Chapter 2: Motion in Chapter 2: Motion in One DimensionOne Dimension

DisplacementDisplacementChange in position of an object

Frame of reference◦A point against which position is measured

Example: A train traveling between stations◦It is in motion when measured against the

track.◦It is stationary when measured against a seat.

DisplacementDisplacementNot equal to distance traveled

Scalar vs. VectorDistance vs. Displacement

Displacement (Displacement (xx))Straight line distance from the initial

position to the final position (change in position)

Can be positive or negative

Displacement (Displacement (xx))What is the

displacement for the objects shown?

Answer: 70 cm

• Answer: -60 cm

Displacement Sign ConventionsDisplacement Sign Conventions

Uses a coordinate system•Right (or east) ---> +◦Left (or west) ---> – ◦Up (or north) ----> +◦Down (or south) ---> –

Speed vs.VelocitySpeed vs.VelocityScalar vs. VectorDistance vs. DisplacementSpeed vs. Velocity

SpeedSpeed

Speed does not include direction while velocity does.

Speed uses distance rather than displacement.

In a round trip, the average velocity is zero but the average speed is not zero.

Average VelocityAverage VelocityDisplacement in a given time

Average Velocity = (ΔX/Δt) = displacement/time interval

Average VelocityAverage Velocity

How would you describe the motion shown by this graph?

Average VelocityAverage VelocityDescribe the motion of

each object.Answers

◦ Object 1: constant velocity to the right or upward

◦ Object 2: constant velocity of zero (at rest)

◦ Object 3: constant velocity to the left or downward

Instantaneous VelocityInstantaneous VelocityVelocity at a single instant of time

Speedometers in cars measure instantaneous speed.

Determined by finding the slope at a single point (the slope of the tangent)

Book Problems

Heather and Matt take a 34 minute walk eastward along a straight road to a store 2.0 km away. What is their average velocity in m/s?

0.98 m/s to the east

Book Problems

Eugene is 75 km due south of Salem. If Joe rides from Salem to Eugene on his bike in 6.00 hr, what is his average velocity?

3.47 m/s to the south

Book Problems

A bus traveled south along a straight path for 3.2 h with an average velocity of 88 km/h, stopped for 20.0 minutes, then traveled south for 2.8 h with an average velocity of 75 km/h. What is the average velocity for the total trip?

20.83 m/s to the south

Book Problems

A bus traveled south along a straight path for 3.2 h with an average velocity of 88 km/h, stopped for 20.0 minutes, then traveled south for 2.8 h with an average velocity of 75 km/h. What is the displacement for the total trip?

490,000 m to the south