• Is a force between two surfaces in contact and prevent their relative motion.
  • Is a force that opposes the sliding motion of a body over a surface.
  • Is measured in Newtons (N).

                        Types of frictional forces.

Dynamic/kinetic/sliding friction.

  • Is the force acting between two surfaces which are in contact and in relative motion.
  • The frictional force opposes the relative motion of the two forces.

Relative/static friction.

  • Is a force of friction between two surfaces in contact and at rest.

                        Factors affecting frictional force.

  1. The force pressing the two surfaces together or normal reaction. Force of friction is directly proportional to the force pressing together the two surfaces.
  2. The nature of the surfaces in contact. Force of friction depends on the surface’s nature. Rough surfaces have higher friction force than smooth surfaces.
  3. When the normal reaction is the same, all the force pressing the two surfaces together is the same.

Coefficient of friction (U).

  • Is the frictional force ratio to that of the normal reaction.

            U =

            U = f/r      or       F = UR

  • There are two types of coefficient friction.
  1. Coefficient static friction U
  • Is the ratio of static friction to the normal force pressing the two surfaces together with normal reaction.

            Us=  =

  1. Coefficient of kinetic friction U
  • The ratio of kinetic friction to the normal reaction when the body is moving at constant velocity.

            Uk=   =

          Uk = Uk= Us


  • Is the force of opposition offered by a fluid to a solid object passing through it.
  • Is the resistance to motion in fluids.
  • Is the force which opposes the relative motion between the layers of the fluid acting on a body.
  • 3 forces acting on the body are;
  2. The viscous drug F due to the liquid acting vertically upwards.
  3. Upthrust U due to the liquid acting vertically upwards.
  • When the object enters the liquid, its weight Mg is greater than the upward forces F and U and the resultant downward force accelerates the body.
  • The viscous drug increases in velocity and at a certain point, the upward forces is equal to the downward forces.

            U + F = W.

  • At this instant, the ball attains a constant/uniform velocity called terminal velocity (Vt). It is a constant velocity with which a boy moves through a fluid when the resultant force on the body due to the fluid is zero or the sum of upward forces is equal to the weight of the object falling the ground.

                                    Stokes law.

  • The force experienced by an object falling in a viscous liquid is directly proportional to
  1. The radius r.
  2. The velocity of the body.
  • These are the factors that affect the force- viscious.


  • The expression holds when.
  1. Radius r of object is small compared to the extent of the liquid surface.
  2. The ball does not create turbulence in the liquid.
  • Then at terminal velocity

Vt, n is given by




                        Application of friction.

  1. Used in walking.
  2. Movement of vehicle require friction between the road and tires.
  3. Moto car jack.
  4. Lighting a fire using a matchstick by sliding against the side of the matchbox.
  5. Sharpening of objects e.g. knife, panga, hoe.
  6. Used in brakes of vehicles.
  7. Transmission of motion with the use of belts in machines.

                        Disadvantage of friction.

  1. Cause machine parts and objects that rub against each other to wear out easily.
  2. Lowers the efficiency of machines as more effort is wasted in overcoming the friction.

                        How to reduce friction.

  1. Use of rollers and ball bearings.
  2. Lubrication using oil or grease or air.
  3. By streamlining the body.

                                    Angle of friction.

  • Consider a body of mass m placed on an inclined plane at °. The forces acting on it Ut at equilibrium balances.



1)fr= UR               2) =                  3)U =       4) =                  5)U =

U =                             R = mg                                     fr= mg           is the angle of friction.


                        Motion of connected particles.

  • Consider a body of mass M1 resting on a smooth horizontal surface and connected to another body of mass M2 by unrescistable string through a pulley.
  • Determine the
  1. Acceleration of the system if M2 moves downwards.
  2. The tension of the string.
  • If there is frictional force then

   a =

fa= T – fr

fr = M2g – T

M2a = M2g – T

M2a = M2g – M1a

M2a + M1a = M2g

a(M2 + M1) = M2g

a =             =



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