Chapter 2: Motion

# Section 1: Describing Motion

Motion

Distance and time are important.

You don’t always need to see something move to know that motion has taken place.

A

**reference point**is needed to determine the position of an object.**Motion**occurs when an object changes its position relative to a reference point.The motion of an object depends on the reference point that is chosen.

After a reference point is chosen, a frame of reference can be created.

A frame of reference is a coordinate system in which the position of the objects is measured.

The

*x-axis*and*y-axis*of the reference frame are drawn so that they intersect the reference point.

**Distance:**how far an object has movedThe

**SI unit of length**or distance is the meter (m). Longer distances are measured in kilometers (km).

Distance and displacement are not the same.

**Displacement**: the distance and direction of an object’s change in position from the starting point.

**Speed**: the distance an object travels per unit of time.Any change over time is called a rate.

The SI unit for distance is the meter and the SI unit of time is the second (s), so in SI, units of speed are measured in meters per second (m/s).

If you are traveling at a constant speed, you can measure your speed over any distance interval.

Usually speed is not constant.

**Average Speed:**the total distance traveled divided by the total time of travel.Average speed describes speed of motion when speed is changing.

It can be calculated using the relationships among speed, distance, and time.

A speedometer shows how fast a car is going at one point in time or at one instant.

**Instantaneous Speed:**the speed at a given point in time.When something is speeding up or slowing down, its instantaneous speed is changing.

Graphing Motion

The motion of an object over a period of time can be shown on a distance-time graph.

Time is plotted along the horizontal axis of the graph and the distance traveled is plotted along the vertical axis of the graph.

The

**slope**of a line on a distance-time graph equals the speed.A

**horizontal line**on a distance-time graph has zero slope, and represents an object at rest.On a distance-time graph, the distance is plotted on the vertical axis and the time on the horizontal axis.

Each axis must have a scale that covers the range of numbers to be plotted.

The slope of a line on a distance-time graph gives the speed of an object in motion

**Velocity**: includes the speed of an object and the direction of its motion.The speed of a storm is not enough information to plot the path. The direction the storm is moving must be known, too.

For an object to have constant velocity, speed and direction must not be changing.

Because velocity depends on direction as well as speed, the velocity of an object can change even if the speed of the object remains constant.

Motion of Earth’s Crust

Geological evidence suggests that Earth’s continents have moved slowly over time.

Changes in the landscape occur constantly as continents drift slowly over Earth’s surface.

Earth is made of layers.

The outer layer is the crust, and the layer just below the crust is called the upper mantle. Together the crust and the top part of the upper mantle are called the lithosphere.

The lithosphere is broken into huge sections called plates that slide slowly on the puttylike layers just below.

The movement of the plates also is changing the size of the oceans and the shapes of the continents.

Plates move so slowly that their speeds are given in units of centimeters per year.

# Section 2: Acceleration

Acceleration, Speed, and Velocity

**Acceleration**: 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.

Acceleration also has direction, just as velocity does.

A change in velocity can be either a change in how fast something is moving or a change in the direction of movement.

Any time a moving object changes direction, its velocity changes and it is accelerating.

Graphs of speed versus time can provide information about accelerated motion.

Calculating Acceleration

To calculate the acceleration of an object, the change in velocity is divided by the length of the time interval over which the change occurred.

To calculate the change in velocity, 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.

The unit for acceleration is a unit for velocity divided by a unit for time.

A speed-time graph tells you if acceleration is a positive or negative number.

Amusement Park Acceleration

Engineers use the laws of physics to design amusement park rides that are thrilling, but harmless.

The highest speeds and accelerations usually are produced on steel roller coasters.

Steel roller coasters can offer multiple steep drops and inver- sion loops, which give the rider large accelerations.

# Section 3: Motion and Forces

What is force?

**Force**: a push or pullEvery push, pull, stretch, or bend results in a force being applied to an object.

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

Force does not always change velocity.

Forces can be balanced and unbalanced.

**Net Force**: When two or more forces act on an object at the same time**Balanced Forces:**Forces on an object that are equal in size and opposite in direction.When two students are pushing with unequal forces in opposite directions, a net force occurs in the direction of the larger force.

Inertia and Mass

**Inertia:**the tendency of an object to resist any change in its motion.If an object is moving, it will have

**uniform motion**. It will keep moving at the same speed and in the same direction unless an unbalanced force acts on it.If an object is at rest, it tends to remain at rest.

The inertia of an object is related to its mass. The greater the mass of an object is, the greater its inertia.

Forces change the motion of an object in specific ways.

Newton’s first law of motion states that an object moving at a constant velocity keeps moving at that velocity unless an unbalanced net force acts on it.

What happens in a crash?

The law of inertia can explain what happens in a car crash.

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

The force needed to slow a person from 50 km/h to zero in 0.1 s is equal to 14 times the force that gravity exerts on the person.

Air bags also reduce injuries in car crashes by providing a cushion that reduces the force on the car’s occupants.

