JAMB Physics Past Questions & Answers - Page 318

1,586.

A load of 20N on a wire of cross-sectional area 8 x 10-7m2, produces an extension of 10m2. calculate Young's modulus for the material of the wire if its length is 3m

A.

7.0 x 1011Nm-2

B.

7.5 x 1011Nm-2

C.

8.5 x 1011Nm-2

D.

9.0 x 1011Nm-2

Correct answer is B

Young's modulus = \(\frac{F}{A}\) \(\div\) \(\frac{e}{L}\)

= \(\frac{20}{8 \times 10^{-7}}\) \(\div\) \(\frac{10^{-4}}{3}\)

= 7.5 x 1011Nm-2

1,587.

An electric water pump rate 1.5 kW, lifts 200kg of water through a vertical height of 6 meters in 10 seconds. What is the efficiency of the pump? [g = 10ms-2, neglecting air resistance]

A.

90.0%

B.

85.0%

C.

80.0%

D.

65.0%

Correct answer is C

Efficiency = output x 100

= \(\frac{\frac{200 {\times} 10 {\times} {6}}{10}}{1500}\) x \(\frac{100}{1}\)

= 80%

1,588.

An object of mass 100g projected vertically upwards from the ground level has a velocity of 20ms-1 at a height of 10m. calculate its initial kinetic energy at the ground level.[g = 10ms-2, neglect air resistance]

A.

10 J

B.

20 J

C.

30 J

D.

50 J

Correct answer is C

Using, v2 = u2 - 2gx

(20)2 = u21/2 x 10 x 10

u2 = 600(m/s)2

initial K.E = - mu2

= \(\frac{1}{2}\) x 600

= 30 J

1,589.

A planet has mass m1 and is at a distance r, from the sun. A second planet has mass m2 = 10m1 and at a distance of r2 = 2r1 from the sun. Determine the ratio of the gravitational force experienced by the planets.

A.

1 : 5

B.

2 : 5

C.

3 : 5

D.

4 : 5

Correct answer is B

g = \(\frac{GM}{R^2}\)

\(\frac{G_1M_1}{r^2_1}\) = \(\frac{G_2M_2}{r^2_2}\)

\(\frac{G_1M_1}{r^2_1}\) = \(\frac{G_2 {\times}10 M_1}{(2r_1)^2}\)

\(\frac{G_1}{G_2}\) = \(\frac{2}{5}\)