UNIVERSITY of PLYMOUTH

 

MECH115: Engineering Science 1

 

Tutorial examples: - Friction                                                  [Use g = 9.81 m/s² where required]

 

 

  1. The slope angle required to cause one piece of wood to begin to slide over another is measured at 34º. What is the coefficient of friction between the wood surfaces?  [0.675]

 

  1. The force required to slide a block of hardened steel of mass 10 kg over a flat cast iron surface (dry) is found to be 24.5 N. When the two faces were lubricated the force reduced to 7.85 N. Find the coefficients of friction between surfaces dry and lubricated and find the percentage reduction in friction because of lubrication.              [0.25, 0.08, 68% ]

 

  1. A railway wagon is found to roll down an incline of 1 in 125 at a steady speed. Assuming there is negligible air resistance to its motion what is the coefficient of rolling friction of its steel wheels on the steel rails.           [0.008]

 

  1. The angle of placing one's foot on the ground when walking is approximately 12º from the vertical. If the coefficient of friction between foot and ground reduces to 0.15 (such as when stepping on a banana skin) show that there is a serious risk of slipping. [ans]

 

  1. The coefficient of static friction for rubber on dry tarmac is approximately 1.0, whereas the coefficient of dynamic friction is approximately 0.91. If a car is travelling at 70 mph find the difference in stopping distance between the two situations of when the tyres are just on the point of skidding (such as can be achieved with ABS braking systems) and when they actually skid.  [4.92 m]

 

  1. Find the force (in tonnes) required to drag a boat of weight 1.5 tonnes up a ramp angled at 33º to the horizontal if the coefficient of sliding friction between the boat bottom and the ramp is 0.17.    [1.03 tonnes]

 

  1. The coefficient of friction between the feet of a ladder and the floor is approximately 0.3. If the contact with the wall is considered smooth, find the angle at which the ladder is just on the point of slipping with a load half way up it.     [31º]

 

  1. It is often thought that heavy items placed on the floor of a vehicle at less at risk of sliding around when the vehicle accelerates or decelerates than are light items. Show that this is not true, and the tendency to slide depends wholly on the coefficient of static friction between the item and the floor. [ans]

 

  1. A lubricated plain bearing of 50mm diameter supports a load of 206 N. Its estimated coefficient of sliding friction is 0.0075. If the shaft is rotating at 580 RPM, find the frictional torque and hence the power required to overcome friction.         [ 0.0386 Nm, 2.35 W]

 

 

 

 

 

115tut_friction