Forces, Pulleys, and Elastic Forces Terms

What does a force measure?

An interaction between two objects.

How many objects are involved in an interaction?

Exactly two objects.

What is a free-body diagram (FBD) used for?

To visualize the external interactions/forces acting on the chosen system.

What is Newton's 1st Law?

If the net force is zero, velocity is constant: ΣF = 0 → v = constant.

Does ΣF = 0 mean the object must be stationary?

No. It can be stationary OR moving at constant velocity.

What do unbalanced external forces determine?

They determine the CHANGE in an object's motion, not its current motion.

What is Newton's 2nd Law?

a = ΣF/m, or equivalently ΣF = ma.

How does mass affect acceleration for the same net force?

More mass/inertia means less acceleration for the same net force.

What is Newton's 3rd Law?

Interactions are mutual and equal in size: F₁,₂ = −F₂,₁.

Do Newton's 3rd-law force pairs cancel on one FBD?

No. The two forces act on different objects.

What is weight?

The force of gravity: Fg = mg.

What is tension?

A pulling force along an object such as a rope, string, cable, or chain.

Which direction does tension point?

Along the rope/string and away from the object being pulled.

What is the normal force?

A support force that points perpendicular to a surface.

What is friction?

A surface force that points parallel to the surface and is due to the roughness of the surface.

Does the normal force always equal mg?

No. Its value depends on the other forces acting on the object.

What is static equilibrium?

The object has zero acceleration, so the net force is zero: ΣF = 0.

For an object in equilibrium, what must be true in each direction?

ΣFx = 0 and ΣFy = 0.

How do you decide whether to use sine or cosine for a force component?

The component adjacent to the given angle uses cosine; the component opposite the angle uses sine.

What does a negative force component mean?

The force component points in the negative direction of the coordinate system; the force's magnitude itself is not negative.

What should you do first when solving a force problem?

Choose the system and draw a free-body diagram of the external forces acting on it.

What should you do after drawing the FBD?

Choose axes, resolve angled forces into components, then apply Newton's 2nd Law in each direction.

What do pulleys do?

Pulleys redirect tension in a rope or string.

For an ideal pulley, how does tension compare on both sides?

The tension has the same magnitude on both sides when the pulley's mass can be ignored.

How can a pulley system multiply applied force?

Multiple rope segments can support the load, so their tension forces combine to give increased output force.

What is Hooke's Law?

Fspring = −kΔx.

What does k represent in Hooke's Law?

The spring constant; it measures the spring's stiffness.

What does Δx represent in Hooke's Law?

The change in position from the spring's equilibrium position.

What does the negative sign in Fspring = −kΔx mean?

The spring force always points opposite the change in position. The spring constant k itself is not negative.

Why can't constant-acceleration equations generally be used for spring motion?

Spring force changes with position, so acceleration also depends on position.

How do springs in parallel combine?

They have the same displacement, their forces add, and kT = k₁ + k₂.

How do springs in series combine?

They have the same force, their displacements add, and 1/kT = 1/k₁ + 1/k₂.