# Thread: questions in physics help me

1. ## questions in physics help me

Q 1 :
Two masses 5 kg and 10 kg are suspended from ends of a string (mass less inelastic )

passing over a smooth pulley . Calculate the acceleration of the masses

------------------------

This is the first body acting by two force on is T other is gravity
Now we write the equation

m1g  T = m1a

( I write m1g ) because T < than m1g )

Now the second body
T - m2g = m2a
( I write - m2g because T > than gravity )

m1g  T = m1a
T - m2g = m2a
>
,,,m1g- m2g=m1a+m2a

a = (m1  m 2)/ ( m1 + m 2 ) X g

= - 735.75

To find T
m1g  T = m1a
5 X 9.81 T = 5 (- 735.75 )
49.05  T = -3678.78
-T = -3678.78 - 49.05
T = 3727.8

2. Originally Posted by r-soy
Q 1 :

m1g – T = m1a

( I write –m1g ) because T < than m1g )

Now the second body
T - m2g = m2a
( I write - m2g because T > than gravity )

m1g – T = m1a
T - m2g = m2a
……………………>
,,,m1g- m2g=m1a+m2a

a = (m1 – m 2)/ ( m1 + m 2 ) X g

= - 735.75

To find T
m1g – T = m1a
5 X 9.81 –T = 5 (- 735.75 )
49.05 – T = -3678.78
-T = -3678.78 - 49.05
T = 3727.8
why do you mean by the red colored statements. Can you describe more

3. Sorry I write wrong now i correct >>

In like this question required to write equation but as techer said you must beware about (- ) and ( + ) in that

First m1g – T = m1a
( I write –T ) because T less than m1g ( No m1g + T = m1a)

Now the second body
T - m2g = m2a
( I write - m2g because - m2g less than T)

4. it's not because T is less than m1g, it's because T acts opposite to the force m1g

5. plese expline that for m1 and m2

6. for m1, if consider forces,

T acts upwards, and the gravitational force acts downwards so resultant force would cause the acceleration

upwards,$\displaystyle \sum F = T - m_1g$

apply F = ma upwards

$\displaystyle T - m_1g = m_1a$ (considered a happens upwards)

if m1 accelerates upwards m2 should accelerate downwards then

since the forces act much similarly as above,

apply F=ma downwards,

$\displaystyle m_2g - T = m_2a$

now solve for T and a (and i think there's something wrong with you answer for a..)

7. for m1, if consider forces,

T acts upwards, and the gravitational force acts downwards so resultant force would cause the acceleration

upwards,$\displaystyle \sum F = T - m_1g$

apply F = ma upwards

$\displaystyle T - m_1g = m_1a$ (considered a happens upwards)

if m1 accelerates upwards m2 should accelerate downwards then

since the forces act much similarly as above,

apply F=ma downwards,

$\displaystyle m_2g - T = m_2a$

now solve for T and a (and i think there's something wrong with you answer for a..)

8. I use this a = (m1 – m 2)/ ( m1 + m 2 ) X g

= - 735.75 where is the mistake ?

9. Originally Posted by r-soy
I use this a = (m1  m 2)/ ( m1 + m 2 ) X g

= - 735.75 where is the mistake ?
i think you are just confused what you are doing.

it should be (m1  m 2)X g / ( m1 + m 2 )

where m1=10 and m2=5

you should get a= 3.2666666...ms^-2

then just that to find T.

10. Originally Posted by r-soy
I use this a = (m1  m 2)/ ( m1 + m 2 ) X g

= - 735.75 where is the mistake ?

$\displaystyle \displaystyle {{m_1 - m_2}\over{ m_1 + m_2}}={{10-5}\over{10+5}}= {{5}\over{15}}={{1}\over{3}}$

11. thanks For T is correct

m1g – T = m1a
5 X 9.81 –T = 5 (3.27 )
49.05 – T = 16.35
-T = 16.35 - 49.05
T = 32.7

12. You used the wrong equation. The correct equation was given earlier:

$\displaystyle T - m_1g = m_1a$