Exploring How Design can Overcome Friction

BACKGROUND:

Friction is a force that acts on objects that retards motion of that object. If there were no friction, cars would be able to go much faster than with the same energy used. Friction arises from the irregularities in the surface of sliding objects. The irregularities act as obstructions to an object in motion. Even very smooth surfaces have irregular surfaces on the microscopic level. Not only surface friction has to be dealt with, but air resistance caused by friction of air and object. For example, a modern passenger car reaches 65 kilometers per hour, 80% of the energy is devoted to overcoming air resistance. The direction of the frictional force is always in a direction opposing motion.

Remind students that friction is in fluids as well as solids. To overcome air resistance airplanes and helicopter have to take advantage of how fluids behave. For example, airplanes are common to students, but many do not know why they fly. It is important that they realize that the forces of lift, gravity, thrust and drag are all working on an airplane in the air. Some aid and some impair the airplane's flight.

Gravity pulls down on the plane opposing the lift created by air flowing over the wind. Thrust is generated by the propeller and opposes drag caused by air resistance to the airplane. During take off and flight, thrust force must be greater than drag force and lift force must be greater than or equal to gravitational forces. For landing thrust must be less or equal to drag to slow down to and/or maintain landing speed. Lift must be less than gravity.

LIFT is a force created above the surface of an airplane's wing causing the wing to be "lifted" upward. DRAG is a force which slows the forward movement of an airplane through the air. THRUST is a force created by a power source which gives an airplane forward motion. The propeller on a helicopter pulls the aircraft forward using the same principle as lift on the wing. The helicopter blades are rotating wings. Big propellers lifting the wingless aircraft. Jet aircraft uses explosive force directed to the ear to move aircraft forward. Remember the balloon and string experiment? GRAVITY is a force pulling down on the airplane.

Friction can also be overcome by the design of simple machines. Gears will help reduce friction and also can increase friction, as any owner of a 10 speed bike knows. You can calculate the speed ratio of gear by dividing the teeth on the gear by the teeth on the pinion. The pinion is the smaller gear in a set. For instance if you have a pinion (follower) with 12 teeth and a gear (driver) the gear ratio is 12/24 or ½. If you have more than 2 sets of gears you multiply each of the ratio’s to get the speed ratio. If the smaller of a gear pair (the pinion) is on the driving shaft, the pair acts to reduce speed and to amplify torque; if the pinion is on the driven shaft the pair acts as a speed increaser and a torque reducer. If the driven gear has twice as many teeth as the pinion, for example, the torque of the driven gear is twice the pinion torque, whereas the pinion speed is twice the speed of the driven gear.
PROCEDURE:

You can demonstrate this easily by having an inclined board and try to "roll" objects down. Before you roll the objects, have the students predict which objects will be influenced more by friction than others (i.e. a marble or a toy car.) Discuss which irregular surfaces the object would have to overcome. Remember that gravity is the driving component of objects going toward a flat surface and friction is just retarding the pull of gravity.

If students are not familiar with simple machines, you may want to review material that was presented in lower primary under the Physics and Technology units.

Answers: 1. 1/3; 2. ½; 3. ½; 4. 1/4


OBJECTIVES: Explaining how simple machines help reduce friction. Exploring how design can overcome friction. VOCABULARY: drag lift thrust MATERIALS: worksheet board objects to roll down	Students use a worksheet to explore friction.