Chapter 2: MOTION AND SPEED

Section 1—DESCRIBING MOTION

Motion occurs when an object changes its position.

To know whether the position of something has changed, you need a reference point.

A reference point helps you determine how far an object has moved.

An important part of describing the motion of an object is to describe how far it has moved, which is distance. The SI unit

of length or distance is the meter (m).

Sometimes you may want to know not only your distance, but also your direction from a reference point.

Displacement is the distance and direction of an object’s change in position from a reference point.

DISTANCE VS. DISPLACEMENT

What is speed?

Speed is the distance an object travels per unit of time.

Any change over time is called a rate.

Speed is the rate at which distance is traveled.

CALCULATING SPEED

Speed = distance time

If s = speed, d = distance, and t = time, this relationship can be written as:

s = d t

Suppose you ran 2 km in 10 minutes. Your speed or rate of change of position, would be:

s = d = 2 km = t 10 min

0.2 km/min

CONSTANT SPEED

If an object is in motion and neither slows down nor speeds up, the object is traveling at a constant speed. (Ex. Car traveling on a freeway—CRUISE CONTROL)

CHANGING SPEED

Much of the time, the speeds you experience are not constant. (Ex. Riding a bicycle for 5 km)

CHANGING SPEED

AVERAGE SPEEDDescribes speed of

motion when speed is changing.

AVERAGE SPEED is the total distance traveled divided by the total time of travel.

For the bicycle trip, the total distance traveled was 5 km and the total time was 15 min. or .25 h. The AVERAGE SPEED was: s = d = 5 km =

t 0.25 h 20 km/h

INSTANTANEOUS SPEED

INSTANTANEOUS SPEED is the speed at a given point in time. (Ex. CAR’S SPEEDOMETER)

VELOCITY

VELOCITY includes the speed of an object and the direction of its motion.

Ex. HURRICANE—traveling at a speed of 60 km/h; located 100 km east of your location

Velocity

VELOCITY IS SPEED WITH DIRECTION!

VELOCITY

SPEEDsame

DIRECTIONdifferent

(VELOCITY = DIFFERENT)

VELOCITY

SPEED

constant

DIRECTION

changing

(VELOCITY = CHANGING)

VELOCITY

SPEEDconstant

DIRECTIONchanging

(VELOCITY = CHANGING)

SPEED UNITS

REMEMBER…VELOCITY includes the speed and direction of

an object;

Therefore, a change in velocity can be either a change in how fast something is moving or a change in the direction it is moving.

CHAPTER 2: MOTION AND SPEED

Section 2: ACCELERATION

ACCELERATION is a change in velocity.

Acceleration occurs when an object changes its speed, its direction, or both.

When you think of acceleration, you probably think of something speeding up (positive acceleration);

However, an object that is slowing down also is accelerating (negative acceleration). In both cases, acceleration occurs, because its speed is changing.

Calculating ACCELERATIONRemember…

Acceleration is the rate of change in velocity.The change in velocity or speed is divided by

the length of the time interval over which the change occurred.

Acceleration = change in velocitytime

How is the change in velocity calculated?

Always subtract the initial velocity—(the velocity at the beginning of the time interval)—from the final velocity—(the velocity at the end of the time interval).

Change in velocity = final vel. – initial vel.

Change in velocity = vf – vi

a = (vf – vi) = (units) m/s

t s

UNITS

The SI unit for velocity is meters/second (m/s), and the SI unit for time is seconds (s).

So, the unit for acceleration is meters/second/second. This unit is written as m/s2 and is read “meters per second squared.”

CALCULATING POSITIVE ACCELERATION

Suppose a jet airliner starts at rest at the end of a runway and reaches a speed of 80 m/s in 20 s. Because it started from rest, its initial speed was zero. Its acceleration can be calculated as follows:

a = (vf – vi) = (80m/s-0m/s)= 4 m/s2

t 20s

CALCULATING NEGATIVE ACCELERATION

Now imagine a skateboarder is moving at a speed of 3 m/s and comes to a stop in 2 s. The final speed is zero and the initial speed was 3 m/s. The skateboarder’s acceleration is calculated as follows:

a = (vf – vi) = (0m/s-3m/s)= -1.5 m/s2

t 2s

ACCELERATION…Will always be positive if an object is speeding up

Will always be negative if an object slowing down

Chapter 2: MOTION AND SPEED

Section 3—MOTION AND FORCES

What is a force?

A force is a push or a pull that one body exerts on another.

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

OBVIOUS VS. NOT SO OBVIOUSSome forces are

obvious…the force applied to a soccer ball as it is kicked into the goal

Some forces are not so obvious…the force of the floor being exerted on your feet

OR gravity pulling down on

your body

BALANCED FORCES

When two or more forces act on an object at the same time, the forces combine to form the net force.

What is the net force acting on this box?

The net force on the box is zero, because the two forces cancel each other.

Forces on an object that are equal in size

and opposite in direction are called balanced forces.

UNBALANCED FORCES

When two students are pushing with unequal forces in opposite directions.

A net force occurs in the direction of the larger force.

UNBALANCED FORCES

The students are pushing on the box in the same direction.

The net force is formed by adding the two forces together.

IT IS IMPORTANT TO REMEMBER…

Students often assume that NO MOTION = NO FORCE (not true), but an object’s lack of motion is because the forces acting on it are balanced.

NO MOTION = BALANCED FORCESMOTION = UNBALANCED FORCES

What is inertia?

Inertia is the tendency of an object to resist any change in motion. (NEWTON’S 1st LAW—The Law of Inertia)

QUESTION: Would a bowling ball or a table tennis ball have a greater inertia? Why?

Remember—Mass is the amount of matter in an object, and a bowling ball has more mass than a

table-tennis ball.

The INERTIA of an object is related to its MASS.

The greater the mass of an object, the greater its inertia.

↑ MASS = ↑ INERTIA

British Scientist Sir Isaac Newton (1642-1727) was able to describe the effects of forces on the motion of objects.

These rules are known as Newton’s Laws of Motion.

According to 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 (Part I—Car-CC). Also, if an object is at rest, it stays at rest, unless a net force acts on it (Part II—Soccer ball).

SHORT VERSION—Newton’s 1st Law

• An object will resist any change in motion.

What happens in a car crash?

This can be explained by the law of inertia…

When a car traveling about 50 km/h collides head-on with something solid, the car crumples, slows down, and stops within appproximately 0.1s.

A passenger without a seatbelt…Will continue to move forward at the same

speed that the car was travelingWithin 0.02 s after the car stops, unbelted

passengers slam into the steering wheel, dashboard, etc.

They are traveling at the car’s original speed of 50 km/h