Work And Energy Test

Result

Work And Energy Test - 6

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Question 1
1 / -0

A body moves a distance of 10m along a straight line under the action of a force of 5N. If the work done is 25 J , the angle which the force makes with the direction of motion of the body is-

A
0 °

B
30 °

C
60 °

D
90 °

Solution

F.S cos θ = 25 J; 5.10 cos θ = 25

10 cos θ = 5; θ = 60 º Ans
Question 2
1 / -0

A particle of mass 100 g is thrown vertically upwards with a speed of 5 m/s. The work done by the force of gravity during the time the particle goes up is

A
0.5 J

B
- 0.5 J

C
- 1.25 J

D
1.25 J

Solution

According to the law of motion,

v² = u² – 2gh

h = 1.25 m

So at the speed of 5 m/s a 100 gm particle move up at the height of 1.25 m.

Thus, Work done by the gravity to pull the particle from the 1.25 m height is

W = - mgh

W = - ( 0.1 × 10 × 1.25)

w = - 1.25 J
Question 3
1 / -0

The potential energy of a particle is determined by the expression U = α (x²+ y²), where α is a positive constant. The particle begins to move from a point with the coordinates (3, 3) (m), only under the action of potential field force. Then its kinetic energy T at the instant when the particle is at a point with the coordinates (1, 1) (m) is

A
8 α

B
24 α

C
16 α

D
zero

Solution

As the particle moves only under the action of conservative force so its mechanical energy must be conserved. So ΔT + ΔU = 0 (T stands for kinetic energy )

or, ΔT = – ΔU = U_i – U_f = α (3² + 3²) – α (1² + 1²) = 18 α– 2α = 16 α.