Mechanics and Energy

Question 1

Name the 4 types of Mechanical Motion.

Answer 1

Mechanical devices all have aninputmotion, which transforms into force to make anoutputmotion. The four types of motion are:
- linear
- rotary
- reciprocating
- oscillating

Question 2

What is rotary motion?

Answer 2

Rotary motionis where something moves around an axis or pivot point, eg a wheel.

Motion which is in a continuously circular path is rotary motion, that is, motion about an axis. Spur gears, wheels, fan blades, Ferris wheels, merry-go-rounds and the output shaft of motor are examples of objects which move with rotary motion.

Question 3

What is oscillating motion?

Answer 3

Oscillating motionhas a curved backwards and forwards movement that swings on an axis or pivot point, eg a swing or a clock pendulum.

Oscillatory motion can be termed as the repeated motion in which an object repeats the same movement over and over. In the absence of friction, the oscillatory motion would continue forever; but in the real world, the system eventually settles into equilibrium.

Question 4

What is reciprocating motion?

Answer 4

Reciprocating motionhas a repeated up and down motion or back-and-forth motion, eg a piston or pump.

Reciprocating Motion is the repetitive back and forth, or up and down linear motion. It is found in mechanisms including reciprocating pumps and engines. Two motions that are opposite and contain a reciprocation cycle are known as strokes. To convert a circular motion into a reciprocating motion, a crank is used.

Question 5

What is the energy conservation law?

Answer 5

“Energy cannot be created or destroyed. It can only be transferred from one form to another”

Question 6

Name 6 simple machines.

Answer 6

• Pulley
• Inclined plane/Ramp
• Lever
• Wedge
• Wheel
• Screw

Question 7

How does a worm gear work?

Answer 7

The worm gear acts like a gear with 1 tooth! This gives very large gear ratios.

The velocity ratio of a worm drive is the same as the gear ratio. The velocity ratio is the ratio between the number of teeth on the worm gear to the number of threads on the worm.

𝑉𝑅= 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑡𝑒𝑒𝑡ℎ 𝑜𝑛 𝑤𝑜𝑟𝑚 𝑔𝑒𝑎𝑟/𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑡ℎ𝑟𝑒𝑎𝑠 𝑜𝑛 𝑤𝑜𝑟𝑚

Worms (worm screws) and worm gears (worm wheels) are constructed with non-parallel, non-intersecting shafts oriented 90 degrees to each other. Worms and worm gears are widely used across various industrial applications since they offer significant gear ratios in small spaces

Question 8

State some important facts about friction.

Answer 8

Friction always acts in a direction opposite to impending or actual motion Static friction has a limiting value beyond which it cannot increase.

The limiting value of static friction is given by Ff = μFn.

The value of the coefficient of static friction (μ) depends on the nature and condition of the surfaces in contact, but is independent of the areas in contact.

In general, kinetic friction is less than the limiting static friction.

Question 9

What is the angle of repose?

Answer 9

The angle of repose is the value of the angle of inclination corresponding to impending motion.

𝜇=𝑡𝑎𝑛𝜃

Question 10

What is a simple machine?

Answer 10

A simple machine uses a single applied force to do work against a single load force. Ignoring friction losses, the work done on the load is equal to the work done by the applied force. The machine can increase the amount of the output force, at the cost of a proportional decrease in the distance moved by the load. The ratio of the output to the applied force is called the mechanical advantage.

Question 11

What is mechanical advantage?

Answer 11

Mechanical advantage (MA) is defined as a measure of the force amplification achieved by using a tool, mechanical device or machine system. In simple terms, mechanical advantage is the ratio of the force produced by a machine (output force or load) to the force applied to it (input force or effort).

𝑀𝐴= 𝑙𝑜𝑎𝑑/𝑒𝑓𝑓𝑜𝑟𝑡= 𝐹𝐿/𝐹𝐸

Question 12

What is velocity ratio?

Answer 12

Velocity ratio (VR) is defined as the ratio of the distance moved by the point of application of the effort to the distance moved by the load in a simple machine.

𝑉𝑅= 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑚𝑜𝑣𝑒𝑑 𝑏𝑦 𝑒𝑓𝑓𝑜𝑟𝑡/𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑚𝑜𝑣𝑒𝑑 𝑏𝑦 𝑙𝑜𝑎𝑑= 𝑑𝐸/𝑑𝐿

Question 13

What is efficiency?

Answer 13

Efficiency ( 𝜂 )is a measure of how much of the work you do actually goes to moving the thing you’re trying to move, compared to mechanical advantage which is a measure of how much your force is multiplied.

The efficiency, 𝜂, of a machine or mechanism is the ratio of the mechanical advantage to the velocity ratio and it is commonly expressed as a fraction or percentage. If the efficiency is 100%, then the mechanical advantage is equal to the velocity ratio.

𝜂= 𝑀𝐴/𝑉𝑅

Question 14

How do you calculate a Gear Ratio?

