# Friction I 11/8/13. Bellwork What is the description of friction force? What is the description of friction force? The force exerted by a surface as an.

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Friction I 11/8/13

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

Agenda Go over H.W. Go over H.W. Sports figure video – “That Mu You Do” Sports figure video – “That Mu You Do” Notes Notes

Friction I 11/8/13

Physics Questions Answer True or False, in your notes, for the following questions Answer True or False, in your notes, for the following questions 14 questions total 14 questions total You will be answering them by a show of hands You will be answering them by a show of hands

Question #1 Ben 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. Ben 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 #2 Gravity in the International Space Station is zero. Gravity in the International Space Station is zero.

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

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

Question #5 Friction is caused by surface roughness. Friction is caused by surface roughness.

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

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

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

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

Question #10 Forces can be negative. Forces can be negative.

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

Question #12 Zero velocity means zero acceleration. Zero velocity means zero acceleration.

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

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

Question #1 Ben 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. Ben 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! FALSE! It still will take more force to accelerate a more massive object. It still will take more force to accelerate a more massive object.

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

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

Question #4 Mass and weight have essentially the same meaning. To increase the weight of an object requires an increase in the mass of the object. Mass 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 it’s inertia, or in other words the resistance to changes in motion. 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 it’s inertia, or in other words the resistance to changes in motion.

Question #5 Friction is caused by surface roughness. Friction is caused by surface roughness. True, Friction is mostly caused by chemical bonding between the moving surfaces; it is caused by stickiness. 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 #6 An object will slow down if there is no net force. An object will slow down if there is no net force. False False This 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). This 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 #7 Passive forces exist (tables can exert force). Passive forces exist (tables can exert force). True True Some 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. Some 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 #8 Surface forces can't exceed the weight (active force) on an object. Surface forces can't exceed the weight (active force) on an object. False False In 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. In 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 #9 An object with a constant net force will have a constant acceleration. An object with a constant net force will have a constant acceleration. True True Constant net force will accelerate an objects at a constant rate Constant net force will accelerate an objects at a constant rate

Question #10 Forces can be negative. Forces can be negative. True True The negative means direction (down, left) The negative means direction (down, left)

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

Question #12 Zero velocity means zero acceleration. Zero velocity means zero acceleration. False False A ball thrown straight up at the top of it path has zero velocity, but is accelerating do to gravity (-9.81 m/s 2 ) A ball thrown straight up at the top of it path has zero velocity, but is accelerating do to gravity (-9.81 m/s 2 )

Question #13 Spacecraft need thrusters to keep them going. Spacecraft need thrusters to keep them going. False False Newton’s 1 st Law – An object in motion will stay in motion Newton’s 1 st Law – An object in motion will stay in motion

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

Mass vs. Weight Mass (scalar): Is the amount of matter an object has Unit is the kilogram (kg) Mass (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 Weight (vector): The force of gravity that acts on an object Unit is the Newton (kg · m/s 2 ) Mass and weight are two different quantities! Mass and weight are two different quantities!

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

Types of Friction Static Friction – is the friction force resisting the relative motion of two surfaces that are not moving Static Friction – is the friction force resisting the relative motion of two surfaces that are not moving Kinetic Friction – is the friction force of two surfaces that are sliding past each other Kinetic 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. Inertia (scalar): The resistance of a body or object to change its motion. Mass is the measurement of inertia (kg) Mass is the measurement of inertia (kg)

Force Quantities QuantityTypeUnit AccelerationVectorm/s 2 Acceleration of gravity Vectorm/s 2 Force of gravityVectorN (kg m / s 2 ) MassScalarkg Coefficient of Friction Scalar(ratio)