# friction i 11/8/13. bellwork what is the description of friction force? what is the description of...

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• Friction I11/8/13

• BellworkWhat is the description of friction force?The force exerted by a surface as an object moves across it or makes an effort to move across it.

• AgendaGo over H.W.Sports figure video That Mu You DoNotes

• Friction I11/8/13

• Physics QuestionsAnswer True or False, in your notes, for the following questions14 questions totalYou will be answering them by a show of hands

• Question #1Ben travels to a distant planet. In outer space, with essentially no gravity, he can throw a baseball (mass = 0.5 kg) and a shot put (mass = 4 kg) horizontally at the same speed with the same force.

• Question #2Gravity in the International Space Station is zero.

• Question #3The force of gravity on an object is the same as the weight of the object.

• Question #4Mass and weight have essentially the same meaning. To increase the weight of an object requires an increase in the mass of the object.

• Question #5Friction is caused by surface roughness.

• Question #6An object will slow down if there is no net force.

• Question #7Passive forces exist (tables can exert force).

• Question #8Surface forces can't exceed the weight (active force) on an object.

• Question #9An object with a constant net force will have a constant acceleration.

• Question #10Forces can be negative.

• Question #11Larger (smaller) velocity means larger (smaller) acceleration.

• Question #12Zero velocity means zero acceleration.

• Question #13Spacecraft need thrusters to keep them going.

• Question #14Objects fall at the same rate because they have the same force of gravity acting on them.

• Question #1Ben travels to a distant planet. In outer space, with essentially no gravity, he can throw a baseball (mass = 0.5 kg) and a shot put (mass = 4 kg) horizontally at the same speed with no discernable difference in effort.FALSE! It still will take more force to accelerate a more massive object.

• Question #2Gravity in the space station is zero. FALSEThe reason that the astronauts act weightless is that they're inside a container which is FALLING!

• Question #3The force of gravity on an object is the same as the weight of the object.TrueForce of gravity is the same as the weight.

• Question #4Mass and weight have essentially the same meaning. To increase the weight of an object requires an increase in the mass of the object.FALSE, Weight is the gravitational attraction force on an object from a nearby planet. Mass in chemistry is the amount of matter present. In physics its inertia, or in other words the resistance to changes in motion.

• Question #5Friction is caused by surface roughness.True, Friction is mostly caused by chemical bonding between the moving surfaces; it is caused by stickiness. (When thinking about friction, don't think about grains of sand on sandpaper. Instead think about sticky adhesive tape being dragged along a surface.)

• Question #6An object will slow down if there is no net force.FalseThis is the Aristotelian idea that an object's natural state is rest. Where objects only move when a net force is exerted upon them. This stems from common everyday observations, e.g. students seeing that objects which have been pushed across the floor come to a stop (and not seeing friction as a dissipative force acting on the object).

• Question #7Passive forces exist (tables can exert force). TrueSome students believe that inert objects cannot exert a force. They can alter an object's motion, but they don't exert a force. This is untrue, the table DOES exert a force.

• Question #8Surface forces can't exceed the weight (active force) on an object.FalseIn acceleration situations surface forces are greater then the force of gravity, elevator starting upward, roller coaster at the bottom of a loop, an object hitting the ground.

• Question #9An object with a constant net force will have a constant acceleration.TrueConstant net force will accelerate an objects at a constant rate

• Question #10Forces can be negative.TrueThe negative means direction (down, left)

• Question #11Larger (smaller) velocity means larger (smaller) acceleration.FalseA small velocity that stops quickly has a high accelerationA large velocity that takes a long time to stop has a low acceleration

• Question #12Zero velocity means zero acceleration. FalseA ball thrown straight up at the top of it path has zero velocity, but is accelerating do to gravity (-9.81 m/s2)

• Question #13Spacecraft need thrusters to keep them going.FalseNewtons 1st Law An object in motion will stay in motion

• Question #14Objects fall at the same rate because they have the same force of gravity acting on them.False!So what is the reason? (Will be answered shortly.)

• Mass vs. WeightMass (scalar): Is the amount of matter an object has Unit is the kilogram (kg)Weight (vector): The force of gravity that acts on an object Unit is the Newton (kg m/s2)Mass and weight are two different quantities!

• Coefficient of FrictionCoefficient of Friction (scalar): The ratio between the force necessary to move one surface horizontally over another. It is reduced once the motion has started.Unit is a ratio (unitless)

• Types of FrictionStatic Friction is the friction force resisting the relative motion of two surfaces that are not movingKinetic Friction is the friction force of two surfaces that are sliding past each other

• Inertia Inertia (scalar): The resistance of a body or object to change its motion.Mass is the measurement of inertia (kg)

• Force Quantities