Physics 1010Comprehensive ReviewPart-3General Test Information Monday, Dec 18, 1:30-4:00pm, in this room Equivalent to ~16% of your final grade;that’s more than a letter grade Tuesday: practice exam Thursday: no lecture Today is last clicker set Comprehensive grades before final will beposted by TuesdayPractice Questions; NewtonA. 9.8 m/s2.B. 0.47 m/s2.C. 4.7 m/s2.D. 0.98 m/s2.Answer: mg=(M+m)a, a=mg/(M+m) BF =mgM = 2 kgm = 0.1 kgA cart of mass of 2kg is attached to a hanging weight of mass0.1kg. They accelerate at:Practice Questions; RampsAnswer: AGravity is always verticalYou are pushing a cart up a frictionless ramp at constant velocity. Which vector best represents:The direction of the force of gravity on the cart.Practice Questions; RampsAnswer: DFriction is parallel, reaction is perpendicular to the surface.Since there is no friction all the contact force is perpendicular.You are pushing a cart up a frictionless ramp at constant velocity. Which vector best represents:The direction of the force the surface of the ramp exerts on the cart.Practice Questions; RampsAnswer: EIt’s the opposite of the force you apply on the cart, since the total force has to be zero (constant velocity).You are pushing a cart up a frictionless ramp at constant velocity. Which vector best represents:The direction of the force of gravity and the force that the surface of the ramp exerts on the cart, added together.Practice Questions; RampsAnswer: B(1/2)mv2 = mgh => v = sqrt(2gh) = sqrt(2*9.8*1) m/s = 4.4 m/sAfter you get to the top of the frictionless ramp, at a hight of 1m from the ground, the cart slips from your hands and starts rolling down the ramp. When it reaches the bottom of the ramp, its velocity will be:vA) 19.6 m/s B) 4.4 m/s C) 9.8 m/s D) 3.1 m/sPractice Questions; RampsAnswer: A(1/2)mv2 = µmgL => L = v2/2µg = (4.4)2 / 2*0.3*9.8 m = 3.3mOR: L = v2/2µg = (2gh)/(2µg) = h/µ = 1.0/0.3 m = 3.3 mAfter the cart reaches the bottom of the ramp, it slides on thelevel ground with a coefficient of friction of µ=0.3.For what distance will it slide before it stops:vA) 3.3 m B) 6.6 m C) 1.6 m D) cannot tell, need massFrictional forces oppose motionsliding frictionForce of block on groundApplied forceReaction forceFriction forceFriction forceReaction forceFriction Frictional forces oppose motion. Two types: static and sliding. Sliding friction is caused by imperfections of the slidingsurfaces, and by molecules of the two material stickingtogether. Lubricants work by introducing an intervening layer ofmolecules that slide freely against each other. Sliding friction is proportional to the normal force and dependson the materials involved.tableblockmovefriction forceopposing motionRed and blue molecules stick to each other, although theystick to their own kind more strongly.Surface imperfections “collide” with each other.Frictiontableblockmovefriction forceopposing motionThe molecules of the lubricant (usually a liquid, but somesolids like graphite work on the same principle)penetrate into the imperfections of the surfaces, but slidereadily against each-other.Friction•Static friction can always be just a bit greater than slidingfriction.•Sliding friction force does not change much as speedincreases. Takes about as much force to go at slowconstant velocity as high constant velocity.SLIDING FRICTIONtireroadWhen Rolling, there is no SlidingIf no sliding takes place, what causes theresistance when pulling a heavy load onwheels?Surfaces deform. Deformations generate forces against the motion,transform energy to heat.Greenhouse Effect For one-layer atmosphere, theincindent power, Is, is absorbedby the ground. The ground then radiates backIg Some of Ig gets absorbed by theatmosphere, and some isallowed to escape, IgdIgdGreenhouse EffectIgd The absorbed power gets re-radiated in a random direction,so on average half goes tospace and half back to theground So now we have Is+Ia absorbedby ground, so the ground has tohave a higher T.Greenhouse EffectIgd So viewed from far into space,the Earth radiates Ia+Igd Part of Igd comes directly fromthe ground, through theradiative window If the temp of the groundincreases, this directly radiatedpower increases Since the total power Ia+Igd has to stay the same,increasing the directly radiated power, Igd, results in adecrease, in the power radiated by the atmosphere;so the atmosphere has to cool.Greenhouse Effect The importance of methane (CH4) is enhanced by thefact that it contributes to closing the radiative windowfrom the left (short wavelengths), increasing thechance of a runaway Greenhouse Effect(see Radiative Transfer applet