2.kinematics part1 2009 student

8/6/2019 2.Kinematics Part1 2009 Student

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Chapter2

Chapter2

KinematicsKinematics

Everything in the universe is in motion.

Nothing is permanently at rest.

Everything in the universe is in motion.

Nothing is permanently at rest.


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State what is meant by speed and velocity .State what is meant be uniform acceleration and calculate the valueof an acceleration using change in velocity/time taken .Interpret given examples of non-uniform acceleration.

Calculate average speed = total distance travelled / total timetaken .Plot and interpret a distance-time graph and a speed-time graph.Deduce from the shape of a distance-time graph when a body is:

(i) at rest, (ii) moving with uniform speed, (iii) moving with non-uniform speed.

Learning Outcomes


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Deduce from the shape of a speed-time graph when a body is at: (a) at rest, (b) moving with uniform speed, (c) moving with uniform acceleration,

(d) moving with non-uniform acceleration.Calculate the area under a speed-time graph to determine thedistance travelled for motion with uniform speed or uniformacceleration.

State that the acceleration of free fall for a body near to the earthis constant and is approximately 10 m s -2.Describe the motion of bodies with constant weight falling with or without air resistance, including reference to terminal velocity

Learning Outcomes (cont.)


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Distance is the total length travelled by an object inmotion.Distance is the total length travelled by an object inmotion.

D istanceD istance

Displacement is the distance measured along a straight linein a ____________________.Displacement is the distance measured along a straight linein a ____________________.

D isplacement D isplacement

A

B

D istance vs displacement D istance vs displacement

DistanceDistance DisplacementDisplacement

________ quantity _________ quantity

regardless of itsdirection

dependent on directionof motion


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D istance vs displacement D istance vs displacement

displacement AB

distance AB


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D istance vs displacement ExampleD istance vs displacement Example

A boy walks 40 m due East from Point P and then 30 m due North and finally 40 m due West.

(1) What is the total distance?(2) What is the displacement?

40m

40m

30m30m

E

P

N


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C ritical Thinking Questions

Can you move in such a way that the distance you cover is(a) equal to the magnitude of the displacement?

(b) greater than the magnitude of the displacement?

(c) smaller than the magnitude of the displacement?


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S peed of an object is defined as the distance travelled byS peed of an object is defined as the distance travelled bythe body per unit time.the body per unit time.S peed of an object is defined as the distance travelled byS peed of an object is defined as the distance travelled bythe body per unit time.the body per unit time.

S peed =time taken

distance travelled

S peed S peed

S peed, velocity and accelerationS peed, velocity and acceleration

SI unit: metres per second (_______)SI unit: metres per second (_______)


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A verage speed A verage speed

SI unit of metres per second (m s -1 )SI unit of metres per second (m s -1 )

Average speed =

for most journeys, speed is not constantfor most journeys, speed is not constant



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A boy runs a distance of 100 m in a straight line with a speed of 8.00 m s -1. He then runs back to the starting point with a speed of 4.00 m s -1. What is his average speed in m s -1? In km h -1?

Time taken to run the first 100 m =Time taken to run the next 100 m =Total time taken =

S peed, velocity and accelerationS peed, velocity and acceleration A verage speed A verage speed

Average speed =Average speed =To tal time

To tal distance travelled


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speedometer measures thespeedometer measures theinstantaneous speed of the carinstantaneous speed of the car

I nstantaneous speed I nstantaneous speed I nstantaneous speed is the speed of an object atany instant (any point in time).I nstantaneous speed is the speed of an object atany instant (any point in time).



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Velocity = time takendistance travelled in a stated direction = displacement

Velocity is the distance travelled per unit time in aVelocity is the distance travelled per unit time in a ______ ______

________________ ________________

Velocity is the distance travelled per unit time in aVelocity is the distance travelled per unit time in a ______ ______

________________ ________________

velocity velocity

SI unit of metres per second (m s -1 )SI unit of metres per second (m s -1 )


Questions:

- I f a car travels along a straight roadwithout making any turns, is the speedof the car same as its velocity?

- I f a car travels round a bend, is the speed

of the car same as its velocity?

Questions:

- I f a car travels along a straight roadwithout making any turns, is the speedof the car same as its velocity?

- I f a car travels round a bend, is the speed

of the car same as its velocity?


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H ow do we specify direction?For straight line motion, we can specify one direction as

positive , and the opposite direction as negative

Alternatively,

Upwar d

Downward

Upwar d

Downward

Assign:Upward : positive

Downward: negative

Assign:Upward : negativeDownward: positive

Speed: _______ Velocity: _______

Speed: _______ Velocity: _______

Speed: _______ Velocity: ________

Speed: ______ Velocity: _______


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Diagram shows the path taken by a tennis ball when it isthrown up.Given that the speed of the ball is 5 ms -1 at the indicated

positions, determine its velocity at these positions.

H ow do we specify direction? Example

1 2

So how about the displacement in this case???


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Acceleration = time taken

change of velocity

Acceleration is defined as the rate of change of velocity.Acceleration is defined as the rate of change of velocity.Acceleration is defined as the rate of change of velocity.Acceleration is defined as the rate of change of velocity.

A cceleration A cceleration

SI unit of metres per second square (_________)SI unit of metres per second square (_________)


where

a: accelerationv: final velocity

u: initial velocity

t : time


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ExamplesCalculate the acceleration of

(a) a saloon car which achieves a velocity of 25 m s -1 in 10.0 safter starting from rest,

(b) a sports car which achieves the same speed of 25 m s -1 in6.0 s after starting from rest.


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The driver of a car brakes when the car is travelling at 30 m s -1.

The velocity of the car is reduced to 10 m s -1 after 5.0 s. What isits average acceleration?

Examples


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A cceleration: What does it mean?

The velocity of the car increases by 3 m s -1 every second.We say that the car has an ________________ acceleration of 3 metres per second per second, i.e. 3 m s -2.

Time, t / s Velocity, v / ms-1

0 0.0

1 3.0

2 6.0

3 9.0

4 12.0


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(1) No acceleration(2) Uniform (constant) acceleration(3) Accelerating (positive acceleration)(4) Deceleration / Retardation (negative acceleration)


0 6.0

1 6.0

2 6.0

3 6.0

4 6.0



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0 0.0

1 3.0

2 6.0

3 9.0

4 12.0



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0 0.0

1 3.0

2 7.0

3 13.0

4 20.0



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0 0.0

1 3.5

2 6.5

3 9.0

4 11.0



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0 12.0

1 9.0

2 6.0

3 3.0

4 0.0


2.kinematics part1 2009 student

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