Ch 4 Forces

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

2. 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

3. 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

4. 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

5. 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

6. 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

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

8. 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.

9. 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

10. 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

11. 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

12. 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)

13. 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

14. 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

15. 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)

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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

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