Inertia and Newton’s 1st Law of Motion

Chapter 2.2 + 2.5

Inertia• Galileo found that a force is required to start

and object moving, but once moving, no force is required to keep it moving.

• The tendency of objects that are moving to stay moving and objects at rest to stay at rest is called Inertia

Newton’s First Law of Motion• Sir Isaac Newton (1642 – 1727) was able to

state rules describing the effect of forces on motion of objects that are still true today.

• These rules are known as:

Newton’s Laws of Motion

• Newton’s First Law of Motion is also called

The Law of Inertia

Newton’s First Law of Motion• An object moving at a constant velocity

keeps moving at that velocity unless a net force acts on it.

• If an object is at rest, it will stay at rest, unless a net force acts on it.

• If an object is in motion, it will stay in motion, unless a net force acts on it.

What Happens in a Crash?

• Newton’s Law First Law of Motion• If you are traveling at 50 kilometers per hour

and you hit a tree…

What Happens in a Crash?

• If you are not wearing your seatbelt…• The car is stopped by the tree and you continue

to move forward at a speed of 50 kilometers per hour until you hit the dashboard, steering wheel, windshield, etc.

• You would hit at the same speed you would reach if you fell from a three-story building.

What Happens in a Crash?

• The seatbelt loosens a little upon impact and will hold you.

• If you are wearing your seatbelt…• This allows your body to be slowed down

gradually and decreases the unbalanced force placed on your body.

• The seatbelt could also save you from being thrown outside the car.

Speed and VelocityChapter 2.3 + 2.4

Speed• Galileo was the first to measure speed by

conidering the distance covered and the time it takes.

• He defined speed as the distance covered per unit of time.– The term per means “divided by”

• The equation we use to define speed is:

Speed =

Distance

Time

• Suppose you ran 2 km in 10 min.– What is your rate?

S = d/t

S= 2 km /10 min.

S= 0.2 km/min.

Remember the units!!!

Constant Speed…• What does constant mean?

• If you are driving on the highway and you set your cruise control, you are driving at a constant speed.

• What would a constant speed graph look like?

Constant Speed Graph

Constant Speed• Now think about how an object’s distance

changes over time when moving at a constant speed…

Two Objects with Constant Speed

• Which one is going faster?

• A steeper gradient indicates a larger distance moved in a given time. In other words, higher speed.

• This means the object represented by the yellow line has a greater speed.

Do you always go the same speed?

• No! Most of the time you are increasing speed, decreasing speed, or stopping completely!

• Think about driving a car or riding a bike!

What is average speed?• Total distance traveled over the total time of

travel

• How do you find an average?- Take the total distance traveled and divide it by the total time of travel

What is average speed?• How do you find an average?

• Average speed is the total distance traveled over the total time

• Avg. Speed = Total d / Total t• Avg S = Td/Tt

What is Instantaneous speed?• What does a speedometer in a car do?

– It shows how fast a car is going at one point in time or at one instant.

• Instantaneous speed is the speed at a given point in time.

What is Velocity?• Speed is how fast something is moving.• Velocity is how fast something is moving and in

what direction it is moving.• Why is this important?

– Hurricanes– Airplanes

Speed or Velocity?

• If a car is going around a racetrack, its speed may be constant (the same), but its velocity is changing because it is changing direction.

Speed or Velocity?• Escalators have

the same speed (constant), but have different velocities because they are going in different directions.

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