Linear Motion

Linear Motion • Motion is easy to recognize but can be hard to describe • The following quantities are used to describe motion: speed, velocity and acceleration • Each of these is a rate. A rate is a quantity divided by time. • Motion along a straight line is sometimes called linear motion.

All Motion is relative • All motion is relative to a reference. • This means that we describe motion of an object relative to some other object • In our environment, the reference for motion is the earth’s surface, and speeds are measured relative to the earth • The earth moves at 107,000 km/h relative to the sun

Speed or Velocity What’s the difference between speed and velocity? Speed is a measure of how fast something is moving. It is the rate at which a distance is covered

distance speed = time d s

t

Measured in m/s, km/h, miles/hr, cm/s, etc… http://www.physicslessons.com/speed.jpg

Speed or Velocity We use the terms loosely today, but there is a distinction. Velocity is a speed AND direction. See the arrow?

http://www.sdsc.edu/~sallen/Cars.html

Velocity and Speed • In physics we distinguish between speed and velocity: • Speed refers to how quickly an object moves (a scalar quantity). • Velocity is defined as speed in a given direction or rate of change of position (displacement over time). • Velocity refers to both the speed and direction of motion of an object (a vector quantity). • Negative velocity means the object is moving in the opposite direction • Motion at constant velocity means that both the speed and direction of an object do not change. • In a car, we can change the velocity three ways: gas pedal to speed up, brake to slow down or steering wheel to change direction

Instantaneous Speed • Instantaneous speed is speed at any instant in time. • A speedometer measures speed in ‘real time’ (the instantaneous speed).

Average Speed • Average speed is the average of all instantaneous speeds; found simply by a total distance/total time ratio • The average speed of a trip:

total distance average speed = elapsed time

• For more information http://www.glenbrook.k12.il.us/gbssci/phys/Class/1DKin/U1L1d.

Speed and Velocity Take as an example, a car driving in a circle. The car is always going the same speed, but it’s direction, or velocity, is constantly changing.

From Conceptual Physics

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Acceleration

Acceleration • For its velocity to change, an object must accelerate. • An object accelerates whenever its speed or direction or both change. • Acceleration may be positive (increasing speed) or negative (decreasing speed). • Acceleration is a measure of how quickly the velocity changes: a = ∆ v/t

change of velocity acceleration = time interval

Acceleration Acceleration is a change to velocity, either in the speed of the object or in the direction Velocity is to the right.

Velocity is to the left.

http://www.physics.montana.edu/physed/misconceptions/ acceleration/zerovzeroa/discover.html

Acceleration As velocity is the change of distance with time, acceleration is the change of velocity with time. The units for acceleration are meters per second per second (or m/s2), miles/hr2, etc. Velocity is to the right.

Velocity is to the left. http://www.physics.montana.edu/physed/misconceptions/ acceleration/zerovzeroa/discover.html

Acceleration Equation

Acceleration at constant speed • An object moving in a circle at constant speed is always accelerating (changing direction).

Acceleration Fighter Jet vs. Jet Car