Question

18. A lead pencil consists of a cylinder of wood with a solid cylinder of
graphite fitted into it. The diameter of the pencil is 7 mm, the diameter
of the graphite is 1 mm and the length of the pencil is 10 cm. Calculate
the weight of the whole pencil, if the specific gravity of the wood is
0.7 g/cm³' and that of the graphite is 2.1 g/cm³'.​

Answer 1

Answer:

please refer to the attachment below

Answer 2

Answer:

We have

Diameter of the graphite cylinder =1mm=

10

1

cm

Radius =

20

1

cm

Length of the graphite cylinder =10cm

Volume of the graphite cylinder =(

7

22

×

20

1

×

20

1

×10)cm

3

Weight of graphite = Volume × Specific gravity

Weight of graphite =(

7

22

×

20

1

×

20

1

×10×2.1)gm

=(

7

22

×

20

1

×

20

1

×10×

10

21

)gm=0.165gm

Diameter of pencil =7mm=

10

7

cm

Radius of the pencil =

20

7

cm and length of pencil =10cm

Volume of pencil =

7

22

×

20

7

×

20

7

×10cm

3

Volume of wood =(

7

22

×

20

7

×

20

7

×10−

7

22

×

20

1

×

20

1

×10)cm

3

Volume of wood =

7

22

×

20

1

×

20

1

×10(7×7−1)cm

3

=

7

11

×

20

1

×48cm

3

Weight of wood =(

7

11

×

20

1

×48×0.7)gm

=(

7

11

×

20

1

×48×

10

7

)gm=2.64gm

Total weight =(2.64+0.165)gm=2.805gm

Answered