Applications of Newton’s Laws Chapter 6 Friction, Strings & Springs Translational Equilibrium TIMED QUIZ – Monday? Over Force Balances, Free Body Diagrams, Friction

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

Frictional Forces – Kinetic Friction Kinetic 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 Friction The 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 v o = 1.15 m/s Stops in m So v f is? Value of μ k ? Stopping Δt for different v o ?

Example 6-1 in Textbook Part A: Start with Newton’s 2 nd Law How do you know a x ?

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

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 Springs The 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 pg 162

Homework Problems #22, pg 179