1915, Henry Sincosky of PhiladelphiaIncluded on a list of Feats of Suspended Bodyweight Strength

His Accomplishment: Suspended himself from a rafter by gripping the rafter with the thumb of each hand on one side and the fingers on the opposite side.

Data: • Sincosky’s mass was 79 kg• Coefficient of static friction between hand and rafter can be

approximated to be 0.70

What makes this amazing? Calculate the least magnitude of the force he must apply on the rafter from each thumb or opposite fingers.

(After suspending himself, Sincosky chinned himself on the rafter and then moved hand-over-hand along the rafter. If you do not think Sincosky’s grip was remarkable, try to repeat his stunt!)

Two strategies/concepts are needed to understand this situation:

• Since he didn't fall, the forces on him must be balanced. In other words, he is in equilibrium.

• Friction holds him up, not his applied force, although his applied force directly determines the available force of friction.This type of friction is called static friction, because the surfaces do not move relative to each other.

Fg

Types of Forces

Points downwardUsed for any object with massFg = mg g = 9.8 N/kg on Earth (about 10 N/kg)

FNPoints perpendicular away from surfaceResponds to other forces that are along same lineMain purpose is to determine the force of friction

Ff Points parallel to surface, against motion or against intended motionIf the surfaces move relative to each other: If the surfaces do not move relative to each other: Value depends on what is needed for balance There is a maximum possible value:

FT Points along rope, away from object it pulls onFor ideal ropes (massless, no stretch) same along each section of ropeMust calculate indirectly

FS Points along spring opposite direction of displacementFor ideal springs:

Fapp External object provides the force

appl

ied

forc

e

time

After conducting an inquiry lab, we discussed the graph created when we pulled a block. There were two distinct parts to the motion: When it was not moving and when it moved

STATIC(no motion of surfaces relative to each other)

The applied force equals the force of friction, until a maximum value is reached.

Ff  =  whatever  is  required  to  balance  the  other  forces

appl

ied

forc

e

time

STATIC KINETIC(surfaces move relative to each other)

The force of friction has a constant value, independent of the type of motion.

In our lab, the applied force equaled the frictional force because I told you to pull at constant rate. In MANY situations, the applied force is greater than or less than the frictional force, i.e. the object has an acceleration.

This graph shows a slightly different situation, but the interpretation is similar.

maximum  static  friction

Short review of equilibrium - working with forces on a particle or a system

Find the unknown mass in terms of the other quantities and fundamental constants.

Find the values of tension and the unknown mass.

5.0 kgm

Fg

FappFN

Ff

Data: Sincosky’s mass was 79 kg. Coefficient of static friction between hand and rafter can be approximated to be 0.70. Calculate the least magnitude of the force he must apply on the rafter from each thumb or opposite fingers.

In order to push my couch to the other side of the room, I first had to move it from where it was. If the couch still has a mass of 55 kg, and the coefficient of static friction, µs, is 0.74, what is the minimum horizontal force I needed to apply to get it moving?

A force F is applied to a block (m = 5.0 kg) at rest on a plane inclined at 30°. µs is 0.80 and µk is 0.40(a) What force F must be applied to get the block on the verge of moving?

(b) Using this force, the block begins to move down the ramp. What acceleration does it have?

A wooden pallet carrying a load of 600 kg rests on a wooden floor. µs is 0.28 and µk is 0.17

(a) A forklift driver decides to push it without lifting it. What force must be applied to just get the pallet moving?

(b) After a bit of time, the pallet begins to slide. How fast is the pallet moving after 0.5 seconds of sliding under the same force you calculated in part a?

(c) If the forklift stops pushing, how far does the pallet slide before coming to a stop?