Physics Word Problem

Physics
Tutor: None Selected Time limit: 1 Day

Two objects are connected by a string of
length 4.8 m and negligible mass. The 10 kg
block is placed on a smooth table top(3.6 m from end of table),and the
7kg block hangs over the edge of the table. The 7 kg block is then released from
rest at a distance of 1.2 m above the floor at
time t = 0.The acceleration of gravity is 9.8 m/s^2. The acceleration of the 7kg block is 4.04 m/s^2. The 7 kg block strikes the floor and doesnot bounce.How long dies ittakethe7kgblocktostrikethe floor?
Answer in units of s

Jun 21st, 2015

Thank you for the opportunity to help you with your question! the solution is as such:

The acceleration acting on the m= 7 kg block is (9.8 + 4.04). soit os the total force on it is f= ma

it is f+ 7*(4.04+ 9.8) = 96.88N

now this force acts as tension through the string to the m' 10 kg block , so its acceleration will be a'

now f= m' *a'

so a' = 96.88/ 10 = 9.688 m/s

this acceleration produces a reversing acceleration to the 10 kg block on the table. so the net acceleration on the 7 kg block is 13.84- 9.688 = 4.15 m/s^2

now since the 7 kg block starts from rest, so u=0.

applying, the motion equation, s= u*t + 0.5 *a*(t) ^2

and 1.2 = 0*t + 0.5 *4.15*(t)^2

we get , t= 0.76s

so the time taken is 0.76 seconds

Please let me know if you need any clarification. I'm always happy to answer your questions.awaiting a positive feedback. thank you.
Jun 21st, 2015

incorrect

Jun 21st, 2015

which part?

Jun 21st, 2015

the final answer of s.

Jun 21st, 2015

should i try again?


Jun 21st, 2015

if you cant figure it out its fine, ill complete the question either way. if you want to try again let me know

Jun 21st, 2015

i am trying it again. give me some time

Jun 21st, 2015

hello ., the solution is such:

the same tension acts in both the parts of the string.

the tension is T=[(m1*m2) / ( m1 + m2)] *g

so the tension is 4.117 N

where m1 = 7kg and m2 = 10 kg and g = 9.8

now the acceleration of the falling block is given by a= [m1/( m1+m2)] * g

so a = [7/(7+10)] *9.8

a= 4.035m/ s^2

now 

now since the 7 kg block starts from rest, so u=0.

applying, the motion equation, s= u*t + 0.5 *a*(t) ^2

and 1.2 = 0*t + 0.5 *4.035*(t)^2

we get , t= 0.77s

so the time taken is 0.77 seconds. Please let me know if you need any clarification. I'm always happy to answer your questions.awaiting a positive feedback. thank you.


Jun 21st, 2015

did u get the final answer?

Jun 23rd, 2015

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