# Rotational Motion - Help me out GUYS!

hi guys i got a couple of questions can somebody help me out by telling me the steps of where to go about and i will try to get the answer on my own as i need to understand how to do these questions i will post what i know.

1. a train slows down as it turns, from 90 km/hr to 50 km/hr in 15seconds. the radius is 150m, calculate the acclereation the moment it reaches 50km/hr.

first of all i know the formula is a= v^2 / r
what i dont know is that i am given 90k/hr and 50 km/hr, time and a radius, im thinking maybe i dont use the a=v^2/r formula but perhaps v= u + at ?

2. there is a pictuer of a circle all i am given is radius is 2.50 m, outside the circle is a= 15 m/s^2 and insie the circle is an angle of 30degrees with a line going down. It wants me to calculate radial, tangential accerlearation along with the speed of the particle.

im thinking maybe the a=15 m/s^2 is the overall accleration and maybe use the components of sin30 and cos30 im lost please shed me some light ppl thanks

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dagg3r said:
1. a train slows down as it turns, from 90 km/hr to 50 km/hr in 15seconds. the radius is 150m, calculate the acclereation the moment it reaches 50km/hr.

first of all i know the formula is a= v^2 / r
what i dont know is that i am given 90k/hr and 50 km/hr, time and a radius, im thinking maybe i dont use the a=v^2/r formula but perhaps v= u + at ?
You are in the right track. You find the centripetal acceleration as a = v^2/r and tangential acceleration as a = v-u/t. Add them as vectors and you will get the net acceleration.

dagg3r said:
2. there is a pictuer of a circle all i am given is radius is 2.50 m, outside the circle is a= 15 m/s^2 and insie the circle is an angle of 30degrees with a line going down. It wants me to calculate radial, tangential accerlearation along with the speed of the particle.

im thinking maybe the a=15 m/s^2 is the overall accleration and maybe use the components of sin30 and cos30 im lost please shed me some light ppl thanks
As far as I get it, you're given the net acceleration a of a particle at the edge of the wheel of radius r, crossed by a radial line forming 30degrees with the vertical. So you are asked to find the radial and tangential accelerations of that particle. But still you would need the speed of the wheel.