2.1 Describing Motion

Motion

Distance and time are important

Motion and Position

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 then is determined by the reference point that is chosen

Frame of Reference

A frame of reference is created after a reference point is chosen

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

Distance

Distance - how far an object moved

The SI unit of distance is meter (m)

Displacement

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

Speed

Speed - distance an object travels per unit of time

Calculating Speed

Any change over time is called a rate

Distance is the change in motion, speed is the rate at which distance is traveled

• Speed=distance/time

Motion with Constant Speed

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

Changing Speed

Speed is usually not constant

Average Speed

Average speed describes speed of motion when speed is changing

Average Speed - the total distance traveled divided by the total time of travel

Instantaneous Speed

Speed shown on a speedometer is the instantaneous speed

Instantaneous Speed - the speed at a given point in time

Changing Instantaneous Speed

When something is speeding up or slowing down, its instantaneous speed is changing

If an object is moving with constant speed, the instantaneous speed doesn’t change

Graphing Motion

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

• time is plotted on the horizontal axis

• distance on the vertical axis

Plotting a Distance-Time Graph

On a distance-time graph, the distance is plotted on the vertical and horizontal axis; each axis must have a scale that covers the range of numbers to be plotted

Velocity

Velocity - the speed of an object and the direction of its motion

Since 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

Changes in earth’s landscape occur constantly as continents drift slowly over earth’s surface

Moving Continents

Like an onion, earth is made up of many layers

The outer layer is called the crust

The layer below the crust is called the upper mantle

Together the crust and the top part of the upper mantle make up the lithosphere

2.1 Summary

Position and Motion

• The position of an object is determined relative to a reference point

• Motion occurs when an object changes its position relative to a reference point

• Distance is the length of the path an object has traveled. Displacement is the distance and direction of a change in position

Speed and Velocity

• Speed is the distance an object travels per unit time and is given by this equation s=d/t

• The velocity of an object includes the object’s speed and its direction of motion relative to a reference point

Graphing Motion

• On a distance-time graph, time is the horizontal axis and distance is the vertical axis

• The slope of a line plotted on a distance-time graph is the speed

2.2 Acceleration

Acceleration, Speed, and Velocity

Acceleration is the rate of change of velocity

When the velocity of an object changes, the object is accelerating

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

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

Speeding Up and Slowing Down

Acceleration involves not only speeding up, but also slowing down

Acceleration has direction, just like velocity

If the acceleration is the same direction as the velocity the speed increased and the acceleration is positive; if the speed decreases, the acceleration is in the opposite direction from the velocity and the acceleration is negative

Changing Direction

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

Earth is constantly accelerating constantly as it orbits the Sun in a nearly circular path

Calculating Acceleration

To calculate the change in velocity subtract the initial velocity from the final velocity

Since velocity has units of m/s, and time has units of s

• the SI unit for acceleration is m/s2

Calculating Positive Acceleration

(no notes) aka my brain is too dumb to figure it out pg. 50

Calculating Negative Acceleration

(no notes) aka my brain is too dumb to figure it out pg. 50

Amusement Park Acceleration

Steel roller coasters can offer multiple steep drops and inversion loops, which give the riders larger accelerations

2.2 Summary

Acceleration, Speed, and Velocity

• Acceleration is the rate of change of velocity

• A change in velocity occurs when the speed of an object changes, or its direction of motion changes, or both occur

• The speed of an object increases if the acceleration is in the same direction as the velocity

• The speed of an object decreases if the acceleration and the velocity of the object are in opposite directions

Calculating Acceleration

Acceleration can be calculated by dividing the change in velocity by the time according to the following equation:

a =vf−vi/t

• The SI unit for acceleration is m/s2

If an object is moving in a straight line, the change in velocity equals the final speed minus the initial speed

2.3 Motion and Forces

What is force?

force - a push or pull

Sometimes a force being applied is obvious, while other times it isn’t

Changing Motion

A force can cause the motion of an object to change

Balanced Forces

Force does not always change velocity

Net force - formed 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

Unbalanced Forces

If an object is being pushed with unequal forces on each side, the object will be moved in the direction of the larger force

Inertia and Mass

Inertia - the tendency of an object to resist any change in its motion

The velocity of an object remains constant unless a force changes it

Newton’s Laws of Motion

Sir Isaac Newton (1642-1727) was a British scientists

He stated rules that describe the effects of forces on the motion of objects; the rules are known as Newton’s laws of motion

Newton’s First Law of Motion

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

• If an object is at rest, it stays at rest 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 0.1 s.

Safety Belts

Wearing a safety belt reduces the force exerted on the person, and can also prevent them from being thrown out of the car.

Airbags also reduce injuries in car crashes by providing a cushion

2.3 Summary

What is Force?

• A force is a push or pull on an object

• The net force on an object is the combination of all the forces acting on the object

• When the forces on an object are balanced, the net force on the object is zero

• Unbalanced forces cause the motion of objects to change

Inertia and Newton’s First Law of Motion

• The inertia of an object is the tendency of an object to resist a change in motion

• The larger the mass of an object, the greater its inertia

• Newton’s first law of motion states that the motion of an object at rest or moving with constant velocity will not change unless an unbalanced net force acts on the object

• In a car crash, inertia causes an unrestrained passenger to continue moving at the speed of the car before the crash

Ch. 2 Study Guide (Reviewing Main Ideas)

2.1 Describing Motion

1. Motion is a change of position of a body. Distance is the measure of how far an object moved. Displacement is the distance and direction of an object’s change in position from the starting point

2. A reference point must be specified in order to determine an object’s position

3. The speed of an object can be calculated from this equation:

   s =dt

4. The slope of a line on a distance-time graph is equal to the speed

5. Velocity describes the speed and direction of a moving object

2.2 Acceleration

1. Acceleration occurs when an object changes speed or changes direction

2. An object speeds up if its acceleration is in the direction of its motion

3. An object slows down if its acceleration is opposite to the direction of its motion

4. Acceleration is the rate of change of velocity, and is calculated from this equation:

   a =vf − vi/t

2.3 Motion and Forces

1. A force is a push or a pull

2. The net force acting on an object is the combination of all the c=forces acting on the object

3. The forces on an object are balanced if the net force is zero

4. Inertia is the resistance of an object to a change in motion

5. According to Newton’s first law of motion, the motion of an object does not change unless an unbalanced net force acts on the object