2.7 Kinetic and Static Friction

Friction is a force that opposes motion between surfaces in contact. It plays a fundamental role in everyday physics, from walking to driving a car. When surfaces slide against each other, kinetic friction acts to oppose this motion. When surfaces are at rest relative to each other, static friction prevents them from beginning to move. Both types depend on the normal force and the properties of the materials in contact.

Understanding friction is crucial in mechanics. The coefficient of static friction is typically higher than kinetic friction, which explains why it's harder to start moving an object than to keep it moving. Mastering friction calculations allows us to predict how objects will behave in various scenarios, from boxes sliding down ramps to vehicles braking on different road surfaces.

Kinetic Friction Between Surfaces

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Relative Motion and Friction

Kinetic friction occurs when two surfaces slide against each other, like a sled moving across snow or a book sliding across a table.

• It always opposes the motion of each surface relative to the other
• Acts in the direction directly opposite to the sliding motion
• Remains constant during sliding (assuming conditions don't change)
• Is independent of the contact area between surfaces

This last point is often counterintuitive - a brick sliding on its narrow side experiences the same friction force as when sliding on its wide side (assuming the same normal force).

Magnitude of Kinetic Friction Force

The kinetic friction force follows a simple mathematical relationship:

$F_k = \mu_k N$

Where:

• $F_k$ represents the kinetic friction force (in Newtons)
• $\mu_k$ stands for the coefficient of kinetic friction (unitless)
• $N$ denotes the normal force exerted by the surface on the object (in Newtons)

The coefficient of kinetic friction $\mu_k$ is a property of the specific pair of materials in contact. For example:

• Rubber on concrete: $\mu_k \approx 0.8$ (high friction)
• Metal on metal: $\mu_k \approx 0.2$ (moderate friction)
• Ice on ice: $\mu_k \approx 0.03$ (very low friction)

The normal force $N$ is always perpendicular to the contact surface. On a horizontal surface, it equals the object's weight ($N = mg$). On an inclined surface with angle $\theta$, it becomes $N = mg \cos\theta$.

Static Friction Between Surfaces

Contacting Surfaces at Rest

Static friction is an "adjustable" force that matches the applied force up to its maximum value.

• If you push lightly on a heavy box, static friction pushes back with exactly the same force
• This is why the box doesn't move until you push hard enough

Unlike kinetic friction which has a constant value during sliding, static friction varies in magnitude. It provides just enough opposition to prevent motion, adjusting itself to match any applied force up to its maximum value.

Prevention of Slipping or Sliding

Static friction works to maintain the status quo between surfaces at rest relative to each other.

• It can change both its magnitude and direction as needed
• It always acts parallel to the surfaces in contact
• It can only oppose motion up to a certain maximum value
• Once this maximum is exceeded, the surfaces begin to slip

For example, when you walk, static friction between your shoes and the ground prevents your feet from slipping backward. When you push a heavy bookshelf, static friction between it and the floor prevents it from moving until you push hard enough.

Static vs Kinetic Friction Coefficients

The relationship between static and kinetic friction explains why it's harder to start moving an object than to keep it moving.

• The coefficient of static friction ($\mu_s$) is almost always greater than the coefficient of kinetic friction ($\mu_k$)
• The maximum static friction force is calculated as: $f_{s,max} = \mu_s N$
• Static friction can take any value from zero up to this maximum: $0 \leq f_s \leq \mu_s N$
• Once motion begins, friction immediately drops to the kinetic value

This explains the common experience of objects "breaking free" and then accelerating suddenly when you push them hard enough to overcome static friction.

Practice Problem 1: Kinetic Friction on a Horizontal Surface

Solution

To solve this problem, we need to find the kinetic friction force using $F_k = \mu_k N$.

First, we need to determine the normal force. Since the box is on a horizontal surface: $N = mg = 5.0 \text{ kg} \times 9.8 \text{ m/s}^2 = 49 \text{ N}$

Now we can calculate the kinetic friction force: $F_k = \mu_k N = 0.3 \times 49 \text{ N} = 14.7 \text{ N}$

Therefore, the kinetic friction force acting on the box is 14.7 N, directed opposite to the box's motion.

Practice Problem 2: Static Friction and Motion

Solution

To determine if the crate will move, we need to compare the applied force with the maximum static friction force.

First, calculate the normal force: $N = mg = 10.0 \text{ kg} \times 9.8 \text{ m/s}^2 = 98 \text{ N}$

The maximum static friction force is: $f_{s,max} = \mu_s N = 0.4 \times 98 \text{ N} = 39.2 \text{ N}$

Since the applied force (35 N) is less than the maximum static friction force (39.2 N), the crate will not move.

In this case, the static friction force exactly balances the applied force: $f_s = 35 \text{ N}$

This demonstrates how static friction adjusts to match the applied force when that force is below the maximum static friction threshold.