Speed and velocity

Learning objectives

Students will be able to• Define speed• Define velocity• Calculate speed and velocity

WARM UP

Usain Bolt Wins Olympic 100m Gold - London 2012 Olympics.mp4

Speed• Distance moved per second or rate of

change of distance• It is scalar quantity• Unit-m/s• Average speed=Distance travelled/time

taken

Velocity

• Velocity measures rate of change of displacement

• Average velocity=Displacement/time taken• It is a vector quantity.• Unit- m/s

CONTINOUS ASSESSMENT1.What does the speedometer of a car measure?

2.A car travels a distance of 2oom and returns to the original position after 5min.What is the speed and velocity of the car?3.What is the eqn. for speed?4.What is the eqn. for time taken?

Final assessment1.a)Define speed b) Define velocity

(page42)

HOME WORK

• Page.33 Example.2

GRAPHSDisplacement –time graph

a) Body at rest

t

S

t(s) 0 1 2 3 4

S(m) 2 2 2 2 2

b)Body moving with steady velocity

Gradient=Gradient of displacement –time graph gives velocity. Steeper the line ,larger the velocity.

t

s

t(s) 0 1 2 3 4

S(m) 0 5 10 15 20

Time

ntDisplaceme

c)Accelerating body

a=2m/s²

Gradient at different time instants are different, as the velocity is changing at every instant.Gradient at any instant gives the instantaneous velocity

t

st 0 1s 2s 3s 4s

s 0m 1m 4m 9m 16

d)Decelerating body

t

s

Velocity –time grapha) Body moving with steady velocity

t

v

b)Body moving with steady acceleration

• Gradient=• Gradient of velocity –time graph gives

acceleration. • Steeper the line ,larger the acceleration.

t

v

t(s) 0 1 2 3 4

v(m/s) 0 5 10 15 20

onacceleratiTime

locityChangeinve

c)Body moving with steady deceleration

• Gradient=• Gradient of velocity –time graph gives

acceleration. • Steeper the line ,larger the deceleration.

t

v

t(s) 0 1 2 3 4

v(m/s) 20 15 10 5 0

ondecelerationacceleratiTime

locityChangeinve/

Area under the velocity –Time graph gives the displacement

Calculate the total displacement of the given body using the given velocity time graph.

OA-accelerated motion; Displacement = 100mAB-Steady velocity ; Displacement = 400mBC—decelerated motion; Displacement = 50mTotal displacement = 100+400+50=550m

Page43. Qn . 8

a) acceleration= gradient of OA =change on y/change on x =y2-y1/X2-X1 =2-0/3-0 =2/3 =1.5m/s²b) Decelerates, its velocity becomes zero and it reaches the maximum height.c)At point C the velocity of the lift is zero. The height=Displacement=area under OABC = ½ × 3 × 2 + 6 × 2 + ½ × 3 × 2 =3+12+3=18m.d)The lift starts moving down. First it accelerates in the downward direction.Then it starts decelerating and come to rest e) 18-6=12m

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