Question

A 125 kg mail bag hangs by a vertical rope 3.3 m long. A postal worker then displaces the bag to a position 2.2 m sideways from its original position, always keeping the rope taut.
1) What horizontal force is necessary to hold the bag in the new position?
2) As the bag is moved to this position, how much work is done by the rope?
3) As the bag is moved to this position, how much work is done by the worker?

Answer 1

Answer:

1) the required horizontal force F is 1095.6 N

2) W = 0 J { work done by rope will be 0 since tension perpendicular }

3) work is done by the worker is 1029.4 J

Explanation:

Given that;

mass of bag m = 125 kg

length of rope $l$ = 3.3 m

displacement of bag d = 2.2 m

1) What horizontal force is necessary to hold the bag in the new position?

from the figure below; ( triangle )

SOH CAH TOA

sin = opp / hyp

sin$\theta$ = d / $l$

sin$\theta$ = 2.2/ 3.3

sin$\theta$ = 0.6666

$\theta$ = sin⁻¹ ( 0.6666 )

$\theta$  = 41.81°

Now, tension in the string is resolved into components as illustrated in the image below;

Tsin$\theta$ = F  

Tcos$\theta$ = mg

so

Tsin$\theta$ / Tcos$\theta$ = F / mg

sin$\theta$ / cos$\theta$ = F / mg

we know that; tangent = sine/cosine

so

tan$\theta$ = F / mg

F = mg tan$\theta$

we substitute

Horizontal force F = (125kg)( 9.8 m/s²) tan( 41.81° )

F = 1225 × 0.8944

F = 1095.6 N

Therefore, the required horizontal force F is 1095.6 N

2)  As the bag is moved to this position, how much work is done by the rope?

Tension in the rope and displacement of mass are perpendicular,

so, work done will be;

W = Tdcos90°

W = Td × 0

W = 0 J { work done by rope will be 0 since tension perpendicular }

3) As the bag is moved to this position, how much work is done by the worker

from the diagram in the image below;

SOH CAH TOA

cos = adj / hyp

cos$\theta$  = ($l$ - h) / $l$

we substitute

cos$\theta$  = ($l$ - h) / $l$  = 1 - h/$l$

cos$\theta$ = 1 - h/$l$

h/$l$ = 1 - cos$\theta$

h = $l$( 1 - cos$\theta$ )

now, work done by the worker against gravity will be;

W = mgh = mf$l$( 1 - cos$\theta$ )

W = mf$l$( 1 - cos$\theta$ )

we substitute

W = (125 kg)((9.8 m/s²)(3.3 m)( 1 - cos41.81° )

W = 4042.5 × ( 1 - 0.745359 )

W = 4042.5 × 0.254641

W = 1029.4 J

Therefore,  work is done by the worker is 1029.4 J