1. Friction -A force which opposes motion. There are two types of friction: 1.Static Friction – the force exerted on a stationary object in order to get the object to move. If you hooked a spring scale on to a block and started to pull slowly you would see that the force meter would keep increasing until the object begins to move. That is the maximum static frictional force. We can calculate the co-efficient of static friction between the two surfaces be doing just that and using the following formula: F fs ≤ µ s F N F N is the normal force.

2. How much force would be necessary to push a 240 kg stationary wooden crate across a wooden floor? F fs ≤ µ s F N Note: µ s for wood on wood is.5 F fs ≤.42 (240 kg x 9.81m/s 2 ) F fs ≤ 988.85 N Note: The surface area of friction does not matter it only matters between the two materials present.

3. 2. Kinetic Friction – the force exerted on a stationary object in order to keep the object to moving with constant speed. F fk = µ k F N How much force would be necessary to keep the 240 kg wooden crate moving across the wooden floor at constant speed? F fk = µ k F N Note: µ k for wood on wood is.2 F fk =.3 (240 kg x 9.81m/s 2 ) F fk = 706.32 N

4. Rolling Friction The friction necessary for the wheel, sphere, cylinder,… to grip the ground and prevent spinning. F r = ( µ r /r) F N When a force applied to a wheel is not enough to overcome the static force of friction, the wheel will start to roll. If the force is greater than the static resistance, the wheel will slide or spin. It will also roll, but not at the same rate as with static friction. The radius of the tire does matter. The larger the tire the smaller the rolling friction. Hence, sports cars have larger thin tires.

5. Fluid Friction 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 to the fluid, there is a friction force that resists the motion. Examples where fluid friction occurs are water flowing through a hose, an airplane flying through the atmosphere and oil lubricating moving parts. If the viscosity or thickness of the fluid is great, there may be no movement due to static friction. One example is trying to move heavy grease through a hose. You need to apply a great pressure to finally break the static friction and start the grease moving. Once a fluid moves through a hose or an object is moving along a fluid, the resistance is considered kinetic friction. The grease will still move much slower than a fluid with low viscosity, like water. Causes of fluid friction are turbulence effects from surface roughness and deformities, molecular attraction or adhesion between the materials, and deformation resistance of the fluid. This deformation resistance is called its viscosity.