# Phy (F.4 )

A sports car accelerates along a straight line from rest at a rate of 6ms-2 for 4s and then its acceleration changes to -4ms-2 foe 2s .

(a) Find the maximum speed of the car . Express the answer in kmh-1.

(b)Find the final speed of the car.Express the answer in kmh-1

(c)Find the average acceleration of the car. Express the answer in ms-2

Rating

The above equations are right ,but he still had not changed the answer for question(1)and (2) to kmh-1

(a) Find the maximum speed of the car . Express the answer in kmh-1.

First we have the following equation:

v=u+at

V is the maximum speed of the car, u is the initial speed of the car, a is the acceleration, and t is the time

as the starting speed is 0, so u=0

v=u+at

v=0+(6)(4)

v=24ms-1

v=24/1000*60*60

= 86.4km -1

so the maximum speed is 86.4km -1

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(b)Find the final speed of the car.Express the answer in kmh-1

after reaching the maximum speed of v=24ms-1,it started to decelerate:

v=u+at

v=24+(-4)(2)

v=16ms-1

=16/1000*60*60

=57.6kmh-1

so the final speed is 57.6kmh -1

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c)Find the average acceleration of the car. Express the answer in ms-2

a=Δv/Δt

a=(final speed-initial speed)/(time taken over the movement)

a=(16 -0)ms-1/(4+2)s

a=2.67ms-2

so the average acceleration of the car is 2.67ms-2

• (a) v=u+at

v=0+6*4

v=24ms-1

v=24*3600/1000 kmh-1

v=86.4kmh-1

(b) v=u+at

v=24+(-4)(2)

v=16ms-1

v=16*3600/1000 kmh-1

v=57.6kmh-1

(c)v=u+at

16=0+6a

a=2.67ms-2 (to 3 sig. fig.)

Source(s): calculator