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ANSWER SCRIPT

ENT142 Engineering Dynamics / ENT252 Dynamics

Peperiksaan Pertengahan Semester Kedua Sidang Akademik 2012/2013

Question 1

A train starts from station A and for the first kilometer, it travels with a uniform acceleration.

Then, for the next two kilometers, it travels with a uniform speed. Finally, the train decelerates

uniformly for another kilometer before coming to rest at station B. Assume the time for the whole

journey is six minutes.

(a) Prove that;

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(15 Marks/Markah)

Answer:

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(b) Evaluate the maximum speed of the train. Draw the vt graph. (5 Marks/Markah)

Answer:

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Question 2

Figure 1 shows the 800 kg car at B is connected to the 350 kg car at A by a spring coupling.

Neglect the mass of the wheels.

Figure 1

(a) Evaluate the stretch in the spring if the wheels of both cars are free to roll. (8 Marks/Markah)

Answer:

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(b) Evaluate and compare the stretch in the spring with your answer in (a) if the brakes are

applied to all four wheels of car B, causing the wheels to skid. Take (k)B = 0.4. Justify

your answer.

(12 Marks/Markah)

Answer:

PART B Answer ALL questions.

Question 3

The 50-kg crate is hoisted up the 30 incline by the pulley system and motor M (see

Figure 2). The crate starts from rest and, by constant acceleration, attains a speed of 4 m/s

after traveling 8 m along the plane. The motor has an efficiency of = 0.74.

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Figure 2

(a) Evaluate the power that must be supplied to the motor at the instant the crate has

moved 8 m. Neglect friction along the plane.

(10 Marks/Markah)

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(b) If the coefficient of kinetic friction between the plane and the crate is k = 0.3,

evaluate the power that must be supplied to the motor and compare with your

answer in (a). Justify your answer. (10 Marks/Markah)

Answer:

Justification: the difference in power inputs without and with the presence of friction is 42.5

percent where the problem in (b) requires more power to overcome the frictional force occured on

the contact sliding surface.

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