2. Force of Friction
• The force that resists the sliding of one
object over another is called force of
friction.
• Friction acts in a direction opposite to the
direction in which the body moves or
tends to move.
3. Force of Friction
• Static friction: If the man in the figure is trying to pull the block but is still unable to
move it because of the force of friction. Since the block is static, we call this friction
as static friction.
𝑓𝑠,𝑚𝑎𝑥 = 𝜇𝑠𝑁
• Kinetic friction: If the block starts to move on the surface, the force of friction still
tries to stop the motion of the block. Now the block is in motion, therefore, the
friction is called the kinetic friction.
𝑓𝑘 = 𝜇𝑘𝑁
• N is the normal reaction and 𝜇𝑠 and 𝜇𝑘 are the coefficients of static and kinetic
friction, respectively. Their values depend on the nature of surfaces in contact.
• In every case 𝜇𝑠 > 𝜇𝑘. This shows that static friction is always greater than the
kinetic friction.
19. If an object is in uniform circular motion, its acceleration is
always directed radially inward. Because of this, the
acceleration associated with uniform circular motion is
called centripetal acceleration (meaning “center seeking”
acceleration) and is written as
From Newton’s second law, we can write
𝐹𝑛𝑒𝑡 = 𝑚𝑎
𝐹𝑛𝑒𝑡 = 𝑚
𝑣2
𝑅
Therefore, for an object in uniform circular motion, the net force acting on it will provide the
centripetal force.