# Chapter 2: Motion

# Section 1: Describing Motion

Motion

Distance and time are important.

You don’t always need to see something move to know that motion has taken place.

A

**reference point**is needed to determine the position of an object.**Motion**occurs when an object changes its position relative to a reference point.The motion of an object depends on the reference point that is chosen.

After a reference point is chosen, a frame of reference can be created.

A frame of reference is a coordinate system in which the position of the objects is measured.

The

*x-axis*and*y-axis*of the reference frame are drawn so that they intersect the reference point.

**Distance:**how far an object has movedThe

**SI unit of length**or distance is the meter (m). Longer distances are measured in kilometers (km).

Distance and displacement are not the same.

**Displacement**: the distance and direction of an object’s change in position from the starting point.

**Speed**: the distance an object travels per unit of time.Any change over time is called a rate.

The SI unit for distance is the meter and the SI unit of time is the second (s), so in SI, units of speed are measured in meters per second (m/s).

If you are traveling at a constant speed, you can measure your speed over any distance interval.

Usually speed is not constant.

**Average Speed:**the total distance traveled divided by the total time of travel.Average speed describes speed of motion when speed is changing.

It can be calculated using the relationships among speed, distance, and time.

A speedometer shows how fast a car is going at one point in time or at one instant.

**Instantaneous Speed:**the speed at a given point in time.When something is speeding up or slowing down, its instantaneous speed is changing.

Graphing Motion

The motion of an object over a period of time can be shown on a distance-time graph.

Time is plotted along the horizontal axis of the graph and the distance traveled is plotted along the vertical axis of the graph.

The

**slope**of a line on a distance-time graph equals the speed.A

**horizontal line**on a distance-time graph has zero slope, and represents an object at rest.On a distance-time graph, the distance is plotted on the vertical axis and the time on the horizontal axis.

Each axis must have a scale that covers the range of numbers to be plotted.

The slope of a line on a distance-time graph gives the speed of an object in motion

**Velocity**: includes the speed of an object and the direction of its motion.The speed of a storm is not enough information to plot the path. The direction the storm is moving must be known, too.

For an object to have constant velocity, speed and direction must not be changing.

Because velocity depends on direction as well as speed, the velocity of an object can change even if the speed of the object remains constant.

Motion of Earth’s Crust

Geological evidence suggests that Earth’s continents have moved slowly over time.

Changes in the landscape occur constantly as continents drift slowly over Earth’s surface.

Earth is made of layers.

The outer layer is the crust, and the layer just below the crust is called the upper mantle. Together the crust and the top part of the upper mantle are called the lithosphere.

The lithosphere is broken into huge sections called plates that slide slowly on the puttylike layers just below.

The movement of the plates also is changing the size of the oceans and the shapes of the continents.

Plates move so slowly that their speeds are given in units of centimeters per year.

# Section 2: Acceleration

Acceleration, Speed, and Velocity

**Acceleration**: 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.

Acceleration also has direction, just as velocity does.

A change in velocity can be either a change in how fast something is moving or a change in the direction of movement.

Any time a moving object changes direction, its velocity changes and it is accelerating.

Graphs of speed versus time can provide information about accelerated motion.

Calculating Acceleration

To calculate the acceleration of an object, the change in velocity is divided by the length of the time interval over which the change occurred.

To calculate the change in velocity, 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.

The unit for acceleration is a unit for velocity divided by a unit for time.

A speed-time graph tells you if acceleration is a positive or negative number.

Amusement Park Acceleration

Engineers use the laws of physics to design amusement park rides that are thrilling, but harmless.

The highest speeds and accelerations usually are produced on steel roller coasters.

Steel roller coasters can offer multiple steep drops and inver- sion loops, which give the rider large accelerations.

# Section 3: Motion and Forces

What is force?

**Force**: a push or pullEvery push, pull, stretch, or bend results in a force being applied to an object.

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

Force does not always change velocity.

Forces can be balanced and unbalanced.

**Net Force**: When two or more forces act on an object at the same time**Balanced Forces:**Forces on an object that are equal in size and opposite in direction.When two students are pushing with unequal forces in opposite directions, a net force occurs in the direction of the larger force.

Inertia and Mass

**Inertia:**the tendency of an object to resist any change in its motion.If an object is moving, it will have

**uniform motion**. It will keep moving at the same speed and in the same direction unless an unbalanced force acts on it.If an object is at rest, it tends to remain at rest.

The inertia of an object is related to its mass. The greater the mass of an object is, the greater its inertia.

Forces change the motion of an object in specific ways.

Newton’s first law of motion states that an object moving at a constant velocity keeps moving at that velocity unless an unbalanced net force acts on it.

What happens in a crash?

The law of inertia can explain what happens in a car crash.

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

The force needed to slow a person from 50 km/h to zero in 0.1 s is equal to 14 times the force that gravity exerts on the person.

Air bags also reduce injuries in car crashes by providing a cushion that reduces the force on the car’s occupants.