Chapter 5Matter in Motion

Section 1

Measuring Matter

Measuring Motion

Motion is one of the key topics in physics. Everything in the universe moves. It might only be a small amount and very slow, but movement does happen. Don't forget that even if you are standing still, the Earth is moving around the Sun and the Sun is moving around our galaxy. The movement never stops.

The Domino Derby

Open Your Book to page 107 and in your DOS try to respond to the 4 questions listed in “What Do You Think?”

We’ll revisit them later.

Now Let’s Investigate!

Measuring Motion

How do we know something is in motion?

Observing motion

Things are constantly in motion all around us…even if we cannot see them.

We can observe an object in motion when we compare it to a reference point that stays still. We can then describe the motion in terms of direction such as north, south, east, or west.

Observing motion

Sometimes a reference point is moving for example, when you see an airplane flying through the sky while you are riding in a car. You can tell the airplane is in motion because it changes position in relation to you.

What is Speed?

• Speed is a scalar quantity (Scalars are quantities which are fully described by a magnitude (or numerical value) alone. ) which refers to "how fast an object is moving." Speed can be thought of as the rate at which an object covers distance.

• A fast-moving object has a high speed and covers a relatively large distance in a short amount of time.

• A slow-moving object has a low speed and covers a relatively small amount of distance in a short amount of time.

• An object with no movement at all has a zero speed.

Speed depends on Distance and Time

• The rate at which an object moves is its speed.

• Speed depends on the distance traveled and the time taken to travel that distance.

Average v Instantaneous Speed

• Animation

Instantaneous Speed - the speed at any given instant in time. Average Speed - the average of all instantaneous speeds; found simply by a distance/time ratio.

Let’s practice a little more

If you walk for 1.5 hours and travel 7.5 km, what is your average speed?

A bird flies at a speed of 15m/s for 10 s, 20 m/s for 10 s, and 25 m/s for 5 s. What is the bird’s average speed?

Recognizing Speed on a Graph

This graph shows distance traveled over time. Notice time is the independent variable and is graphed on the x-axis

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Velocity

• Velocity is a vector quantity (Vectors are quantities which are fully described by both a magnitude and a direction. )which refers to "the rate at which an object changes its position."

Velocity: Direction Matters

• The speed of an object in a particular direction is the object’s velocity.

• SPEED AND VELOCITY DO NOT MEAN THE SAME THING! Velocity must include a reference direction. For example, the car is traveling 650 km/h south.

Determining Resultant Velocity

Acceleration

• Acceleration is a vector quantity which is defined as the rate at which an object changes its velocity. An object is accelerating if it is changing its velocity.

• Sports announcers will occasionally say that a person is accelerating if he/she is moving fast or speeding up. Yet acceleration has nothing to do with going fast. A person can be moving very fast and still not be accelerating.

Acceleration

• Acceleration has to do with changing how fast an object is moving. If an object is not changing its velocity, then the object is not accelerating.

• Remember that velocity changes if the speed changes, or the direction changes, or if both change

Acceleration

• The data at the right are representative of a northward-moving accelerating object. The velocity is changing over the course of time. In fact, the velocity is changing by a constant amount - 10 m/s - in each second of time. Anytime an object's velocity is changing, the object is said to be accelerating; it has an acceleration. (animation)

Examples of Acceleration

Acceleration or not, and why?

1. You are riding your bike at 9 km/h. Ten minutes later your speed is 6km/h.

2. You ride your bike around the block at a constant speed of 11 km/h.

3. You ride your bike in a straight line at a constant speed of 10 km/h.

Average Acceleration

• The average acceleration (a) of any object over a given interval of time (t) can be calculated using the equation

f= final

i=initial or final

v=velocity

Acceleration Values

Acceleration values are expressed in units of velocity/time. Typical acceleration units include the following:m/s/smi/hr/skm/hr/sm/s2

Visual Representations• Like the study of all of

physics, our study of 1-dimensional kinematics will be concerned with the multiple means by which the motion of objects can be represented. Such means include the use of words, the use of graphs, the use of numbers, the use of equations, and the use of diagrams.

Kinematics is the science of describing the motion of objects using words, diagrams, numbers, graphs, and equations.

Position v Time Graphs

• To begin, consider a car moving with a constant, rightward (+) velocity - say of +10 m/s.

Position v Time Graphs• If the position-time data

for such a car were graphed, then the resulting graph would look like the graph at the right. Note that a motion described as a constant, positive velocity results in a line of constant and positive slope when plotted as a position-time graph.

Position v Time Graphs• Now consider a car moving with a rightward (+),

changing velocity - that is, a car that is moving rightward but speeding up or accelerating.

Position v Time Graphs• If the position-time data

for such a car were graphed, then the resulting graph would look like the graph at the right. Note that a motion described as a changing, positive velocity results in a line of changing and positive slope when plotted as a position-time graph.

Position v Time Graphs

Constant VelocityPositive Velocity

Positive VelocityChanging Velocity

(acceleration)