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Mechanical Power Transmission Fundamentals: Energy, Force, Inertia, Acceleration, Friction, and Work

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  1. MECH1200 Topics: • Introduction to Chapter 3: Mechanical Power Transmission Fundamentals • Energy • Force • Inertia • Acceleration • Friction • Work • Note: Refer to textbook Mechanical Principles and Systems for Industrial Maintenance, Richard Knotek and Jon Stenerson, Prentice Hall, 2006

  2. MECH1200 Introduction • Usually mechanical systems consist of the following elements: • A prime mover: such as en electric motor or an internal combustion engine. • Linking components: such as shafts, gears, belts, joints..etc. • Driven components such as wheels.

  3. MECH1200 Energy Exists in many forms: kinetic, potential, heat, electrical, chemical, ..etc. • Energy cannot be created nor destroyed, but it changes from one form to another. Law of conservation of energy - 1st law of thermodynamics • Some common units of energy: • Joule (J) • Foot-pound force (ft.lb) = 1.356 Joules • KiloWatt-hour (KWH) = 1000 Watt.h = 3600 kJ • Calorie = 4.2 kJ = 4200 J

  4. MECH1200 Force Force: causes an object that has a mass to change velocity. • A vector quantity: It has direction and magnitutde. • Some common units of force: Newton (N) Pound force (lbF) = 4.45 N

  5. MECH1200 Inertia • Inertia: the resistance of an object to any change in its motion state. • Example 1: Object A (1 kg) and object B (10 kg) are each acted upon by a force of 1 N to the right. which would move at a higher acceleration? A 1kg 1 N B 10 kg 1 N Object B has a larger inertia, thus more resistance to change in its rest state, so it has a lower acceleration.

  6. MECH1200 Newton’s Laws of Motion Newton’s First Law (Law of Inertia): A body in rest will stay in rest and a body in motion will stay in motion unless an external force is acted upon it. Newton’s Second Law (conservation of momentum): The net force acting on an object is equal to the rate of change of its momentum. Forcenet = mass × acceleration Newton’s Third Law (action-reaction law): For every action there is a reaction equal in magnitude and opposite in direction. FA = - FB

  7. MECH1200 Acceleration & Deceleration • The change of speed of a body. If the speed decreases, then it is a deceleration. • From Newton’s 2nd Law: • Example 2: • What is the acceleration of object A if a force of 2 N acts on it? • What is the acceleration of object B if a force of 20 N acts on it? A 1kg 2 N B 10 kg 20 N Both objects will move at the same acceleration and acquire the same final speed. Both objects will have an acceleration of 2 m/s2

  8. MECH1200 Acceleration & Deceleration • Example 3: • A vehicle is moving at a speed of 20 ft/s. After 4 seconds, the vehicle was moving at a speed of 15 ft/s. what is the acceleration of the vehicle? • Solution: Thus the vehicle decelerates by 1.25 ft/s2.

  9. MECH1200 Angular Speed • Angular speed: a measurement to describe an object rotating about an axis. • Units of measurement: • RPM • Rad/s Surface velocity (ft/min) = RPM× π × diameter (ft) = RPM ×0.262 × diameter (in inches) Example 4: what is the surface velocity of a 20 in diameter rotor that rotates at 1500 rpm? Solution: = 1500 × 0.262 × 20 = 7860 ft/min

  10. MECH1200 Friction • The resistance to a mechanical force. • Applies to solids and fluids • Types of friction: • Static friction: when the contacting surfaces are at rest. • Sliding friction: when one or both of contacting surfaces is/are at motion. • Rolling friction: when the surfaces are separated by rollers. How to quantify friction? Coefficient of friction relates force to mass of object under force.

  11. MECH1200 Work • Work = force × distance ( W = F × D) • Units of work: N.m or ft.lb • Example 5: • A force of 10 ft.lb is applied on a body at rest that has a mass of 4 lb. Then the force was removed after the body moved 5 ft. 4 kg 10 N What is the acceleration that the body acquires while the force is applied? F = ma thus a = F/m = 10 / 4 = 2.5 ft/s2 b) What is the work performed on the body? W = F×D = 10 × 5 = 50 ft.lb

  12. MECH1200 Exercise 1. Which of the following are vectors and which are scalar quantities: Temperature Pressure Velocity Acceleration Energy Force Inertia scalar scalar vector vector scalar vector scalar

  13. MECH1200 Exercise 2. Match each of the following terms to its unit of measurement: Mass Energy Velocity Acceleration Force Power Angular speed Kilowatt.hour Newton kilowatt ft/s Kg RPM m/s2

  14. MECH1200 Exercise 3. What is the acceleration (g) of the earth’s gravitational force at sea level? Solution: 9.81 m/s2 = 32.2 ft/s2

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