True or False When two surfaces rub, the irregularities of one surface get caught on those of the other surface. True What is friction? Friction is the force that two surfaces exert on each other when the two rub against each other. Friction acts in a direction __ to the objects direction of motion. Opposite The strength of the force of friction depends on what two factors? The types of surfaces involved and how hard the surfaces push together. How is friction involved in sledding and skiing? There is very little friction between a sled or skis and the snow because the snow is relatively smooth. Also, the snow may be partially melted, and fluid friction is much easier to overcome. Why does some friction occur with seemingly smooth surfaces? Even surfaces that seem smooth have irregular bumps that you could see with a microscope, so friction occurs even with smooth-looking objects. Which kind of friction? Friction occurs when an object moves through a liquid or gas Fluid friction Which kind of friction? Friction occurs when solid surfaces slide over each other Sliding friction Which kind of friction? Friction occurs when an object rolls over a surface Rolling friction Which kind of friction? Friction occurs when objects are not moving Static friction Which kind of friction requires more force to overcome, rolling friction or sliding friction? Sliding friction What kind of friction occurs when moving parts have ball bearings? Rolling friction How does oil between machine parts reduce friction? The oil keeps the machine parts from making direct contact, and there is fluid friction between the parts instead of sliding friction. A force that pulls objects toward each other is called __. gravity True or False The force that makes an apple fall to the ground is the same force that keeps Earth orbiting the sun. True The force of attraction between two objects varies with what two factors? Mass and distance between the objects. How is weight different from mass? Weight is a measure of the force of gravity on an object, while mass is a measure of the amount of matter that makes up an object. Weight is usually measured in __. newtons True or False On the moon, your mass would be less than it is on Earth, but your weight would be the same. False When is an object said to be in free fall? An object is in free fall when the only force acting on the object is gravity. Near the surface of Earth, what is the acceleration of an object due to the force of gravity? 9.8 m/s2 Objects falling through air experience a type of fluid friction called __. Air resistance The greatest velocity a falling object reaches is called __. Terminal velocity True or False The greater the surface area of an object, the greater the air resistance. True An object that is thrown is called a(n) __. projectile True or False An object that is dropped will hit the ground before an object that is thrown horizontally from the same height. False Draw arrows that show the forces acting on the falling coconut. Label each arrow with the NAME of the force.