## Average Speed

“A car travels from A to B with a speed of 30km/h and returns back to A with a speed of 50km/h.Find distance,displacement,average speed and average velocity ifdistance between A and B is 100km.”

Distance 200 m

Displacement 0

Average velocity 0

Average speed =$\frac{2v_{1}v_{2}}{v_{1}+v_{2}}$ = 37.5 kmph

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## How wheel + Tyre diameter affects a vehicle’s speed

I have been told a larger wheel/tyre decrease acceleration because its harder to turn the wheel making the car rev slower.

But the same person tells me it will increase the top speed.

My question is if the car has a max rev of 8000rpm but only reaches 7000rpm in its highest gear would a smaller wheel/tyre not make it easier for the car to rev higher thus possible increasing the top speed by using a smaller wheel?

As wheel radius increases, its top speed increases for a given RPM since for a given RPM the speed of the vehicle is directly proportional to its radius.
The distance traveled in one rotation = 2 π R
But now, there is another effect also here. As the radius of tyre increases, its moment of inertia increases and it would be harder for the engine to rotate it. So, the engine will not be able to turn it with the same ease as before, but however, there will be an increase in top speed other conditions remaining the same.
Refer to http://wahiduddin.net/calc/calc_speed_rpm.htm for a calculation of top speed vs tyre radius

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## Another problem from Kinematics

Shibu posted:

“a car is running at a speed “u”. Seeing a child on
the road, the driver applies the break so as to
bring the car to a halt within a distance S. Show
that hte reaction time of the driver is 2S/u”

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## Question on distance, acceleration and velocity

Two cars start simultaneously form rest at the same place. one accelerate uniformly as 0.5m/s until it reaches a speed of 60m/s which it maintains steadily, the other accelarate uniformly at the speed of 80m/s which it maintained steadily. calculate

1. How many seconds will elapse before the cars are levelled and each travelling at equal speed.

2. the distance travelled by either car this time.

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