Applications of Newton’s Laws

Chapter 6Friction, Strings & SpringsTranslational Equilibrium

TIMED QUIZ – Monday?Over Force Balances, Free Body Diagrams, Friction

Frictional ForcesFriction has its basis in surfaces that are not completely smooth:

Frictional Forces – Kinetic FrictionKinetic friction: the friction experienced by surfaces sliding against one another

The static frictional force depends on the normal force:

(6-1)

The constant is called the coefficient of kinetic friction.

Frictional Forces – Kinetic FrictionThe kinetic frictional force is also independent of the relative speed of the surfaces, and of their area of contact.

The static frictional force keeps an object from starting to move when a force is applied. The static frictional force has a maximum value, but may take on any value from zero to the maximum,

Frictional Forces – Static Friction

depending on what is needed to keep the sum of forces zero.

Frictional Forces – Static Friction

(6-2)

where

(6-3)

The static frictional force is also independent of the area of contact and the relative speed of the surfaces.

Example 6-1 in Textbook

• Salt shaker - 50 g• With vo = 1.15 m/s• Stops in 0.840 m• So vf is?

• Value of μk?

• Stopping Δt for different vo?

Example 6-1 in Textbook

• Part A: Start with Newton’s 2nd Law

• How do you know ax?

Example 6-1 in Textbook

• Need force balance in y-direction ( for Normal )

• Why do we need the Normal Force on shaker?

Example 6-1 in Textbook

• Part B: what is the same?

• Use relationship for constant acceleration

Conceptual Checkpoint

• 6-1 pg 155

Conceptual Checkpoint 6-1

Conceptual Checkpoint 6-1

Homework Problems

• #6, pg 178 ( HONORS )

• #10, pg 178 ( AP )

Strings and Springs

When you pull on a string or rope, it becomes taut. We say that there is tension in the string.

Strings and SpringsThe tension in a real rope will vary along its length, due to the weight of the rope.

Here, we will assume that all ropes, strings, wires, etc. are massless unless otherwise stated.

Strings and Springs

An ideal pulley is one that simply changes the direction of the tension:

Strings and Springs

Hooke’s law for springs states that the force increases with the amount the spring is stretched or compressed:

The constant k is called the spring constant.

Translational Equilibrium

When an object is in translational equilibrium, the net force on it is zero:

(6-5)

This allows the calculation of unknown forces.

Translational Equilibrium

Conceptual Checkpoint

• 6-2 pg 159• 6-3 pg 162

Homework Problems

• #22, pg 179