Chapter 2 - -Motion and Speed

Section 1 – Describing Motion

I. Motion

a. Motion occurs when an object changes its position.

i. Relative motion – you are always moving relative to the universe

ii. Distance – How far an object has moved

iii. Displacement – the distance and direction of an objects change in position from a starting point. (or how far away you are from where you started)

b. Describing Speed

i. Speed – the distance an object travels per unit of time = rate.

ii. Calculating speed

1. Speed = distance / time

2. s = d/t

3. SI units for speed = meters/second = m/s

iii. Measuring speed

1. Constant speed- speed that doesn’t change.

a. Example = light

2. Changing speed- speed that either increases or decreases.

3. Average speed-

a. total distance/total time

4. Instantaneous speed- speed an object is going at that instant = speedometer

II. Graphing Motion

a. Distance-time graph -display the motion of an object over a period of time.

  1. Examples:

No MotionMovingGoing Back

ii. Steeper the graph (greater slope) the higher the speed.

III. Velocity

a. Velocity- Speed and direction of an object in motion.

b. Velocity can change even when speed does not change, because the direction can be constantly changing.

Section 2: Acceleration

I. Acceleration = The rate of change of velocity which occurs when an object changes its speed, its direction, or both.

a. Positive acceleration = speed increases.

b. Negative acceleration = speed decreases.

c. Changing direction when the speed is constant also results in acceleration. Ex: earth is accelerating constantly as it orbits the sun in a near circular path

i. Constant acceleration

II. Calculating Acceleration

a. Acceleration = change in velocity / time

i. Change in velocity = final velocity – initial velocity = vf – vi

b. a = (vf – vi)/t

c. SI units for acceleration = meters/second/second = m/s2(meters per second squared).

Section 3: Motion and Forces

I. Motion and Forces

a. Force = a push or pull that one body exerts on another.

i. Forces can cause a change in motion of objects.

ii. Forces do not always change velocity.

iii. Forces can also cancel each other out. = Balanced Force

b. Net Force = Total amount of force acting on an object at the same time.

i. Unequal forces in opposite directions results in a net force in the direction of the larger force = Unbalanced force.

c. Inertia and Mass

i. Newton’s First Law of Motion was invented by Sir Isaac Newton

1. An object in motion (or at rest) will stay in motion (or at rest) unless an outside force acts upon it.

2. Example:

i. Car crash: occupants keep moving forward even though the car is slowing. Passengers will continue moving forward until the force of the steering wheel, dashboard, or windshield acts as a net force against them. = reason for seatbelts

ii. Another name for the first law of motion is the Law of Inertia.

1. Inertia= the tendency of an object to resist any change in motion.

i. Mass is the factor that determines an object’s inertia