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Average Speed

Naman Asks:

“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 = = 37.5 kmph

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?

(Garry asked)

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.
Monster Truck
Refer to http://wahiduddin.net/calc/calc_speed_rpm.htm for a calculation of top speed vs tyre radius

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”

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.

More problems coming in; Try to answer now!

Stuti posted these:

  1. a particle is thrown with  any velocity vertically upward. the distance traveled by particle in last sec of ascent is 1)g 2)g/2 3)g/4 4)cant be calculated
  2. a ball is dropped from a bridge of 122.5 metre above a river. after ball has been falling for 2 sec,a second ball is thrown straight down after it. initial velocity of second ball so that both hit water at sme time is 1)49 m/s 2)55.5 m/s 3)26.1 m/s 4)9.8 m/s.
  3. two cars are moving in same direction with a speed of 30 km/hr. they are separated from each other by 5 km. third car moving in opposite direction meets the two cars after an interval of 4 minutes. the speed of third car is 1)30 km/hr 2)25 km/hr 3) 40 km/hr 4)45 km/hr
  4. a balloon starts rising from ground from rest with an upward acceleration 2 m/s^2. jst after 1 sec , a stone is dropped from it. time taken by stone to strike ground is nearly 1)0.3 sec 2)0.7 sec 3)1 sec 4)1.4 sec

A simple numerical on momentum

Car mass is 1200 kg moving velocity is 108 km/hr. On applying brakes velocity reduced to 36 km/hr. Find change in momentum?

(posted by Aditi)

m=1200 kg

u=108 x 5/18=30m/s


Change in momentum = m(v-u) = 1200 x (30-10)=1200 x 20= 24000 kg m/s

Vector Addition Problem – A Spy Report

A spy report about a suspended car reads as follows.”the car moved 2.00 km towards east,made a perpendicular left turn, ran for 500 m, made a perpendicular left turn, ran for 4.00 km and stopped”. Find the displacement of the car.

Answer: 2061.5 m




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