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MM1MS1-E1
STATICS AND DYNAMICS
Time allowed TWO hours
Candidates may complete the front cover of their answer book and sign their desk card but
must NOT write anything else until the start of the examination period is announced.
Answer ALL questions in Section A and ALL questions in Section B.
Please answer Section A and Section B in separate answer booklets.
Only silent, self contained calculators with a Single-Line Display or Dual-Line Display are
permitted in the examination.
Dictionaries are not allowed with one exception. Those whose first language is not English
may use a standard translation dictionary to translate between that language and English
provided that neither language is the subject of this examination. Subject specific translation
dictionaries are not permitted.
No electronic devices capable of storing and retrieving text, including electronic dictionaries,
may be used.
DO NOT turn examination paper over until instructed to do so
ADDITIONAL MATERIAL: Formula Sheet, Graph paper
INFORMATION FOR INVIGILATORS:
Question papers should be collected in at the end of the exam – do not allow candidates to
take copies from the exam room.
Turn over
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SECTION A
Answer ALL questions in this section
1. The structure shown in Figure 1a is a preliminary design for an overhead crane
support truss in which the position B at which the load P is applied is at a
variable distance from support A, given as a.
Figure 1a
(a) Determine the reaction forces as a function of the variables.
[8]
(b) Show that in region AB the bending moment is given by
1
a
M x xP
L
where x is the distance from A, and determine the bending moment
function as a function of position y measured from C for the region BC.
[10]
(c) The dimensions and loads are now fixed, but the load is now applied to the
crane through a rigid arm, as shown in Figure 1b. Determine the equivalent
forces and moments acting on the beam ABC at point B, and hence the new
support reactions. There is no need to calculate shear forces and bending
moments along the beam.
Figure 1b
[7]
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2. A thin-walled pipe 600 mm in diameter and 4 mm in thickness is used to carry
pressurised gas at a pressure of 2 MPa, leading to a state of plane stress.
(a) Explain the meaning of the term plane stress.
[4]
(b) Show that the principal in-plane stresses are 150 MPa and 75 MPa, and
make a sketch of an element of stress on the pipe to illustrate the directions
of the stresses.
[7]
(c) For this state of plane stress, sketch the Mohr’s circle, and determine the
maximum in-plane shear stress.
[6]
(d) If the maximum in-plane shear stress allowable in the material is 100 MPa,
determine the maximum pressure that can safely be carried by the pipe.
[8]
Turn over
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SECTION B
Answer ALL questions in this section
3. A 4kg object is launched vertically up into the air. The launch mechanism applies
constant 1kN vertical force to the object for a period of 0.25 seconds.
An observer watches the launch from a position 30m away from the launch point,
at the same height as the launch point.
(a) Carefully draw and label a free body diagram for the object during the 0.25
second launch period. What is the constant acceleration applied to the mass
during the launch? What is the velocity of the object just after the launch
period?
[5]
(b) Assuming that there is no air resistance, what is the maximum height that
the object reaches?
[5]
(c) Sketch a diagram showing the position of the object in Cartesian coordinates
(x,y) and polar coordinates (r,θ) from the observer’s point of view, i.e.
showing how x,y,r and relate to the geometry shown in Figure 3a What is
the height of the object, and the distance from the observer to the object
when the observer is looking up at an angle of 70°?
[5]
(d) Sketch diagrams of the polar components of the velocity and acceleration of
the object. What are the values for , , andr r when the observer is looking
up at an angle of 70° and the object is travelling upwards?
[10]
Figure 3a
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4. A geared hoist system for raising and lowering a load is shown in Figure 4a. It
consists of a motor driving a compound gearbox – gear A has 20 teeth, gear B
has 200 teeth and is rigidly fixed to gear C, gear C has 25 teeth, gear D has 200
teeth and is rigidly attached to a hoist pulley of diameter 200 mm. The gearbox
efficiency has been found to be 95%. The hoist pulley is 250mm long and is
made of steel (ρ=7800Kg/m3). Ignore the thickness of the cable and the inertia
of the gearbox itself.
(a) What is the gear ratio from the input (motor) to the output (hoist pulley)?
What is the output torque applied to the hoist pulley if the motor drive
torque is 2Nm?
[5]
(b) The winch is required to lift a 120Kg mass from the ground to a height of 5m.
If a constant torque of 2Nm is applied by the motor, how long will it take to
raise the mass to 5m? What is the speed of the motor (in revolutions per
minute) when the mass is 2cm off the ground?
[8]
(c) If the cable of the hoist system snaps and the 120Kg mass drops from the
rest at a height of 56m, how long will it take reach the floor?
[6]
(d) If the mass does not rebound from the floor, what is the impulse applied to
the mass during impact? What is the average impact force applied to the
mass if the impact lasts 0.1 seconds?