs when an object changes its position

• *Motion doesn’t always have to be seen to have taken place

•Reference Point-used to determine how far something moves–Ex: mailbox while driving

Relative Motion

•Relative Motion–Not all motion is obvious–Everything is in motion relative to something else•Ex: the Earth is always in motion around the sun, etc.

Distance and Displacement

• Distance—describes how far something has moved– SI base unit to measure distance

is METER– Displacement—the distance and

direction of an object’s change in position from the starting point•See figure 2 on pg. 39

Speed

• Speed-the distance an object travels per unit of time– Ex: miles per hour/ feet per

minute, etc.– Rate—any change over time, so--

– **Speed is a RATE– Equation: Speed = Distance /

Time– (typically measured in

m/s)

Practice Calculating Speed-pg. 42-# 1 and 2

• *Most speeds aren’t constant, they either slow down or speed up

• Average speed-describes speed of motion when speed is changing

(driving on a 3 hr. trip)

• Instantaneous Speed-speed at a given point in time– Ex: speed on a speedometer

Velocity

•Velocity--includes the speed of an object AND the direction of its motion

• *Velocity can change EVEN IF speed remains constant

•Change in Velocity—can be EITHER a change in speed OR in direction of movement

Remember:*To Calculate Velocity, it is the same as calculating speed, but you must also report a DIRECTION-north, south, east, west, etc.

*One example of change in velocity—when a race car at a constant speed is rounding a curve, velocity is changing, due to direction of travel changing

Plate Tectonics

• The motion of the Earth’s crust creates the continental drifting that occurs every year

• Plate Tectonics Theory-Pangea• Plates move so slowly that their

speeds are given in units of centimeters per year.

• Australian plate-1 of fastest—moves north at an average speed of 17 cm./yr and San Andreas Fault (in CA) about 1 cm/yr

Section 2—Acceleration

Acceleration-the rate of change of velocity (velocity = speed w/direction)

When the velocity of an object changes, the object is accelerating

So: Acceleration occurs when an object changes its SPEED, its

DIRECTION or BOTH

Acceleration:

If speed is increasing = positive acceleration (acceleration)

If speed is decreasing = negative acceleration (deceleration)

Ex: horse on a carousel-acceleration due to direction only

Equation for finding Acceleration:

Acceleration = change in velocity / time

ORAcceleration = final velocity – initial

velocity / time(usually reported in m/s/s or m/s^2)

(if motion is in a straight line, (no change in direction, use change in speed to calculate

acceleration)

Amusement Park Acceleration:Wooden and steel roller coaster provide different thrills due to

different materialsWooden-swaying effectSteel-more loops, steep inclines, drops, etc.

Section 3: Motion and Forces

Force: a push or pull that one body exerts on another

*A force can cause the motion of an object to change

ex: tennis racket on a ballex: billiards

Balanced forces: forces that are equal, but in opposite directions *they do NOT change the direction of the forceNET FORCE-when two balanced forces are combined the net force will be zero (if equal but in opposite directions)

Unbalanced forces: forces that are unequal and in opposite directions*the greater force will cause an object to move in that directionNET FORCE: with unbalanced forces, will be the DIFFERENCE BETWEEN the two forces

Inertia: The tendency of an object to resist ay change in its motion*If an object is moving, it will keep moving at the same speed and direction unless an unbalanced force acts on it*Or: velocity of the object remains constant unless a force changes it

Newton’s First Law of Motion: (Also called Law of Inertia)*An object in motion will remain in motion unless acted upon by an outside force*An object at rest will remain at rest unless acted upon by an outside forceEx: what happens in a car crash-pg. 55