MACHINES

Detailed Study:

Brief Revision:

https://www.youtube.com/watch?v=rgrsKmy6Ixo

Frequently Tested Topics:

  • Definition and concepts of mechanical advantage, velocity ratio and efficiency

  • Principle of ideal machine

  • Relationship between mechanical advantage, velocity ratio and efficiency

  • Speed and force multipliers

  • Levers in the human body

  • Classes of levers and diagrams

  • Diagrams of block and tackle system for given velocity ratio

  • Types of pulleys and functions

  • Numericals on pulleys

*Note that other aspects of the chapter are equally important.

Important Past Questions:

  • Name the type of single pulley often used as a machine though it does not give any gain in mechanical advantage. What is its purpose? (2013)

  • What is an ideal machine? (2011)

  • What is the purpose of an ideal machine? (2014)

  • Derive a relationship between mechanical advantage, velocity ratio, and efficiency of a machine. (2011, 2014)

  • A scissor is a _________ multiplier. (2015)

  • With reference to the terms mechanical advantage, velocity ratio and efficiency of a machine, name and define the term which will not change for a machine of a given design. (2009, 2016)

  • Why is a jack screw provided with a long arm? (2017)

  • Which class of lever will always have M.A > 1 and why? (2017)

  • Give 2 reasons why the efficiency of a single movable pulley is never 100%. (2003)

  • Explain why scissors for cutting cloth have blades much longer than handles, while shears for cutting metals have short blades and long handles. (2004)

  • How does an 'ideal machine' differ from a practical machine? (2013)

  • Can a simple machine act as a force multiplier and speed multiplier at the same time? (2013)

  • Which class of lever found in the human body is being used by a person:

i) when he holds a load on the palm of his hand?

ii) when he raises the weight of his body on his toes? (2013)

  • Draw a diagram of a lemon crusher, indicating the direction of load and effort. (2015)

  • A block and tackle system has V.R = 5.

i) Draw a neat labelled diagram of a system indicating the direction of load and effort.

ii) Rohan exerts a pull of 150kgf. What is the maximum load he can raise with this pulley system if its efficiency = 75%? (2015)

  • From the diagram below, answer the questions that follow:

machines_edited.jpg

i) What kind of pulleys are A and B?

ii) State the purpose of pulley B.

iii) What effort has to be applied at C to just raise the load L = 20kgf? (Neglect the weight of pulley A and friction.) [2016]

  • A pulley system has 3 pulleys. A load of 120N is overcome by applying an effort of 120N. Calculate the mechanical advantage and efficiency of this system. (2016)

  • A boy uses a single fixed pulley to lift a load of 50kgf to some height. Another boy uses a single movable pulley to lift the same load to the same height. Compare the effort applied by them. Give a reason to support your answer. (2017)

  • A pulley system with V.R = 4 is used to lift a load of 175kgf through a vertical height of 15m. The effort required is 50kgf in the downward direction, (g = 10 N/kg) Calculate:

i) distance moved by the effort

ii) work done by the effort

iii) M.A of the pulley system

iv) efficiency of the pulley system (2017)

  • A stationary wheel barrow has its center of gravity at A. The wheel and the leg are in contact with the ground. The horizontal distance between A and F is 50cm, and that between is 150cm. 

i) What is the direction of the force acting at A? Name the force.

ii) What is the direction of the minimum force at B to keep the legs off the ground? What is this force called?

iii) The weight of the wheel barrow is 15kgf. It holds sand of weight 60kgf. Calculate the minimum force required to keep the leg just off the ground. (2000)

question.png
  • A pair of scissors and a pair of pliers belong to the same class of levers. 

i) which one has M.A less than one?

ii) State the usefulness of such a machine whose M.A is less that one. (2001)

  • A block and tackle pulley system has a velocity ratio 3.

i) draw a labelled diagram of this system. In your diagram, clearly indicate clearly the points of application and direction of the load and effort.

ii) why should the lower block of this pulley system be of negligible weight? (2007)

  • A block and tackle system of pulleys has a velocity ratio 4.

i) draw a labelled diagram of the system indicating clearly the points of application and directions of load and effort.

ii) what is the value of the mechanical advantage of the given pulley system if it is and ideal pulley system? (2013)