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NEWTON’S LAWS 1. 2. 3. 4. Solving Force Problems Draw a FBD Identify direction of movement Write the expression for ΣF = F1+F2-F3… Set ΣF expression equal to mtotanet F1+F2-F3…= mtotanet INERTIA •Newton’s 1st Law •An object wants to keep on doing what it is already doing •In order to change, it needs a net force not equal to zero if å F = 0, then a = 0 INERTIA •If the net force is equal to zero then: 1. an object will stay at rest OR 2. an object will continue to move at a constant velocity. if å F = 0, then a = 0 EXPLORING ST 1 LAW: TORQUE •Translational motion: motion without rotation •Torque can cause rotation •If a force is applied to an object with a fulcrum, you get rotational motion (torque) •Fulcrum: point of rotation (ex. Hinge) Fulcrum TORQUE •Rotational Force is most effective far from the fulcrum •Torque is the combination of force and distance from fulcrum (lever arm = r) t = Fr r A r B r=0 C Which lever arm has greatest torque? TORQUE •Only the perpendicular part of the force is applied to the torque. •This requires a touch of trig. t = Frsinq q r F A A St = F1 F2 E St = F1 F2 F3 F4 ROTATIONAL STABILITY •1st Law, rotational style: •“In the absence of a net torque, an object not rotating will continue to not rotate and an object rotating will continue to rotate at a constant rotational velocity” •This means that if a rotational system is in balance (not moving), the net torque is zero. ROTATIONAL STABILITY •1st Law, rotational style: if St = 0 then a = 0 •Convention: CW is negative, CCW is positive •Which dog has a positive torque? CENTER OF MASS •Where the weight force of an object is located COM CENTER OF MASS •Objects are balanced when the COM is at the fulcrum •WHY? b/c the lever arm = 0 r=0 COM Dr Carlson's Science Theatre CENTER OF MASS •The Weight force of the COM can add a torque if it is not at the fulcrum. 1. What is applying the torque on the left side? The center of mass! Fcm St = 0 = t CM - t W Fw CENTER OF MASS •Objects in freefall will rotate about their center of mass CENTER OF MASS •Concentration of mass •Where the weight force of the object is located •Weight force can add a torque if it is not at the fulcrum. Fweight = ma A father wants to let his 55 lb. daughter play on a seesaw alone. Assume she sits at the end of the 10.0-foot long board. If the board has a weight of 100 lbs where must he slide the board so that the board’s weight will balance her weight? Center of mass r 5ft - r St = 0 t CM = t girl 10ft FCM = 100lbs Fgirl = 55lb A uniform plank 16.0-m long, weighing 300 N, rests symmetrically on two supports 8.00 m apart, as shown. A woman weighing 800 N walks toward the right. How far beyond point B can she walk before the plank tips? 16m rcm= 4m 8m FCM = 300N Center of mass FWoman = 800N St = 0 = t CM - t Woman 0 = FCM rCM - FWomanrWoman 0 = (300N)(4m)-(800N)rWoman A uniform plank 16.0-m long, weighing 300 N, rests symmetrically on two supports 8.00 m apart, as shown. A woman weighing 800 N walks toward the right. How far beyond point B can she walk before the plank tips? rcm= 4m FCM FWoman 0 = (300N)(4m)-(800N)rWoman (300N)(4m) = (800N)rWoman (1200Nm) = rWoman =1.5m 800N To the right of fulcrum B 4. In the previous problem, how far from the right end of the plank should support B be placed so that the woman can walk just to the right end of the plank without causing it to tip? rcm= FCM FWoman MASS WEIGHT •Mass is a measure of how much matter makes up an object • Weight is a Force • Depends on object itself – NEVER CHANGES • Mass is a measure of inertia •Mass is measured in: grams, Kilograms • Depends on object’s location • Weight is measured in: Newtons (N) or pounds (lbs) Mass: 70 kg Mass: 70 kg Weight: 700 Newtons Weight: 112 Newtons (157 pounds) (25 pounds) Mass is Same!! Lab: pg 375-377 Write givens and equations Box answers & include units Mass needs in kg r (distance to fulcrum) in m Mass vs. Weight You can convert mass into weight using Newton’s Second Law F = ma On Earth, a = 9.8m/s2 Static Equilibrium An object is in Static equilibrium if: 1. it is at rest (not moving) 2. the sum of the torques acting on the object are zero 3. the sum of the forces acting on the object are zero St = 0 SF = 0 A uniform plank 16.0-m long, weighing 300 N, rests symmetrically on two supports 8.00 m apart, as shown. Draw a monkey weighing 50 N on top of support B. Fweight Fmonkey Center of mass 1. Label the center of mass Now assume support B is the fulcrum 2. Label the length of the lever arm to the center of mass What is the length of the lever arm to the monkey? 0m Does the Monkey create a torque? No! rCM = 4m Fweight Fmonkey Problems pg 385 Where do you start? 1. 2. 3. 4. 5. Draw a diagram! Label CoM and draw all force vectors Set net torque = 0, (for #10 SF = 0 ) Measure lever arms (r) Useτ= Fr and F=ma to solve for unknown.