Answer 14

𝑉𝑅= 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑡𝑒𝑒𝑡ℎ 𝑜𝑛 𝑑𝑟𝑖𝑣𝑒𝑛/𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑡𝑒𝑒𝑡ℎ 𝑜𝑛 𝑑𝑟𝑖𝑣𝑒𝑟

Or

𝑉𝑅= 𝑟𝑎𝑑𝑖𝑢𝑠 𝑑𝑟𝑖𝑣𝑒𝑛/𝑟𝑎𝑑𝑖𝑢𝑠 𝑑𝑟𝑖𝑣𝑒𝑟=𝑑𝑖𝑎𝑚𝑒𝑡𝑒𝑟 𝑑𝑟𝑖𝑣𝑒𝑛/𝑑𝑖𝑎𝑚𝑒𝑡𝑒𝑟 𝑑𝑟𝑖𝑣𝑒𝑟=𝑐𝑖𝑟𝑐𝑢𝑚𝑓𝑒𝑟𝑒𝑛𝑐𝑒 𝑑𝑟𝑖𝑣𝑒𝑛/𝑐𝑖𝑟𝑐𝑢𝑚𝑓𝑒𝑟𝑒𝑛𝑐𝑒 𝑑𝑟𝑖𝑣𝑒𝑟

Question 15

What is work?

Answer 15

A force is said to do work if, when acting, there is a displacement of the point of application in the direction of the force, thus work is the product of force and displacement. The SI unit of work is the joule (J), which is defined as the work expended by a force of one newton through a displacement of one metre.

Question 16

How can GPE and Kinetic energy be used to solve a problem?

Answer 16

When an object is falling it has both kinetic and potential energy, this is due to the fact it has a velocity and it has not reached the ground thus it still has height. Using the Law of Conservation of Energy which states that energy cannot be created or destroyed in an isolated system, if we know the total energy in the system and assume no energy losses, we can determine the kinetic and potential energy at any point (ie. the mechanical energy stays constant throughout).

Question 17

The frictional force always acts to oppose motion and is always tangential to the surfaces in contact. True or false?

Answer 17

True

Question 18

The frictional force, FF, depends upon the nature and the relative roughness of the surfaces involved. True or false?

Answer 18

True

Question 19

A retarding force applied to a moving object does negative work on the object. True or false?

Answer 19

True

Question 20

The energy stored in an object is the measure of its capacity to release gravitational forces. True or false?

Answer 20

False

Question 21

In which of the following is work not done:

A. A body changes kinetic energy
B. A body changes potential energy
C. A body moves against a resistance
D. A body moves at a constant speed in space.

Answer 21

D

Question 22

Mike applied 10 N of force over 3 m in 10 seconds. Joe applied the same force over the same distance in 1 minute. Determine who did the most work.

a) MIKE
b) JOE
c) BOTH DID THE SAME
d) NO WORK WAS DONE BY EITHER

Answer 22

C

Question 23

The point at which metals become totally plastic is called:

a) yield stress
b) Youngs modulus
c) plastic limit
d) strain length

Answer 23

a

Question 24

A force of 545 N is exerted on the rope of a pulley system and the rope is pulled 10.0m. This work causes an object weighing 2520N to be raised 1.5m. The efficiency of this machine is:

a. 22%
b. 15%
c. 45%
d. 69%

Answer 24

d

Question 25

Two boys use a wheel and axle to raise a mass of 750kg. The radius of the wheel is 0.50m and the radius of the axle is 0.04m. If the efficiency of the machine is 62% and each boy exerts an equal force, approximately how much force must each apply?

a. 475 N
b. 295 N
c. 950 N
d. 680 N

Answer 25

a

Question 26

A drone starts from rest and accelerates at a constant rate vertically from the ground to a height of 100m, reaching a velocity of 20 m/s, it continues to climb vertically at a constant velocity until it reaches a height of 1 km. The drone then continues on at the same velocity heading south for 2 km at a constant height of 1 km before returning directly to its starting point, near the end of its journey the drone decelerates at 5 m/s2 so that it comes to a stop at its initial start position.
Calculate the time taken for the drone to complete its journey.

Answer 26

Answer: Leg 1: 55 s, Leg 2: 100s, Leg 3: 113.8s, Total time: 268.8s

Question 27

Explain why an engineer would recommend solar-powered water pumps for crop irrigation to a
community in a developing country. For three different types of technology knowledge, identify
two factors of each to support your explanation.

Answer 27

• explains appropriately using
  1 relevant environmental factor [1 mark]
  another relevant environmental factor [1 mark]
• explains appropriately using
  1 relevant economic factor [1 mark]
  another relevant economic factor [1 mark]
• explains appropriately using
  1 relevant sustainability factor [1 mark]
  another relevant sustainability factor [1 mark]

The engineer would have considered the local environmental conditions. Effective use of solar energy requires that most days are cloud free. This would allow the solar-powered pumps to operate efficiently. On overcast, cloudy days when rain is likely, the solar pumps would operate less often, saving water.
The engineer would also have considered the economics of the use of solar water pumps. The initial cost of the pumps is possibly high when compared to other methods, but running costs are low, which allows the community to transport water cheaply and efficiently.
It would be important to have a sustainable water supply, as solar pumps are able to supply large volumes of water. The engineer would have considered the sustainability of the solar pumps and the irrigation system, as breakdowns in the system would need to be repaired. This aspect of implementation of any mechanical system would need to be considered in the maintenance of the system.

Question 28

A crane requires 4355 W to vertically lift a full scrap metal bin a distance of 20 m from the ground at
constant velocity over a period of 90 s.

Determine the mass of the full scrap metal bin to the nearest whole unit.

Answer 28

The mass of the full scrap metal bin is 2000 kg.