A steel ball is attached to a string and moving in uniform circular motion on a horizontal plane. Its velocity increases by a factor of 3. The corresponding tension in the string increases by 6 4 N . What was the original tension force in N of the string?
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We know, from the centripetal force, that, if F is the force of tension, then F ∝ v 2 , since the tension is the source of the centripetal acceleration. Therefore, if v is multiplied by 3 , F is multiplies by 9 .
Let the original tension force, in Newtons, be $T$. Then, we have: 9 T = T + 6 4 ⟹ T = 8 , so our answer is 8 N .
One comment here on the use of the words "centripetal force". I know that in this problem you know what you're doing, so I'm not worried about that. However, "centripetal force", as if there is some extra force out there called the centripetal force can be confusing to some students and I've even seen a magical F c appear on free body diagrams. There is no such thing a centrifugal force, nor is there such a thing as centripetal force. There are only real forces (tension, normal, etc.) that provide centripetal acceleration.
on FBDs, you don't label the centripetal and centrifugal forces?
Not usually since they are net forces, not actually forces acting on the body.
using centripetal force formular, F=mv2/r, i use 2 in place of squaed let the initial force be F=mv2/r _equation 1, then the new force, F+64=(m(3v)2)/r _equation 2, subtracting equation 1 from 2, we get 64=9mv2/r - mv2/r=8mv2/r, therefore, mv2/r=64/8=8N, remember mv2/r=F which is the original tension force.
Let T be the original tension in the string.
mv^2 / r=T
m(3v)^2 / r=T + 64
(9mv^2) / r = T + 64 , We also know that mv^2/r=T and we can sub that into this equation which gives,
9T = T + 64
8T=64
T=8
Therefore the original tension was 8N.
Tension in the string will be (m v 2 )/r. If T is the original tension, then T = (m v 2 )/r and the new tension T' = (m ( 3 v ) 2 )/r = 9T. It is given that 9T-T=64. Thus 8T = 64 => T = 8.
The tension causes the centripetal acceleration of the ball. The magnitude of centripetal acceleration experienced by the ball is a c = r v 2 . Thus, the initial tension experienced by the ball is T i = r m × v 2 .
When the velocity is tripled, the centripetal acceleration is now nine times the original centripetal acceleration, i.e. the tension experienced by the ball is now T f = r 9 × m × v 2 .
The increase in tension is thus T f − T i = r 8 × m × v 2 = 6 4 N .
The initial tension is thus T i = r 8 × m × v 2 × 8 1 = 8 6 4 = 8 N .
In the original condition the tension force cancels with the centripetal force so T = m v^2 / R then the velocity is increased by a factor of 3 so the velocity now becomes 3v. as the velocity increases, we are told that the Tension force also increases by 64 N so T + 64 = m (3v)^2 / R T + 64 = 9 m v^2 / R from the first equation we get T + 64 = 9T 64 = 8T T = 8 N
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T = R m v ² → T ′ = T + 6 4 = R + d R m ( 3 v ) ² *
Dividing the 2 equations:
T T + 6 4 = 9 ∴ T 6 4 = 8 ∴ T = 8 , ,