Investigating the Factors Affecting the Speed of a Car After Freewheeling Down a Slope (Annotate this article)

Size: px

Start display at page:

Download "Investigating the Factors Affecting the Speed of a Car After Freewheeling Down a Slope (Annotate this article)"

Christine Freeman
6 years ago
Views:

1 Investigating the Factors Affecting the Speed of a Car After Freewheeling Down a Slope (Annotate this article) Sir Isaac Newton formulated three Laws relating to the motion of objects. A moving object covers a particular distance in a particular time. This is called the Speed of the object and is expressed as meters/second i.e. the distance covered in meters in one second. It is a Scalar quantity as it only has magnitude. If however the same speed is expressed with the object moving in a particular direction e.g. due north, it will be called the Velocity of the object. It again is expressed as meters/second but having both magnitude and direction it is a Vector quantity. Newton described that an object that is stationary will stay stationary until a force is applied to it and an object that is in motion will stay in motion in a straight line until it is acted upon by a force. This is Newton s First Law of Motion. Average Velocity (m/s) = Displacement taken place (m) Time taken (s) The soapbox derby is an American tradition. As a youth-based racing program, the derby has been around since Soapbox cars, also known as gravity racers or coaster cars, are vehicles without motors. They are propelled by gravity and capable of carrying one passenger. Back in the day, the chassis or frame and wheels of a racer were built out of wooden crates and roller skates. These days use of other materials is encouraged. In fact, there is a special racing format in the United States known as the Ultimate Speed Challenge (USC). Developed in 2004 to preserve the tradition of innovation, creativity and craftsmanship in soapbox design, the goal of the event is to attract the most creative entries that are still super-fast. Sheri Lazowski, the first repeat USC winner, won her 2 nd victory in 2011 with the blistering time of seconds. What is speed anyway? Simply put, speed is a rate of motion. It s calculated by dividing the distance an object moves by the time it takes to move that distance. For example, a car that travels 100 miles in 1 hour is said to be traveling at a speed of 100 miles per hour. If a car changes its speed while traveling, it is accelerating. Given that the course is a 989-foot hill; can you figure out her average speed in miles per hour? Her peak speed must have been even faster. All objects need force, to overcome their inertia or mass and change their speed. Force is a push or a pull, which can make an object start moving when it is stationary, or change its shape or its direction of motion. It is measured in Newtons (N). One Newton is the force that gives a mass of 1Kg an acceleration of 1meter/sec 2. N=Kg m/sec 2 The force that gets coaster car moving is gravity. However, even though all cars are subject to the same gravity, they encounter different amounts of friction (an opposing force) within their systems, so they do not end up traveling the same speed. Friction is the reason coaster cars have different speed changes while traveling because the frictional force is in the opposite direction to the direction of the force produced by gravity. One source of friction is air resistance because the coaster cars must push through air molecules. Another source of friction is the rubbing of the tires against the road. Some tires may produce less friction but they may also provide less traction (the ability for tires to grip the road). So, there can be trade-offs between friction and traction. Yet more friction is produced by the rotating axle. More axles allow a vehicle to support more weight, but that advantage must be traded off against the additional friction. Another consideration is an axle produces more friction the faster it turns. When an external force acts upon a moving object it changes its velocity. The rate at which this velocity is changed is called acceleration (if the velocity is increased) or deceleration (if the velocity is decreased by an opposing force). This is the Second Law of Motion. Or Force = mass x the acceleration. And acceleration is expressed in m/s 2. Acceleration(m/s 2 ) = Change in Velocity V f- V i (m/s) Time taken t f -t i (s) It is the property of matter that it opposes any change in an object s fixed position. This is called Inertia. The greater the mass of an object the greater will be the force of Inertia. This indicates that to make an object move from its stationary position, it will take a stronger force if the mass of the object is more as compared to the force needed to move a lighter object with less mass. Similarly the force needed to change the velocity (acceleration) of a moving object will depend upon it mass. The stronger the force applied the more will the acceleration be. The more the mass of an object, the less will the acceleration of the object be when the same amount of force is applied. Newton s Third Law is for every action there is an equal and opposite reaction.

2 So one can express that acceleration (a) is directly proportional to the Force (F) applied to an object and inversely proportional to the mass (m) of the object. Force (N) = mass (kg) acceleration (m/s 2 ) Rearrange the formula and solve for mass= Rearrange the formula and solve for acceleration=. When (a) goes up the (F) goes. When the (m) goes down the (a) goes. A moving object keeps moving due to a force called Kinetic Energy (KE). Energy can neither be created nor destroyed (Law of Conservation of energy). It merely changes from one form to another. The energy present in a moving object is called Kinetic energy. It is expressed as: KE = ½ mv 2 Where KE is the Kinetic energy, m is the mass of the object in Kg and v is the velocity of the moving body. This indicates that higher the velocity of a moving object, the higher is the kinetic energy in the object. Before an object can start to move from a stationary position, it either has to possess some energy inside it or has to be forced to start moving. A body that has energy due to its position or condition is called the Potential Energy (PE). An object above the surface of the earth is considered to have Gravitational Potential Energy (GPE). This is when an object gains energy by being lifted to a height above the ground. In your experiment you will be placing a penny racer on a ramp at a height above the surface of the earth. It is the potential energy in the racer that will let it come down the slope when it is let go. Work done by force = force x height or = mass x gravity x height Mass is measured in Kg. Gravity in Newton/kg and height in meters This indicates that if the mass of the object is increased and the height is kept constant and with the force of gravity being constant, it will take more force to place the object to a height and the object would hold more potential energy. Similarly, if the height is increased and the mass of the object is kept constant, more potential energy will be held by the object at higher level than at lower levels. When an object is realized from a height, it accelerates down the slope due to its Gravitational potential being converted into Kinetic energy (called Energy Conversion), until it reaches a flat surface when it continues to move (First Law of Motion) at a constant velocity until it is stopped mechanically or by friction due to its contact with the surface it is moving on. At any one time the forces acting on the racer are: The force of gravity pulling the racer towards the earth. 1)Label the forces acting on the car next to the arrows: 2)Label the following at its greatest: a) PE, b) PE & KE, c) KE Opposing force from the ramp pushing the racer upwards Kinetic energy making the racer go down the runway The force of air resistance and friction opposing the run of the racer

