Newton’s LawsJune 5, 2012

Sample Problem 1A tetherball leans against a smooth, frictionless post. The string is 1.4 m long and the ball has a radius of 0.11 m with a mass of 0.27 kg. What is the tension in the rope? What force does the pole exert on the ball?

Sample Problem 2Two blocks are held in place on a frictionless incline. The mB = 10 kg and mA = 12 kg. The incline is tilted at an angle of 37o to the horizontal. Determine the tension in each rope.

FrictionFriction is a resistive force. Kinetic friction is resistive to motion and static friction resists motion in the first place.

In general, static friction is greater than kinetic friction

Kinetic Friction

Will always point in a direction opposite to the direction of motion

Ff = µkFn

Sample Problem 3

A box of mass 25 kg is pushed across a floor at a constant speed of 2 m/sec. What force is needed to push the box if there is a coefficient of friction of 0.2 between the box and the floor?

Sample Problem 4The weight of the crate is 500 N and there is a coefficient of friction between the crate and the floor of 0.4. If the worker pulls with a 400 N force, what is the acceleration of the crate?

Sample Problem 5

A pig slides down a chute inclined at 30o. If there is a coefficient of friction between pig and slide of 0.6, what is the acceleration of the pig down the slide?

Static FrictionResists the desire of an object to move.

Range of possibilities though we will typically look at the maximum case.

Static Friction

Static FrictionHarder to predict which way it will go.

Ffs < µsFn

Typical cases are the limits where Ffs = µsFn

Sample Problem 6A person on a stationary 125 N box with a force of 75 N at 30o below the horizontal. Determine the coefficient of static friction if the box just starts to move.

Sample Problem 7If the coefficient of static friction between the box and the truck bed is 0.6, what is the greatest acceleration the truck can have without the box slipping?