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Static Friction and Kinetic Friction Physics Problems With Free Body Diagrams
The Organic Chemistry Tutor
Overview
This video explains the concepts of static and kinetic friction, detailing how they oppose motion and the conditions under which each applies. It covers the formulas for calculating these forces, emphasizing that static friction has a maximum value while kinetic friction is constant when an object is sliding. The video demonstrates these principles through several example problems, including calculating friction forces, determining the minimum force to initiate sliding, and finding acceleration when friction is present. A key focus is on how the normal force can change based on applied forces, affecting friction calculations, especially when forces are applied at an angle.
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Chapters
- Static friction prevents an object from moving when a force is applied.
- Kinetic friction opposes the motion of an object that is already sliding.
- Static friction is represented by an inequality (Fs <= μs * N) indicating a range of possible values.
- Kinetic friction is represented by an equation (Fk = μk * N), indicating a constant value when sliding occurs.
- The coefficient of static friction (μs) is generally greater than the coefficient of kinetic friction (μk).
Understanding the difference between static and kinetic friction is crucial for predicting whether an object will move and how it will behave once it starts moving under the influence of applied forces.
Pushing a heavy box: initially, static friction prevents it from moving. Once pushed hard enough to slide, kinetic friction opposes the ongoing motion, and it feels easier to keep it moving than to start it.
- The normal force (N) is the force exerted by a surface perpendicular to that surface.
- On a horizontal surface with no other vertical forces, the normal force equals the object's weight (N = mg).
- The maximum static friction is calculated as Fs_max = μs * N.
- The kinetic friction is calculated as Fk = μk * N.
- Static friction adjusts its value to match the applied force up to its maximum limit.
Accurate calculation of normal force is fundamental to determining both static and kinetic friction, which directly impacts the net force and subsequent motion of an object.
For a 5 kg box on a horizontal surface (μs=0.4, μk=0.2), the normal force is 49 N. The maximum static friction is 19.6 N, and kinetic friction is 9.8 N. If you push with 10 N, static friction opposes with 10 N; if you push with 19.6 N, static friction opposes with 19.6 N; if you push with 20 N, the box slides, and kinetic friction opposes with 9.8 N.
- To make an object begin to slide, the applied force must exceed the maximum static friction.
- The minimum force required to start sliding is equal to the maximum static friction.
- Once sliding begins, static friction is replaced by kinetic friction.
- The net force determines the acceleration of the object (F_net = ma).
These problems demonstrate the practical application of friction formulas to determine the conditions necessary to overcome static friction and initiate movement.
A 15 kg box requires a minimum horizontal force of 51.45 N (μs=0.35) to start sliding. If pushed with 90 N, the acceleration is 4.04 m/s² (using μk=0.2).
- The coefficient of static friction (μs) can be found if the maximum static friction and normal force are known.
- The coefficient of kinetic friction (μk) can be found if the kinetic friction force and normal force are known, or by using the net force and acceleration.
- When calculating acceleration, the net force is the applied force minus kinetic friction.
- The relationship μs > μk generally holds true.
This section provides methods to calculate the friction coefficients themselves, which are inherent properties of the surfaces in contact, and to determine acceleration when kinetic friction is acting.
For an 8 kg box, if 65 N is needed to start it moving (μs=0.829), and it accelerates at 1.4 m/s² when pushed with 65 N, the coefficient of kinetic friction is 0.686.
- When a force is applied at an angle, its components (horizontal and vertical) must be considered.
- The horizontal component of the applied force contributes to overcoming friction.
- The vertical component of the applied force can alter the normal force.
- If the vertical component pulls upward, it reduces the normal force, thus reducing friction.
- If the vertical component pushes downward, it increases the normal force, thus increasing friction.
This addresses more complex scenarios where applied forces are not purely horizontal, requiring a more detailed analysis of force components and their impact on the normal force and friction.
A 30 kg box pulled by a 150 N force at an angle (e.g., 30 degrees) has a horizontal component (150*cos(30)) to overcome kinetic friction (μk=0.25). The upward vertical component (150*sin(30)) reduces the normal force from mg to mg - Fy, resulting in a normal force of 219 N and an acceleration of 2.505 m/s².
Key takeaways
- Static friction is a variable force that matches the applied force up to a maximum, preventing motion.
- Kinetic friction is a constant force that opposes motion once an object is sliding.
- The maximum static friction is always greater than or equal to kinetic friction for the same surfaces.
- The normal force is critical for calculating friction and can be affected by other applied forces.
- Forces applied at an angle must be resolved into components to correctly analyze their effect on motion and friction.
- Understanding friction is key to analyzing the dynamics of objects in contact with surfaces.
Key terms
Static FrictionKinetic FrictionCoefficient of Static Friction (μs)Coefficient of Kinetic Friction (μk)Normal Force (N)Applied ForceWeight Force (mg)Net ForceAccelerationForce Components
Test your understanding
- What is the fundamental difference between static and kinetic friction, and when does each type apply?
- How does the normal force influence the magnitude of both static and kinetic friction?
- Explain why static friction can have a range of values, while kinetic friction typically has a single value when an object is sliding.
- How would applying a force at an upward angle to a box affect the normal force and the resulting friction compared to applying the same force horizontally?
- What is the minimum condition required for an object to begin sliding, and how is this related to the coefficients of friction?