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SOLID FRICTION A-LEVEL PHYSICS

This Unit is about friction and how it can be calculated.

SOLID FRICTION

There are 2 types of friction i.e.

  • Static friction
  • Kinetic friction / sliding friction

Static friction opposes the tendency of one body sliding over the other. Kinetic opposes the sliding of one body over the other.

Limiting friction is the maximum friction between on two surfaces.

Laws of solid friction

  • Frictional force between 2 surfaces always oppose their relative motion or attempted motion.
  • For given pair of surface in contact, the limiting frictional force is proportional to the normal reaction.
  • For two surfaces in contact, the sliding frictional force is proportional to the normal reaction and independent of the relative velocity of these surfaces.
  • The frictional force is independent of the area of contact of the given surface provided the normal reaction is constant.

Molecular Theory and the laws of solid friction.

On a microscopic level, even a highly polished surface has bumps and hollow. It follows that when 2 surfaces are put together, the actual area of contact is less than the apparent area of contact.

FRICTION

At points of contact like a,b,c, small  cold-welded joints are formed by the strong adhesive  forces between the molecules in the two surfaces.

These joints have to be broken before one surface can move over the other.

This accounts for law 1.

The actual area of contact is proportional with the normal force (reaction). The frictional force which is determined by the actual area of contact at the joints is expected to be proportional to the normal force.

This accounts for law 1 and 3

If the apparent area of contact of the body is decreased by turning the body so that it rests on one of the smaller side, the number of contact points is reduced. Since the weight of the body has not altered, there is increased pressure at the contact points and this flattens the bumps so that total contact area and the pressure return to their original values.

Therefore, although the apparent area of contact has been changed, the actual area of contact has not.

This accounts for law 4

Coefficient of static friction

Consider a block resting on a horizontal surface

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The block is in equilibrium under the action of its weight W and normal reaction R.

Suppose a string is fixed to the block and the tension (T) in the string increased gradually, the static frictional force Fs; which posses the tendency of the block to ride over the surface comes in play. In equilibrium Fs=T.

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The value Fl of Fs at which the block starts moving is called the limiting frictional force (0<Fs Fl)

The ration of the limiting frictional force to the normal reaction is called the co-efficient of static friction µs

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Measurement of coefficient of static friction, µs

Method 1: Using a tilting plane.

A block is placed on a plane and the plane is tilted until when the block begins to slide. The angle of θ of inclination of the plane surface to the horizontal is measured.

The co-efficient of friction is given by µs = tan θ

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When the block is at the point of sliding

Fs = Wsinθ ………….(i)

R = Wcosθ…………..(ii)

(i) (ii)

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Method 2: To determine the co-efficiency of static friction.

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Masses are added to the scale pan until the block just slides. The total mass m of the scale pan and masses added is noted. The prodecures is repeated for different values of R obtained by adding known weights to the block.

A graph of mg against R(Mg) is plotted.

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The slope of the graph is µs

Co-efficient of kinetic (dynamic friction.

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Weights are added to a scale pan S and each time, body is given a slighht push.

At one stage, body continues to move with a constant velocity and kinetic frictional force F is then equal to the weight of the scale pam  together with the pan’s weight.

On dividing F by the weight of body, the co-efficient of dynamic friction canbe calculated.

Example 1

A car of mass 200kg moving along a straight road at a speed of 96kmh-1 is brough to rest by steady application of the brakes in a distance of 80m. find the co-efiecient of kinetic friction between the tires and the road.

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A car of mass 1.5 x 103kg and tractive pull 3.5×103N climbs a truck which is inclined  at an angle of 300 to the horizontaa. The speed of the car at the bottom of the incline is

20ms-1 and the coffient of sliding friction is 0.25, calculate

  • The distance travelled along the incline before the car comes to a halt.
  • The time taken ttravelling along the incline before the car comes to a halt.

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THIS VIDEO IS ABOUT FRICTION AND ITS TYPES

 

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