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# Mechanics Watch

1. Hi can anyone give me some help on this question i'm not sure where to start

A particle, of mass 8kg, is attached to one end of a length of elastic string. The particle is placed on a smooth horizontal surface. The other end of the elastic string is attached to a point O fixed on the horizontal surface.

The elastic string had natural length 1.2m and modulus of elasticity 192N.

The particle is set in motion on the horizontal surface so that it moves in a circle, centre O, with a constant speed 3m/s.

Find the radius of the circle.

i've ended up with a quadratic which i solved and selected the positive solution
2. Use
3. (Original post by jonathan3909)
Use
is v revolutions per minute? how do i convert it?
also is F the centripetal force?
4. (Original post by hey_its_nay)
is v revolutions per minute? how do i convert it?
also is F the centripetal force?

F is the centripetal force (tension in the string) , v is the constant speed and m is the mass.
5. (Original post by jonathan3909)
F is the centripetal force (tension in the string) , v is the constant speed and m is the mass.
and the tension in the sting also equals ?x/l
?=192, l=1.2 but is x(the extention) r-l?
6. (Original post by hey_its_nay)
and the tension isn the sting also equals ?x/l
?=192, l=1.2 but is x(the extention) r-l?
Modulus of elasticity has no use here.The centripetal forces is the tension = 80N.
7. (Original post by jonathan3909)
Modulus of elasticity has no use here.The centripetal forces is the tension = 80N.
can you show me how you found the tension please?
8. (Original post by hey_its_nay)
and the tension in the sting also equals ?x/l
?=192, l=1.2 but is x(the extention) r-l?
Moreover in these types of questions , the length of the string is taken as the radius and since it is attached to a fixed point , the answer must be 1.2m!(think of it for yourself)
9. (Original post by hey_its_nay)
can you show me how you the tension please?
Tension = mg
= (9.81 x 8)
= 78.48N ~ 79N
10. (Original post by jonathan3909)
Moreover in these types of questions , the length of the string is taken as the radius and since it is attached to a fixed point , the answer must be 1.2m!(think of it for yourself)
but its an elastic string so as it starts spinning around in a circle its going to stretch
im confused
11. (Original post by jonathan3909)
Tension = mg
= (9.81 x 8)
= 78.48N ~ 79N
its a horizontal circle why are you using g
12. i've ended up with a quadratic which i solved and selected the positive solution

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