Motion of masses connected by a string over a pulley

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SUMMARY

The discussion focuses on the dynamics of two particles, A and B, connected by a string over a pulley, where A has mass m and B has mass 2m. Key calculations include the speed of the particles just before B hits the ground, determined to be v = √(2gh/3), and the impulse exerted by B on the ground, calculated as Impulse = m * √(2gh/3). The conversation also addresses the motion of B after it hits the ground, emphasizing the importance of conservation of momentum when the string becomes taut again. The participants conclude that the motion continues indefinitely, leading to an infinite total distance traveled by B.

PREREQUISITES
  • Understanding of Newton's laws of motion
  • Familiarity with kinematic equations, specifically v² = u² + 2ad and ΣF = m.a
  • Knowledge of impulse and momentum concepts
  • Basic principles of dynamics involving pulleys and connected masses
NEXT STEPS
  • Study the application of conservation of momentum in inelastic collisions
  • Explore the dynamics of systems involving pulleys and strings in greater detail
  • Investigate the effects of impulse on motion in various physical scenarios
  • Learn about the differences between elastic and inelastic collisions in physics
USEFUL FOR

Students studying classical mechanics, physics educators, and anyone interested in understanding the dynamics of connected mass systems and the principles of motion involving pulleys.

  • #31
ii) Impulse on ground when mass B impacts: 2mΔv ?
 
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  • #32
neilparker62 said:
ii) Impulse on ground when mass B impacts: 2mΔv ?

Yes, sorry. The mass should be 2m.For (v) :
Distance of B = twice the distance when it accelerates upwards + decelerate to stop (when A hits the ground)

Speed of B when A hits the ground
v^2 = (\frac{1}{3} \sqrt{\frac{2gh}{3}})^2 + 2 . \frac{g}{3} . 2h
v^2 = \frac{38gh}{27}

Distance traveled by B when decelerating:
v^2 = u^2 - 2gd
0 = \frac{38gh}{27} - 2gd
d = \frac{19h}{27}

So total distance
= 4h + 2 . \frac{19h}{27}
= \frac{146h}{27}

For (vi), I think I need to use formula of sum to infinity of geometric progression but I can not find the ratio. The distance traveled by B:
a. when released from rest until hits the ground = h
b. between first hit and second hit = \frac{146h}{27}
c. between second hit and third hit (repeating the same process as (v)) = \frac{1298h}{243}

How to find the total distance?

Thanks
 
Last edited:

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