Questions: A) List Newton s 3 Laws: 1. 2. 3. B) Define each of the bolded words.

3 Questions: A) List Newton s 3 Laws: B) Define each of the bolded words. Speed: Velocity: Acceleration: Gravity: Force: Newton (N): Inertia: Kinetic Energy: Potential Energy: Gravitational Potential Energy: Work: Friction: Chassis: Mass: You will be building a penny racer. Read the following article by a former NASA engineer who explains how you can use science to succeed at your next racer derby. For seven years, I worked at NASA on the Mars Curiosity rover. It is just like a pinewood derby car, except it has six wheels, it s nuclear powered and it shoots lasers. My Cub Scout son and I decided we would take the science principles I used while building stuff at NASA and apply them to making his pinewood derby car. Take a look at some of those science principles in this video and check out my list of the most important steps for making fastest pinewood derby car possible. Youtube:

4 Seven Steps for Making a Fast Derby Car 1. Max out your derby car s weight and make sure the heaviest part is in front of the rear axle. Too heavy, it may have trouble starting to move, try experimenting with the weight to see how it changes the motion of your car. You want your car s weight to be balanced and you want to make sure your center of mass is in the correct location. This is the most important step. Science shows if you do this correctly, you will beat a derby car built exactly the same except with the weight toward its front by 4.6 car lengths. It works because the farther back the weight is, the more potential energy you have because your center of mass is higher up on the track. (Don t put it too far back, or your derby car will become unstable and pop a wheelie.) 2. Use lightweight wheels. This is illegal in some races, but if it s not in yours, this is a must-do step that will give you a 2.1- car-length advantage at the finish line versus a car with normal wheels. It works because heavy wheels take away from the kinetic energy (the energy something has due to its motion), which makes the derby car slower. 3. Use bent smooth axles. Bending your axles with a bending tool will make the wheels ride up against the nailhead, which creates less friction than if the wheel is bouncing around and rubbing against the derby car body. See video for details. The wheel and axle is one of the six simple machines. Machines make work easier by changing either the size and/or the direction of an input force. A wheel and axle combination consists of two circular objects of different sizes, with the larger wheel turning around the smaller axle. 4. Railride. Railriding means your derby car stays in the center track to keep the car from bouncing around. The straighter your car drives, the better. This helps reduce friction and saves energy for speed. See video for details. If your car swerves too much it will decrease your time, and hence your speed. Swerving is a type of acceleration, which the car must overcome. 5. Create a derby car that is reasonably aerodynamic, meaning its design cuts down on drag caused by air. No need to get crazy here, but simply having a wedge-shaped derby car instead of the standard block out of the box will equal a 1.4-car advantage at the finish line. 6. Ride on three wheels by raising one wheel off the track. (Check the rules to make sure this is allowed in your race.) You will move faster if you have to get only three wheels rotating, giving you a 1.1-car advantage over an identical pinewood derby car riding on four wheels. 7. Reduce Friction. You want to have your axels and wheels rolls as smoothly and as easily as possible. Any contact to contact will cause friction and slow your car down. The friction between the wheels and the surface below, plus the friction of the axle rubbing in the axle supports, slows the car. For each rotation, the axle travels a shorter distance around than the wheel. The shorter turning distance and smaller diameter of the axle means less energy is lost as the car moves. Be sure to engineer your car to reduce friction and if possible lubricate around each wheel and on the axle.

5 QUESTIONS: 1. Do you think a light car will travel faster or a heavier car? Why? 2. Do you think the height of the ramp will affect the speed of your car? Would you rather start your car higher on the ramp or lower on the ramp? Why? 3. Where do you think friction will come from on your car? How can you reduce friction on your car? 4. How does your car move? Where is the force coming from? 5. After reading the article, how will you design your car? a. Wheels: You can make the wheels out of coins, cardboard, or plastic. The axels will be made from straws, toothpicks, and glue. How big will you make your wheels? What will your wheels be made from? How many wheels will your car have? How far apart will you place your wheels from each other? How far from the chassis will you place your wheels? b. Chassis: We are making the chassis out of popsicle sticks or tongue depressors. How long will your chassis be? How wide will your chassis be? c. Mass: How heavy will you make your car? What will you use to increase the mass of your car? Where do you plan on placing mass on your car? d. Aerodynamics: What will the shape of your car be? Draw a Picture with of your car. With measurements. Design and Color it.

1. Two forces are applied to a wooden box as shown below. Which statement best describes the effect these forces have on the box?

1. Two forces are applied to a wooden box as shown below. Which statement best describes the effect these forces have on the box? A. The box does not move. B. The box moves to the right. C. The box moves