Motion Physical Science Chapter 11. 2 Motion Can be described as a change in position. Physical Science chapter 11

Motion

Physical ScienceChapter 11

Physical Science chapter 11 2

Motion

Can be described as a change in position.


Distance vs. displacement

Distance is how far you traveled What your odometer reads

Displacement is the distance between your starting point and your ending point, regardless of your path As the crow flies


Speed

How fast something moves The rate of change in position The rate of motion

Rate means change over time


Velocity vs. speed

Velocity describes both speed and direction.

Both “how fast” and “which way” something is moving


Constant speed

Speed that does not vary Cruise control


Average speed

Total distance traveled divided by total time of travel

Used when the object in motions makes changes in speed, including starts and stops.


Calculating speed

If you know distance and time, you can calculate average speed.

distancespeed

time

t

dv


Example

You skate a distance of 100 m in 67 s. What is your average speed?

Known: d = 100 m t = 67 s

Unknown v = ?


Example continued

t

dv

s 67

m 100v

s

m 5.1v


You try

Find the speed in meters per second of a baseball thrown 38 m from third base to first base in 1.7 s.


Example

Find the velocity in meters per second of a swimmer who swims 110 m toward the shore in 72 s.


You try

Calculate the displacement in meters that a cyclist would travel in 5.00 h at an average velocity of 12.0 km/h to the southwest. Remember to include direction.


Instantaneous speed

The rate of motion at a given instant Speedometer


Graphing speed

When distance is graphed on the y-axis and time is graphed on the x-axis, we call this a distance-time graph.

The slope of a distance-time graph is the speed.

See pages 370–371 Discuss the practice problems


Acceleration The rate of change of velocity. Acceleration occurs if either the

speed or direction changes, or if both change.

“Slowing down” is a change in speed, so when something slows down, it is accelerating.


Acceleration

The size of acceleration depends on both the change in velocity and the time interval over which the change takes place

If the change in velocity is large, the acceleration will be large.

If the time interval is short, then acceleration will be large.


Calculating acceleration

The triangle is the Greek letter delta, and stands for “change in”

final initialv va

tv

at


Units on acceleration

Written as m/s2

second

ondmeters/sec

second

1

second

meters

2second

meters


Examples

1. Natalie accelerates her skateboard along a straight path from 0.0 m/s to 4.0 m/s in 2.5 s. Find her average acceleration.

2. Find the average acceleration of a northbound subway train that slows down from 12. m/s to 9.6 m/s in 0.8 s.


Acceleration on graphs

On a speed vs. time graph (speed on the y-axis and time on the x-axis) Acceleration is the slope

On a distance vs. time graph (distance on the y-axis and time on the x-axis) A curved line indicates acceleration A straight line indicates a constant

velocity (no acceleration)


Discuss

1. Identify the straight-line accelerations below as either speeding up or slowing downa. 5.7 m/s2b. -29.8 m/s2c. -2.43 m/s2d. 9.8 m/s2

2. A turtle swimming in a straight line twoard shore ahs a speed of 0.50 m/s. After 4.0 s, its speed is 0.80 m/s. What is the turtle’s average acceleration?


Force

A push or a pull one object exerts on another


Fundamental forces

Gravity Electromagnetic force

Like charges repel; opposite charges attract

Strong force Holds nucleus together

Weak force Involved with radioactivity


Contact forces vs. field forces

Contact forces Objects must be touching

Field forces Don’t need to be touching Gravity, magnets, electric charges


Balanced forces

Forces that are equal in size and opposite in direction are called balanced forces.

Tug-of-war tie


Net Force

When forces are unbalanced, there is a net force.

A net force always causes a change in velocity.

Could be a change is speed, a change in direction, or both.


Friction

The force that opposes motion between two surfaces that are touching each other.

Enables us to walk, hold things, etc.


Friction

The amount of friction depends on two things The kinds of surfaces The force pressing the surfaces together

More force more friction


Static Friction

Between surfaces that aren’t moving Largest frictional force between any

two objects


Kinetic Friction

Friction between moving surfaces Sliding friction Rolling friction

Usually less than sliding friction


Discuss

What is a force? What are balanced forces? What is friction? Does a net force of zero mean that

there is no force at all?

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Motion Physical Science Chapter 11. 2 Motion Can be described as a change in position. Physical Science chapter 11