section 2.1 – describing motion motion and position a reference point is needed to determine the...
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Motion
Motion
Motion
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Section 2.1 – Describing Motion
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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?
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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).
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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
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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
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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!
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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)
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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
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Section 2.2 - Acceleration
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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.
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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.
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Speeding Up…• If the acceleration is in the same direction as the velocity,
the speed increases and the acceleration is positive.
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Slowing Down…• If the speed decreases, the acceleration is in the opposite
direction from the velocity, and the acceleration is negative.
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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.
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Calculating Acceleration
Vf = final velocityVi = initial velocity t = time
Units = m/s2
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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.
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Example with Positive Acceleration
• The airliner is speeding up, so the final speed is greater than the initial speed and the acceleration is positive.
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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.
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Example with Negative Acceleration
• The skateboarder is slowing down, so the final speed is less than the initial speed and the acceleration is negative.
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Section 2.3 – Motion and Forces
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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)
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Net Force
Net force – the sum of the forces acting upon an object
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Balanced Forces
Balanced forces - occurs when the forces acting upon an object are equal in size but opposite in direction
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Unbalanced Forces
Unbalanced forces – when unequal forces act on the same object; the object moves in the direction of the larger force
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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?