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Types of Friction Mr. V. Perez Grade 6 Science Friction • The amount of friction between two surfaces depends on two factors: 1) The kinds of surfaces that the objects are made of 2) The force pressing the surfaces together • If two surfaces are in contact, welding or sticking occurs where the bumps touch each other. Microwelds are where the bumps meet. They are the source of the friction. • To move one surface over the other, a force must be applied to overcome the microwelds • Once the object is moving, friction slows it or stops it Static Friction • If a box is too heavy for you to lift, push, or pull it, then it is not moving. We can also say that it has zero acceleration. Why? The net • Another force that cancels your lift, push, or pull force is 0 (applied force) must be acting on the box • The force is the friction • Static friction is the due to the microwelds frictional force that prevents between the bottom of the two surfaces from sliding box and the floor past each other Static Friction • Arrows that show the direction of the frictional force are drawn in the opposite direction of the force (push or pull) • Why isn’t the potted tree able to move? • The net force is zero. This means the push is equal to the opposing force – which is the static friction. • The force (or push) is not strong enough to overcome the microwelds between the bottom of the pot and the floor • Baseball players use batting gloves or pine tar to increase static friction • Lets say you ask a friend Sliding Friction to help you move the box • Pushing together, the box moves. • Enough applied force has been exerted to overcome the microwelds between the floor and the bottom of the box • If you stop pushing, the box comes to a stop • This is because the box • Sliding friction is the slides across the floor, and force that opposes the another force - sliding friction - opposes the motion of two surfaces motion of the box sliding past each other Sliding Friction • In this example a force (in the form of a push) is put on the potted tree • The tree accelerates in the direction of the push (from right to left) • What is the force that acts on the push? • Sliding Friction • Sliding Friction is the frictional force that prevents the potted tree from continuing to stay in motion • Rubber pads are used under rugs to add sliding friction, which stops you from falling • As a wheel rolls over a Rolling Friction surface, the wheel digs into the surface, causing both the wheel and the surface to be deformed • Static friction acts over the deformed area where the wheel and surface are in contact, producing a frictional force called The wheels are deformed or “squared” rolling friction • Much less force is • Rolling friction is the frictional needed to roll an object force between a rolling object than to slide it (or drag it) and the surface it rolls on • This is why pens are • Tires on a car or bicycle will easier to use than pencils get very hot due to rolling friction Ball bearings Rolling Friction in these wheels • Rolling greatly reduce friction by Friction takes replacing place at the sliding friction point of rolling with rolling contact due to friction the bent shape of the wheel and the ground • Skates, skateboards, bicycles, and automobiles use ball bearings to replace sliding friction with rolling friction • Grease or oil is often used on the ball bearings to limit the amount of friction (that is from unwanted heat energy) • Up to 20% of the energy output from an engine is lost to the heat from friction. Oil is used to limit the friction and prevents the parts of the engine from not working properly Fluid Friction • Fluid Friction opposes the motion of an object through liquids and gases. • You feel fluid friction when you stir a cake batter. The faster you stir, the greater the fluid friction. • Air resistance is the fluid friction on an object moving A flying squirrel takes through air. Bicyclists, advantage of air resistance to speed skaters, and skiers slow its fall and increase the wear slick racing suits to distance covered in the jump reduce air resistance. • Soap is a lubricant that Another example of air is used on screws. resistance is a parachute. • It adds fluid friction to overcome static friction Fluid Resistance • Fluid resistance is when a solid object is in contact with a fluid (such as a liquid or gas) and a force is applied to either the object or the fluid, there is a frictional force that resists the motion. • Examples of where fluid friction occurs include: 1) water flowing through a hose 2) an airplane flying through the atmosphere 3) oil lubricating moving parts on an engine 4) a parachute that is opening by a skydiver or a dragster • It is harder to move thick grease through a hose than it is to move water through a hose • It is harder to ride a bicycle on sand or gravel path than it is to ride it on concrete or black-top path because there is more resistance from sand and gravel • Fluid resistance has less resistance than sliding friction Air Resistance • Air resistance is a friction- like force that opposes the motion of objects that move through the air • Air resistance causes objects to fall with different accelerations and different speeds • The amount of air resistance on an object depends on the speed, size, • Air resistance, not the and shape of the object object’s mass, is why feathers, • The larger the surface area leaves, and pieces of paper exposed to the air – the fall more slowly than pennies, greater the air resistance acorns, and apples. What will happen when the hammer and feather are dropped at the same time on the moon? The hammer and the feather will fall at the same rate because there is no air resistance, or friction, in a vacuum. What will happen when the hammer and feather are dropped at the same time on Earth? On Earth, the hammer will land first because there is more air resistance on the feather than the hammer.
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