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# A Level Circular motion question? watch

1. Could someone give me a solution?

A garden strimmer uses a piece of string rotating at 100rev s^-1 to trim grass. If the length is 15cm calculate the instantaneous velocity at the end of the chord.

Thanks
2. (Original post by liverpool2044)
Could someone give me a solution?

A garden strimmer uses a piece of string rotating at 100rev s^-1 to trim grass. If the length is 15cm calculate the instantaneous velocity at the end of the chord.

Thanks
You know that the linear velocity is the distance covered in time, v=(2*pi*R)/T, where 2*pi*R is the circular distance (circumference), R is the radius of the orbit and T is the time period (time taken for one rotation). Since the angular velocity is w=theta/t (where theta is the angle travelled in radians and t is the time taken to travel that angle), or w=(2*pi)/T, the above equation becomes v=wR so you can simply substitute the numbers into the equation.

Note that 1 revolution is 2pi radians.
3. (Original post by Mehrdad jafari)
You know that the linear velocity is the distance covered in time, v=(2*pi*R)/T, where 2*pi*R is the circular distance (circumference), R is the radius of the orbit and T is the time period (time taken for one rotation). Since the angular velocity is w=theta/t (where theta is the angle travelled in radians and t is the time taken to travel that angle), or w=(2*pi)/T, the above equation becomes v=wR so you can simply substitute the numbers into the equation.

Note that 1 revolution is 2pi radians.
Thanks alot mate, could you also help with this?

A fairground round makes 9revs per minute. What is the angular speed?
4. (Original post by liverpool2044)
Thanks alot mate, could you also help with this?

A fairground round makes 9revs per minute. What is the angular speed?
I gave you the angular velocity formula as w=theta/t (although you might have seen the equation as w=(2pi)/T, the other equation is a shortcut), so you just need to work out the angular distance, that is 9*2pi, and the time taken for it, that is 1minute*60)
5. (Original post by Mehrdad jafari)
I gave you the angular velocity formula as w=theta/t (although you might have seen the equation as w=(2pi)/T, the other equation is a shortcut), so you just need to work out the angular distance, that is 9*2pi, and the time taken for it, that is 1minute*60)
I got 0.94 is that about right? Thanks in advance
6. (Original post by liverpool2044)
I got 0.94 is that about right? Thanks in advance
That is correct!
7. (Original post by liverpool2044)
I got 0.94 is that about right? Thanks in advance
I have not checked it but yeah, that should be correct
8. Cheers
9. (Original post by liverpool2044)
Cheers
Don't forget the units
10. Isn't it
100(2pi*r)/t = 100(2pi*0.15)/1

because of the 100 revs per second. which then equals to 30pi = 94ms-1
not 0.94?
11. (Original post by Siddhart1998)
Isn't it
100(2pi*r)/t = 100(2pi*0.15)/1

because of the 100 revs per second. which then equals to 30pi = 94ms-1
not 0.94?
0.94 rad/s is the answer to the second question asked after the first question. Your answer is for the first one
12. (Original post by Mehrdad jafari)
0.94 rad/s is the answer to the second question asked after the first question. Your answer is for the first one
Ah okay.
Safe G 👊

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