Ch 4 Forces Unbalance forces will cause changes in speed and or direction of an object motion

Force Push or pull cause a change in motion of an object speed up or slow down or change direction. units: Newton = N = kgm/s2 1N = force that causes a 1kg mass to acceleration 1m/s2 1N=1kgm/s2

Weight Magnitude of the gravitational force exerted on an object 1lb=4.448N, 1N=0.225lb contact force: physical contact between objects field force: not physical contact, gravity, ele- force

Force diagram diagram of the object(s) involved in a situation and the forces exerted on the object(s) magnitude of  motion affected by magnitude and direction of the force

Force is a vector Use arrows & affect the object in the center free-body diagram shows the forces on object first isolate and identify all the forces acting on the object ( pg 123 steps free-body diagram) 13690N 5800N Results? 775N 14700N

Newton’s 1st Law object at rest remains at rest, object in motion continues in const. motion unless acted upon by a net force inertia: tendency of an object to maintain its state of motion Fnet=0 anet=0 , net force can be determine by a change in motion

External force interaction between the objects and environment net external force: vector sum of all forces acting determine the net acceleration

Board work An ag. student designed a support to keep a tree up-right. 2 wires attracted at rt angle to each other are 30.0N and 40.0N. Determine the placement and force of the 3rd wire. 50.0N 143ofrom F2 and 127o from F1 A kite acted on by a 9.8N force downward has a wind exert a 45N force at 50o above the hor. Find the angle and force on the string. 38N at 40o above the hor.

Equilibrium The state of a body in which there is no change in its motion Fnet = 0: closed vector diagram, last vector’s head attached to the tail of the 1st vector Force that brings an accelerating object into equlibrium must be equal and opposite to the force causing the object to acceleration Fx=0 and Fy=0

Newtons 2nd Law The acceleration of an object is directly proportional to the net force acting on the object and inversely proportional to the mass. F=ma: F= vector sum of all external forces acting on the object

Board work A 7.5 kg bowling ball initially at rest is dropped from the top of a 11m building. If it hits the ground 1.5s later, find the net force on the falling ball. -73.3N Space shuttle astronauts experience an acceleration of about 35m/s2 during take off. What force does a 75 kg astronaut experience during an acceleration of this magnitude? 2600N

Cont. Two people pull on a 208 kg boat. If they pull in the same direction the boat has an acceleration of 1.52m/s2. If they pull in opposite direction the boat has an acceleration of .518m/s2 to the left. What is the magnitude of the force exerted by each person on the boat? F1 = 212N ( a= 1.019m/s2) F2 = 104N ( a = .501m/s2)

Newton’s law components sum of forces x direction: Fx = max y direction: Fy = may if F = 0 and a=0 then v const. or zero

Board work A 75.0 kg man is hanging from 2 wires that make a 40.0o and 60.0o angle with the vertical. What is the tension of each wire? 480.N and 648N Fnet = T - Wt (mg) Fnet = manet A rope can support a 90.0 kg person. If this rope is used to lift a 60.0 kg load, what is the maximum upward acceleration that can be achieved without breaking the rope? 4.90m/s2

Newton’s 3rd law Forces always exist in pairs If two bodies interact the magnitude of the force exerted on object 1 by object 2 is equal to the magnitude of the force simultaneously exerted on object 2 by 1 and in the opposite direction for every action there is an equal and opposite reaction F1= - F2 m1a1= - (m2a2) Newton’s 3rd law

Action/reaction forces act on different objects do not result in equilibrium because they act on different objects

Every day forces Disregarding air resistance, will a bullet shot from a gun go father horizontally on earth or on the moon? The bullet will travel farther on the moon because the bullet is accelerated downward more slowly than on earth. gearth=-9.81m/s2 gmoon=-1.6m/s2

The normal force(FN) contact force exerted by one object on another in a direction perpendicular to the surface of contact FN always perpendicular to surface but not always opposite gravity (nonhorizontial surface) FN Fg FN (cos)Fg Fg

The force of friction friction opposes the applied force Fs(static friction): the force exerted on a motionless body by its environment to resist an external force Until the applied force is large enough to move the object, Fs=-F Fnet = F = difference between Fs & F

Kinetic friction(Fk) the force exerted on a moving object is less than Fs Once the object is moving: Fnet = F - Fk manet = ma -mmg (FN) Object stationary: surfaces are cold welded, moving objects not cold welded friction depends on the surfaces in contact

Fk,Fs & FN  = coefficient of friction, detemined by the contacted surfaces table 4-2 pg 138 s = Fsmax/FN k = Fk/FN Board work: A 91 kg refrigerator is placed on a ramp. The refrigerator begins to slide when the ramp is raised to 34o and slides at a constant speed at 27o. Cal. s and k. s = .67 and k = .51

Board work 2 students are sliding a 225kg sofa at constant speed across a wood floor. One student pulls with a force of 225N at a angle of 13o above the horizontal and the other student pushes with a force of 25N at 23o below the horizontal. k? k = .11

Air resistance form of friction ( FR): increase FR with increase speed. FR = Fg ( terminal velocity) a=0, vel constant free fall: vterminal ~ 200mph