Motion

Chapter 2

Section 1

Describing Motion

Motion and Position• Motion – when an object changes its position

relative to a reference point• Position of an object is determined relative to

a reference point.

Motion

• Distance – how far an object has moved– SI unit for distance will be the meter (m)

• Displacement – distance and direction of an object’s change of position from a starting point

Distance and Displacement

• The length of displacement and distance will be the same if the motion is in a single direction.

Distance and Displacement

• Distance would be the path that was travelled… while displacement is distance and position from the starting point

Speed

• Speed is the distance an object travels per unit of time. Speed is a rate.– Rate – any change over time

• Speed = distance (in meters)time (in seconds)

• Speed will be measured in rates of meters per second (m/s)

Units of speed

• Sometimes it will be more convenient to measure speed in other units

• Examples– Rocket going into space – km/s– car driving down the highway – km/h– geological plate movements – cm/year– baseball being thrown – m/s

Constant vs. Changing speed

• Speed that does not change over time is a constant speed

• Usually speed will not be constant. When the speed is not held at a constant rate this is called changing speed.

Average Speed

• Speed of motion when sped is changing.– This will use the total distance travelled divided by

the total time of travel.

• Average Speed = total distancetotal

travel time

Instantaneous Speed

• The speed at any given point in time.

Graphing Motion

• A distance-time graph displays motion of an object over time.– Plot distance on a vertical axis (y-axis)– Plot time on a horizontal axis (x-axis)

• The slope of the line plotted on the distance-time graph is the speed.

• Board example

Velocity

• Velocity - The speed and direction of an object’s motion

• Ex. A race car has a constant speed and is going aroundan oval track. Even though the speed remains constant,the velocity changes b/cthe direction of the car’s motion is changing constantly.

Section 2

Acceleration

What is acceleration?

• Acceleration is the rate of change of velocity.• When the velocity of an object changes, the

object is accelerating.– Acceleration occurs when an object changes its

speed, its direction, or both.

Types of Acceleration

1. Positive acceleration – speed is increasing2. Negative acceleration – speed is decreasing3. Changing Direction

• When an object changes speed or direction, it is accelerating.

Velocity

Velocity

Acceleration

Acceleration

When the speed of the car is increasing the car has positive acceleration.

When the speed of the car is decreasing the car has negative acceleration.

Calculating Acceleration

• Acceleration = change in velocitytime

• A = Vf – Vi

t

• Change in velocity = final velocity – initial velocity

• V = Vf - Vi

Calculating Acceleration

• Unit for acceleration – meters per second squared– m/s2

• There can be positive or negative acceleration.– Positive acceleration will have a final velocity that

is more than the initial velocity.

Section 3

Motion and Forces

Motion and Forces

• Force – a push or a pull that one body applies to another

Force

• A force can cause an object’s motion to change

• When two or more forces combine at the same time, they create a net force

• Balanced forces are equal in size and opposite in direction

• Unbalanced forces are unequal in size and/or are not in the same direction

Force Changes Motion

Net Force

Newton’s First Law

• An object at rest will stay at rest unless acted on by another force. And an object in motion will stay in motion unless acted on by another force.– Aka the Law of Inertia

Inertia

• Inertia – an object’s resistance to any change in motion– Objects with greater mass have greater inertia

Newton’s Law of Inertia in Action