8.2:
Static Friction
Consider a man trying to push a couch on a slightly rough floor. When the applied force is zero, the couch is in equilibrium.
As the applied force increases, the frictional force increases linearly, keeping the couch in equilibrium. This shows the relation between the applied force and frictional force is linear.
When the applied force reaches a certain point, the couch is about to slip and the frictional force decreases slightly. It remains constant for some time, then decreases as the applied force increases.
The maximum value of static frictional force, known as the limiting static frictional force, is proportional to the normal force with a coefficient of static friction, which depends on the roughness of contacting surface.
At this point, the couch is on the verge of slipping and is in unstable equilibrium, known as the point of impending motion.
The angle made by the resultant reaction force with the normal force is known as the angle of static friction.
8.2:
Static Friction
Static friction is a force that opposes the relative motion or tendency of motion between two surfaces in contact. It plays a crucial role in our daily lives, from walking on the ground to driving a car.
For example, consider a scenario where a truck is connected to a car by a rope, ready to tow it along a road. When no external force is applied by the truck, the car remains stationary and is said to be in static equilibrium. In this case, the forces acting on the car, such as gravity and the normal force from the road, are balanced.
When the truck applies the to move the car, the frictional force between the car's tires and the road surface increases linearly, keeping the car in equilibrium.
As the applied force increases, there comes a point when the car slips and starts to move. At this moment, the frictional force between the car and the road decreases slightly due to the transition from static friction to kinetic friction.
Once the car has slipped, the frictional force remains relatively constant. However, as the applied force from the truck continues to increase, the frictional force opposing the car's motion eventually starts to decrease.
The range of static friction where the car remains stationary and in equilibrium, despite increasing applied force is known as the region of impending motion. In this region, the static frictional force is less than the normal force exerted on the car by the road surface.
When the car is on the verge of slipping, the maximum value of static frictional force, called the limiting static frictional force, comes into play. It is proportional to the normal force acting on the car, with a coefficient of static friction that depends on the roughness of the contacting surfaces (in this case, the car's tires and the road).
As the car approaches the point of slipping, the angle formed by the resultant reaction force and the normal force is referred to as the angle of static friction. This angle measures how close the car is to sliding and can be used to determine the effectiveness of the static friction between the car and the road.
Suggested Reading
- Hibbeler, R.C. (2016). Engineering Mechanics: Statics. Fourteenth Edition, New Jersey: Pearson. Pp. 403.
- Meriam, J.L., Kraige, L.G. and Bolten, J.N.(2016). Engineering Mechanics: Statics and Dynamics. Eighth Edition, Singapore: John Wiley & Sons. Pp. 334.