PhysicsPhysics

Motion, Speed, and Velocity

SC Standards Covered

PS-5.1 Explain the relationship among distance, time, direction, and the velocity of an object

PS – 5.2Use the formula v=d/t to solve problems related to average speed or velocity

PS – 5.6Represent linear motion of objects on distance – time graphs

Motion

• When an object changes its position, motion has occurred.– Distance- How far an object has moved.– Displacement- How far an object has moved in

relation to its starting point.– Consider direction

Example: Two runners travel along the same straight path in a straight line for 500 meters. At the end of the run their distances are the same but their displacements are different. How can this be so?

Speed- Distance an object travels per unit of time

Relationships between speed, distance, and time:

Speed = Distance/ Time = d/ t » Constant Speed- speed does not change

over time» Average Speed- speed of motion when

speed is changing

Avg Speed = Total Distance/ Total Time

» Instantaneous Speed- speed at any given moment in time (speedometer)

Speed

• The units for speed: • meters/second (m/sec) • kilometers/hour (km/hr)• Speed that does not change is constant speed

Graphing Motion

• Graph distance on the y-axis and time on the x-axis

Slope = rise = distance = speed run time

Distance - Time Graph

• If something is not moving, a horizontal line is drawn.

• If something starts out slow and then speeds up, its change in speed can look like this.

Learning Checkpoint

This graph shows several stages of motion:

• Stage 1: 100 m in 10 s

• Stage 2: 50 m in 10 s

• Stage 3: 150 m in 20 s

Calculate the speed as indicated by each of the colors.

Calculate the average speed.

What is the total distance?

What is the displacement?

Solution

Stage 1: S= d/ t100 m/ 10 s= 10 m/s

Stage 2: S= d/t50 m/ 10 s= 5 m/s

Stage 3: S= d/t150 m/ 20 s= 7.5 m/s

Ave Speed= Tot d/ Tot t300 m/ 40 s= 7.5 m/s

Distance = 300 metersDisplacement = 0 meters

Velocity

• The speed and direction of an object’s motion.–88 km / hr southwest

Acceleration

• The change in velocity• Acceleration is measured in m/sec/sec or m/sec2

• Formula is:• (final velocity - original velocity)/time

Deceleration vs. Acceleration

• A decrease in velocity is deceleration or negative acceleration

• A distance-time graph for acceleration is always a curve

Centripetal Acceleration

• Acceleration directed toward the center of circular path

Frames of Reference

• The object or point from which movement is determined

• Movement is relative to an object that appears stationary

• Earth is the most common frame of reference