how to make a mousetrap car

Mousetrap cars and friction

By Doc Fizzix // Published Nov 17, 2005

A ball rolling across the floor will eventually slows to a stop. The reason the ball slows to a stop is because of friction. Friction is a force that always opposes motion in a direction that is opposite to the motion of the object. An object that sliding to the right experiences a force of friction towards the left. If it was not for friction, an object would roll or move forever, as long as there was nothing—like a wall—to stop its motion. Your mouse trap car is affected by friction in the same way as a rolling ball, friction acts against the motion of the moving ball and will slow it to a stop. Friction occurs anytime two surfaces slip, slide, or move against one another. There are two basic types of friction—surface friction and fluid friction. In some situations fluid friction is called air resistance. A ball falling through the air is affected by fluid friction and a block sliding on a table is mainly affected by surface friction as well as a little air resistance. The greater the amount of friction between two surfaces, the larger the force that will be required to keep an object moving. In order to overcome friction, a constant force is needed. In order to maintain a constant force, there must be a supply of energy. A ball which is given an initial push will roll until all its energy is consumed by friction, at which point it will roll to a stop. The smaller the forces of friction acting against a moving object (like a ball or mouse-trap car), the farther it will travel on its available energy supply. Eliminating all forms of friction is the key to success no matter what type of vehicle you are building.

Surface friction occurs between any two surfaces that touch or rub against one another. The cause of surface friction is mutual contact of irregularities between the touching surfaces. The irregularities act as obstructions to motion. Even surfaces that appear to be very smooth are irregular when viewed microscopically. Luckily, during motion surface friction is unaffected by the relative speed of an object; even though the speed of an object may increase, the force of surface friction will remain constant. This means that the same force is required to slide an object at a slow or fast rate of speed on a given surface.

The amount of friction acting between two surfaces depends on the kinds of material from which the two surfaces are made and how hard the surfaces are pressed together. Ice is more slippery than concrete; therefore, ice has less friction or less resistance to slippage. A heavier brick is harder to push and has more friction than a lighter brick only because the heavier brick pushes into the ground with more force or weight.

Minimizing surface friction on a mouse-trap car allows its wheels to spin with less resistance, resulting in a car that travels faster, farther and wastes less energy. The most common area where surface friction will occur is between the axle and the chassis.

Friction is not restricted to solids sliding over one another, friction also occurs in liquids and gases, collectively called fluids. Just as the friction between surface friction depends on the nature of the surfaces, fluid friction depends on the nature of the fluid. For example, friction is greater in water than it is in air. But unlike the surface friction, fluid friction depends on speed and area of contact. This makes sense, for the amount of fluid pushed aside by a boat or airplane depends on the size and the shape of the craft. A slow-moving boat or airplane encounters less friction than fast-moving boats or airplanes. Wide boats and airplanes must push aside more fluid than narrower crafts. If the flow of fluid is relatively smooth, the friction force is approximately proportional to the speed of the object. Above a critical speed this simple proportion breaks down as fluid flow becomes erratic and friction increases dramatically.

The amount of air friction or fluid friction depends on the speed and the shape of a moving object. The faster an object moves, the more collisions that occur with particles of the fluid, causing increased friction. The shape of a moving object, its aerodynamic, determines the ease of flow of the fluid around the moving object. Fast cars are designed and shaped to cut through the air with less friction so they can move faster. Trucks have a special cowling that increases their aerodynamics and allows air to flow more easily over the trailer. Increased aerodynamics saves energy. Fish have aerodynamic shapes that allow them to move through the water with less effort. Keep in mind that there are situations in which you would want to increase the air resistance. A good example is the use of a parachute on a dragster to help it stop the vehicle or the flaps on an airplane to help slow it down.

Because the force of air resistance increases as the speed of an object increases, faster moving mouse-trap cars will have more air resistance acting against them, causing them to use more energy and come to rest sooner than a similarly built slower-moving mouse-trap car. Keeping this in mind, good aerodynamics will improve performance of any vehicle, no matter what type of car you are building. This means that your car must be smooth with few points of air drag. Inspect the body for flat surfaces on leading edges that could catch air, thus increasing the air drag. Rounding the leading edges of your vehicle will allow for smoother movement of air around your vehicle. Cars made from wood need to be sanded smooth. Sanding will remove any unwanted irregularities, thus decreasing the force of air resistance acting on your car once it is in motion. Tires should be thin. Thin tires are more aerodynamic and slice through the air more smoothly. Wider tires will have more air drag than narrower tires. Therefore, try to pick thin tires when you are building your mouse-trap car.

Friction is not always a bad thing. Without friction you could not move anywhere. As you walk forward, it is the friction on the bottom of your feet that allows for the grip you need to move. It is the friction between the road and your tires that keeps the wheels from slipping and allows your vehicle to move. This type of friction is called traction and is a desired form of friction. What type of wheels work best-: fat tires, thin tires, knobby tires, or smooth tires? Knobby tires have good traction if the road surface is rough, but the trade-off is that they are inefficient and use more energy than smooth wheels. When pressure is placed directly on a knob, more force compresses the tire causing the rubber and air inside the tire to heat up. The energy it takes to compress the rubber and air in the tire is lost as heat. Knobby tires become hotter with continued use. Race cars use smooth tires called slicks. Smooth wheels will be more energy efficient and allow a car to travel faster. On mouse-trap cars, smooth tires will allow a vehicle to travel a longer distance.

See the following related articles to get started:

• basic mousetrap vehicle propulsions (how to make it move)
• how to make a good distance traveler
• how to make a good speed-trap racer