Physics Chp 8

• Torque

• This is the combination of force applied and distance from an axis point and can cause rotation.

• The force and distance from the axis must be perpendicular.

• If the torque can cause counter clockwise rotation then it’s positive

• Clockwise is negative

Fd The force and distance have to be perpendicular. If at an angle use the angle and sine or cosine to get only the component that is perpendicular!

Units are Nm

• What is the net torque on a 4 m teeter totter if you, 58 kg, and your friend, 45 kg, are on opposite ends with the fulcrum (pivot) in the center?

• τ =(58kg)(9.8m/s2)(2m) –(45kg)(9.8m/s2)(2m)

• τ = 255 Nm

0 m2m 2m

58 kg 45 kg

• Center of mass

• Moment of Inertia – Tendency to try to stay rotating– Different for each object as well as how it rotates– Ex. A ball around it’s edge vs it’s center

• In equilibrium

• ΣFx = 0 Σ Fy = 0 Σ τ = 0

• Steps to solve• 1) Free-body diagram• 2) Choose convenient axis, pivot point• 3) Apply the force equations• 4) Apply torque equation• 5) Solve

• If you, 55 kg, climb a 6.0 m, 25 kg ladder and stand on the rung that is 5.6 m up and the ladder is 40o from the ground. How much force does the wall apply to the ladder assuming no friction with the wall? How about the ground?

Fw

Fyou

Flad

Fgy

Fgx

6.0m

5.6m

3.0m

40o

• ΣFx = Fgx – Fw = 0 Fgx = Fw

• ΣFy = Fgy – Flad – Fyou = 0• Fgy = Flad + Fyou = • Fgy = (25kg)(9.8m/s2) + (55kg)(9.8m/s2)• Fgy = 784 N

• Στ = Fw(6.0m)sin40o – Fyou(5.6m)sin50o – Flad(3.0m)sin50o = 0

• Fw(6.0m)sin40o= (55kg)(9.8m/s2)(5.6m) sin50o + (25kg)(9.8m/s2)(3.0m)

sin50o

• Fw = 750 N

• Fgx = Fw = 750 N

• Simple Machines

• Changes the force applied to an object or the distance it’s moved (or both)

• Work remains the same (ignoring friction)

• Lever

• Inclined Plane

• Wheel and Axle

• Wedge

• Pulleys

• Screw

• Just an incline plane wrapped around itself

• All machines provide a mechanical advantage or why use them (not Rube Goldberg Machines)

out

in

FMA

F in

out

dMA

d

• Real Machines

• Friction causes a loss in the energy to do work

• Efficiency out

in

Weff

W