Physics

L2

www.slideshare.net/DMS_physics

Motion is relative

• Motion = change in position of an object• System of Reference

Speed

• A measure of how fast something is moving (see p.12)

• Average speed is the total distance divided by the time interval

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v =total distance

time interval

[m/s, km/h]

Instantaneous speed

• A measure of how fast an object is moving at a certain time

• speedometer• odometer

Over to you

• P.13 / Q.1

• P.13 / Q.2

Velocity• has speed and direction

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(instantaneous) velocity :r v = 300km /h [SW ]

average velocity :r v = 300km /h [SW ]

(instantaneous) speed : v = 300km /h

average speed : v = 300km /h

Scalars and Vectors

• Scalars = have only one value– Ex: time, speed, temperature

• Vectors = have two or three values (magnitude and direction)– Ex: velocity, force, acceleration

Over to you (p.14/Q)

• The speedometer of a car moving northward reads 60 km/h. It passes another car that travels southward at 60 km/h.

– Do both cars have the same speed?– Do they have the same velocity?

Constant and changing velocity

• Constant velocity– both speed and direction are constant– The motion occurs in a straight line– Uniform (linear) motion

• Changing velocity– Either one of speed or direction are

changing, or both

Over to you

• Q: What controls are used to change velocity in a car?

Over to you

A: gas pedal, brake, steering wheel

Acceleration

• Is the rate at which velocity is changing– Acceleration is a vector

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acceleration =change in velocity

time interval

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ra =

Δr v

t=

r v −

r v 0

t

units km/h ⋅s, m/s ⋅s = m/s2

Uniform Circular Motion

• Speed is constant

• Velocity is changing

• Acceleration points towards the center of the motion

Classwork

• Consider a car that is leaving drops of oil on the road. Show how the drops would look if– The car is traveling at constant velocity– The car is accelerating (increasing its

velocity)– The car is decelerating (reducing its

velocity)

Constant speed (zero acceleration)

Positive acceleration (accelerating)

Negative acceleration (decelerating)

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ra

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ra

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rv

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rv

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rv

Answer

Example (p.16/Q1)

– Suppose a car moving in a straight line steadily increases its speed each second, first from 35 to 40 km/h, then from 40 to 45 km/h, then from 45 to 50 km/h. What is its acceleration?

– Answer:

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(change in)v = (50 − 45)km /h = (45 − 40)km /h = (40 − 35)km /h = 5 km /h

t =1s

a =change in( )v

t=

5km /h

1s= 5km /h ⋅s

Its acceleration is 5 km/h⋅s

Classwork

• P.16 / Q2– In 5 seconds a car moving in a straight line

increases its speed from 50 km/h to 65 km/h, while a truck goes from rest to 15 km/h in a straight line. Which undergoes greater acceleration? What is the acceleration of each vehicle?

Homework assignment

• P. 25 / 12– What is the acceleration of a car moving

along a straight-line path that increases its speed from 0 to 100 km/h in 10s ?

Homework assignment

• P. 25 / 13– By how much does the speed of a vehicle

moving in a straight line change each second when it is accelerating at 2 km/h.s?

End L2