8 Friction and Mechanical Applications Free Online FlashCards

What are the two components of contact force in the Coulomb dry-friction model?

In the Coulomb model, contact force has a normal component perpendicular to the surfaces and a friction component tangent to them.

How does static friction behave before slipping begins?

Static friction adjusts as needed from zero up to its limit: $0 \leq F_s \leq \mu_s N$.

What is the static-friction condition at impending slip?

At impending slip, static friction reaches its limiting value: $F_s = \mu_s N$.

What equation models kinetic friction during sliding?

During relative sliding, the simplified model gives $F_k = \mu_k N$, where $\mu_k$ is the coefficient of kinetic friction.

What motion does friction oppose at a contact?

Friction opposes relative motion or impending relative motion at the contact, not necessarily the body's overall motion.

How should contact forces between two bodies appear on their free-body diagrams?

Draw a separate free-body diagram for each body, showing the forces acting on that body. Contact forces between two bodies are equal and opposite and appear on their respective diagrams.

What does a negative friction result mean?

If an assumed friction direction gives a negative result, the actual friction direction is opposite to the assumed one.

How is a block's weight resolved on an incline?

For an incline at angle $\theta$, the weight components are $W_{\parallel}=mg\sin\theta$ and $W_{\perp}=mg\cos\theta$.

What balances a resting block's downslope weight on an incline?

With no applied force along the plane, static friction balances the downslope weight component: $F_s=mg\sin\theta$, if this is within the friction limit.

At impending downslope slip, how does incline angle relate to $\mu_s$?

For the simple setup at impending downslope slip, $mg\sin\theta=\mu_smg\cos\theta$, so $\mu_s=\tan\theta$.

How should a wedge system be analyzed?

Draw a free-body diagram for the wedge and each body it contacts; include each contact's normal and friction forces, then apply equilibrium to each body.

How can a wedge use input force, and what role can friction play?

A narrow wedge can amplify an input force to separate or lift other bodies, while friction can resist driving or help the wedge hold position.