Force and Friction II
Friction

Friction can be defined as the force that acts between two surfaces in contact and tends to resist the motion of one body over the other. It is simply the force that opposes relative motion between surfaces in contact.

Laws of solid friction
  1. Friction tends to oppose relative motion between two surfaces in contact. It acts opposite the direction of the motion.
  2. Friction is dependent on the nature of the surface in contact
  3. Friction is independent of the area of the surfaces in contact.
  4. The coefficient of static friction is greater than the coefficient of kinetic friction.
  5. Friction increases to the extent of the force that causes the motion.
  6. The friction of the moving object is proportional and perpendicular to the normal force
  7. Kinetic friction is independent of velocity
$$ F = µR $$ $$ where, $$ $$ F = \text{friction} $$ $$ µ = \text{coefficient of friction} $$ $$ R = \text{Normal force} $$ $$ R = mg $$
Types of friction
  1. Fluid friction: This is the force that opposes a motion that takes place within fluids or in between fluids. It is also called Viscous drag. An example is the movement of honey. It also reduces the motion of objects in air which is air resistance.
  2. Static friction: Static friction exists between a stationary object and the surface on which it is resting. It prevents an object from moving against the surface. It is the force that must be overcome for a body at rest to move over another body. For example, static friction prevents a book falling from a table slightly tilted.
  3. Kinetic or dynamic friction: This is force that must be overcome for a body to move with uniform speed over another body.
  4. Sliding friction: This is the force that resists the sliding of a body over another. It is the force that must be overcome for a body to slide over another.
  5. Rolling friction: This is the force that opposes the motion of a ball or wheel. It is the weakest type of friction.
Calculations

Example 1: A metal block of mass 5kg lies on a rough horizontal platform. If a horizontal force of 8N applied to the block through it's centre of mass just slides the block on the platform, what is the coefficient of limiting friction between the block and the platform?​(JAMB)

Solution

$$ F = µR $$ $$ \text{Sliding friction, F} = 8N $$ $$ mass = 5kg $$ $$ R = mg = 5 × 10 = 50N $$ $$ µ = \frac{F}{R}$$ $$ µ = \frac{8}{50}$$ $$ µ = 0.16N $$

Example 2: When a box of mass 25kg, is given an initial speed of 3m/s it slides along a horizontal floor a distance of 2.5m before coming to a rest. what is the coefficient of kinetic friction between the box and the floor? (g = 10m/s²)(WAEC)

Solution

$$ F = µR $$ $$ \text{sliding friction, F} = ma $$ $$ u = 3 $$ $$ s = 2.5m $$ $$ v = 0 $$ $$ m = 25kg$$ $$ g = 10m/s² $$ $$ \text{To get a } $$ $$ v² = u² + 2as $$ $$ a = \frac{v² - u²}{2s} $$ $$ a = \frac{0 - 3²}{2 × 2.5} $$ $$ a = \frac{-9}{5} $$ $$ a = 1.8m/s² $$ $$ F = ma $$ $$ F = 25 × 1.8 $$ $$ F = 45N $$ $$ R = mg $$ $$ R = 25 × 10 = 250N $$ $$ µ = \frac{F}{R} $$ $$ µ = \frac{45}{250} $$ $$ µ = 0.18 $$

Example 3: What is the coefficient of friction between a wood of mass 1.6kg ans a horizontal surface, if the limiting frictional force is 8N?

Solution

$$ µ = \frac{F}{R} $$ $$ R = mg $$ $$ R = 1.6 × 10 $$ $$ R = 16N $$ $$ µ = \frac{8}{16} $$ $$ µ = 0.5 $$

Friction Calculator


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Summary