Ask the students: "Would you slide further on a sidewalk, on the grass or on a frozen lake? Why?" (Answer: On a frozen lake, because the ice provides less friction than the sidewalk or grass.) Next, show photographs of bobsledders to the class. Point out that the sleds have a very smooth shape and that the riders tuck themselves down very low in the sled. Ask the students, "Why do the bobsledders crouch down so low?" (Answer: To reduce friction and make the sled go faster.) Explain that the scientific reason is to reduce drag, or make the sled more aerodynamic. Describe to the students that the drag force depends upon the shape of the object. Ask for some other examples of objects that are designed to move very quickly (such as, racecars, rockets, airplanes, etc). What do they have in common? (Answer: They are all shaped with smooth edges.) Explain that the position the athletes (such as, speed skaters, cyclists, ski jumpers, etc.) hold is important and determines how fast they can go. Students may be familiar with this and other examples from watching the Winter Olympics.

Friction also plays an important role in all the previous examples. Ask the students, "What would happen if they sprinkled sand on the bobsled track?" (Answer: The sleds would go much slower.) What if the tracks were made out of concrete? (Answer: The sleds would probably not slide at all, but if you pushed hard enough to move the sled, the metal skids on the sled would probably get hot.) Friction changes the energy of motion (kinetic energy) into heat. You can experience this by rubbing your hands together very quickly. Friction on ice is very low, which means that the sleds can move quickly. Many of us know that ice is very slippery, which you know if you have ever slipped and fallen on a patch of ice! In a brainstorming session, ask the students to suggest sports in which friction and drag are factors.

Mechanical and aerospace engineers work together when designing automobiles or airplanes. Mechanical engineers must be careful in designing engines because they have many parts that are constantly moving and rubbing against each other. If enough friction occurs, the engine could overheat and lockup or wear down and break. We add oil to engines to reduce friction and prevent such a disaster. Aerospace engineers design low-drag airplanes and cars to improve gas mileage. Drag wastes energy by making planes and cars harder to move forward. Friction and drag dissipate useful energy, forcing us to pay for more gasoline, oil and maintenance.

Friction is not always a bad thing; sometimes engineers use friction to their advantage. For example, anti-lock brakes (ABS) were designed to take advantage of static friction. Recall that static friction is always stronger than kinetic friction. ABS prevents the wheels from locking up and causing the tires to slide against the road. It keeps the tires on the brink of sliding, maximizing their static friction, and brings a car to a stop faster. Another example of engineers taking advantage of friction is in the design of parachutes. By maximizing drag, the skydiver can land on the ground safely, which is much better than the alternative of minimizing their drag.

Assign the students the following activity as a way to verify that static friction is stronger than kinetic friction. Fill a box with books or other heavy items and have the students try to slide it across the floor. Have the students describe the amount of effort it took to move the box. To initially move the box, they will find they must gradually increase how hard they are pushing on the box until it starts sliding. Once the box is moving, the students should feel how it takes less force to keep the box sliding than to start it sliding. Have the students write a journal entry explaining which type of friction is stronger and why.

The effects of lubrication can be felt in a simple demonstration. Have the students rub their hands together quickly. They should feel heat being generated by the friction. Now give the students a small amount of hand lotion or wet their hands with water. It should be easier to rub their hands together and produce less heat (due to reduced friction).