Motion

Motion

Motion

Section 2.1 – Describing Motion

Motion and Position• A reference point is needed to determine the position of

an object.• Motion occurs when an object changes position relative to

its reference point.• Motion depends on its point of reference.

• Question – are you moving right now?

Distance and Displacement

Distance – how far an object has moved

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

The SI unit for distance and displacement is the meter (m).

Distance and Displacement

Example: A runner runs from the start line 50 m north plus 30 m in the opposite direction.

What is the distance?

What is the displacement?

START

50 m N 30 m S

SpeedSpeed – the distance an object travels per unit of time

Average speed – total distance traveled divided by the totaltime of travel

Instantaneous speed – speed at a given point in time

Remember to use the metric system!m/s (meters/second) or kilometers/hourkm/h

Velocity

Velocity – includes the speed of an object and the directionof its motion

• Because velocity depends on direction as well as speed, the velocity of an object can change even when its speed does not!

Car is driving 30 km/hr but it’s velocity is constantly changing!

Equation to Find Speed

Speed (in meters per second) = distance (in meters) / time (in seconds)

s = d/t

Units – meters/second (m/s) or kilometers/hour (km/h)

Graphing Motion• Motion can be shown on a distance vs. time graph

• Time on x-axis• Distance on y-axis

• On a distance vs. time graph the slope describes the speed• Steeper slope = faster speed• Horizontal line = zero speed = object at rest

Section 2.2 - Acceleration

What Is Acceleration?• Acceleration is the rate of change of velocity.

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

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

• So acceleration occurs when an object changes its speed, its direction, or both.

Speeding Up and Slowing Down• When you think of acceleration, you probably think of

something speeding up. However, an object that is slowing down also is accelerating.

• Acceleration also has direction, just as velocity does.

Speeding Up…• If the acceleration is in the same direction as the velocity,

the speed increases and the acceleration is positive.

Slowing Down…• If the speed decreases, the acceleration is in the opposite

direction from the velocity, and the acceleration is negative.

Changing Direction…• Any time a moving

object changes

direction, its velocity

changes and

it is accelerating.

• The speed of

the horses in this

carousel is constant,

but the horses are

accelerating because their

direction is changing

constantly.

Calculating Acceleration

Vf = final velocityVi = initial velocity t = time

Units = m/s2

Example with 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.

Example with Positive Acceleration

• The airliner is speeding up, so the final speed is greater than the initial speed and the acceleration is positive.

Example with Negative Acceleration

• Imagine that a skateboarder is moving in a straight line at a constant speed of 3 m/s and comes to a stop in 2 s.

Example with Negative Acceleration

• The skateboarder is slowing down, so the final speed is less than the initial speed and the acceleration is negative.

Section 2.3 – Motion and Forces

Force

Force – a push or pull

• Sometimes it is obvious a force is being applied and sometimes it is not• Forces cause the motion of an object to change• Forces are measured in Newtons (N)

Net Force

Net force – the sum of the forces acting upon an object

Balanced Forces

Balanced forces - occurs when the forces acting upon an object are equal in size but opposite in direction

Unbalanced Forces

Unbalanced forces – when unequal forces act on the same object; the object moves in the direction of the larger force

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

The greater the mass of an object, the greater its inertia.• Think about playing ping pong with a bowling bowl…

what’s the problem?

•Newton’s First Law of Motion (aka Law of Inertia)

An object moving at a constant velocity keeps moving at that velocity unless an unbalanced net force acts upon it.

How does this apply to a car crash?Why can a seat belt prevent serious injury?