PHYSICS 197 Section 1 Chapter N4 Statics October 18, 2017

Forces from Motion: Overview Apply Newton’s 2nd law of motion Knowing the forces involved are useful in many practical physical contexts, e.g.

Introduction to Statics The simplest motion is no motion. Statics problem: Any problem involving an object at rest. By Newton’s 2nd law, the net external force on the object must be zero. Simple principle: Balance the forces.

Clicker Question N4T.1 A weight hangs from a string but is pulled to one side by a horizontal string, as shown in the figure. The tension force exerted by the angled string is Less than the hanging object’s weight Equal to the hanging object’s weight Greater than the hanging object’s weight Depends on other factors

Answer N4T.1 A weight hangs from a string but is pulled to one side by a horizontal string, as shown in the figure. The tension force exerted by the angled string is Less than the hanging object’s weight Equal to the hanging object’s weight Greater than the hanging object’s weight Depends on other factors

Practice Problem

Demo: Sharing the Weight In both of the cases depicted below, the same weight is supported by two strings. In which case are the students holding the strings exerting a force of greater magnitude? Case A Case B Both the same

Demo: Sharing the Weight In both of the cases depicted below, the same weight is supported by two strings. In which case are the students holding the strings exerting a force of greater magnitude? Case A Case B Both the same Smaller component of tension pointing upwards. So need larger tension to balance the weight.

Clicker Question N4T.2 A person would like to pull a car out of a ditch. This person ties one end of a chain to the car’s bumper and wraps the other end around a tree so that the chain is taut. The person then pulls on the chain perpendicular to its length, as shown in the figure. The magnitude of the force that the chain exerts on the car is Much smaller than the force the person exerts on the chain About equal to the force the person exerts on the chain Much bigger than the force the person exerts on the chain. Need more information.

Answer Much smaller than the force the person exerts on the chain About equal to the force the person exerts on the chain Much bigger than the force the person exerts on the chain. Need more information.

Demo: Hangover How far out is it possible to overhang blocks from the edge of a table without them toppling? Is it better to make Big displacements at the bottom of the tower and smaller at the top Uniform displacements all the way up the tower Smaller displacements at the bottom and larger at the top? The Leaning Tower of Lire

With One Block Let’s start with one block. As long as its center-of-mass remains over the table, the block is stable. So we can slide the block maximum half-way over the edge.

With Two Blocks

With Three Blocks

With Four Blocks

With n Blocks

Infinite Tower! Divergent, but very slowly! Try at home! More details: http://datagenetics.com/blog/may32013/index.html

Static Problems Involving Torque Consider a situation where multiple forces act at different positions on an extended object at rest. These forces exert torques on that object. If the object is at rest, its angular momentum is conserved. So the net torque must also be zero (along with the net force, as per Newton’s 2nd law): Recall

Demo: Mass Hanging from Stick

Demo: Mass Hanging from Stick

Practice Problem and Demo

Solution