The Irresistible Pull Of Gravity

Published Date: 02 Nov 2017

Stride length. It’s extremely important to take your stride length into consideration while you run, as different stride lengths will result in different speeds. You can think of these strides as gears on a bike. Let’s say you’re starting off a race. You don’t want to use long energy-expending strides; you’d want to switch to ‘low gear’. Quick, short steps are the best to start your run off, using the least amount of energy as possible. Now let’s say you’re interested in finding out how much distance you can cover over a specific amount of time. Velocity is defined as distance traveled over change in time, (delta d over delta t), so, you essentially are aiming for a high velocity. You want to maximize the number of strides that you’re able to take during that time interval. Would you stay in ‘low gear’, and continue to take short steps? Of course not! Large strides would make you capable of achieving a high velocity. Alright so, when running, you’re using up a lot of energy; why? Because your body’s doing work! In physics, work is notated as W= Fd, where F is a constant force of magnitude on a point and d is the distance in the direction of the force. When dealing with stride lengths, power should be taken into consideration. Power is expressed in the equation P= delta W/delta t, where W is work done, and delta t is the period of time it’s done over. When using short strides, you are doing less work as you’re propelling yourself to a lesser extent in contrast to if you were taking longer strides. In taking longer strides, you would be increasing the value of delta t, as you would be taking more time, and the work done would increase as well, because the distance covered would be greater.

Gravity. What is gravity? Gravity is how physical bodies attract each other with a force that is proportional to their masses, and inversely proportional to the square of the distance between them. Its numerical value is -9.8m/s^2 and is notated by Fg (force of gravity). When you’re running, gravity acts on your center of gravity by pulling it towards the core of the Earth. You would think this only occurs when you’re moving, but even as you stop to take a break from your run, and you’re just standing around, gravity is still acting upon you, but another force, called the normal force, (notated as Fn) works in the opposite direction, perpendicular to the ground, and is equal to Fg. In races, you want to think of yourself as falling forwards. Since gravity is the constant acceleration toward the center of the earth, it will always pull you down towards that center of gravity, and you will run faster. You’ll have great running posture, and the structure of your body will be carrying the burden of your weight; your muscles can focus only on moving you forward and won’t have to work to also keep you upright. It’s important to keep in mind that you must lean forward from your ankles up and not from the waist. If you are leaning forward from the waist, you’re running upright and are consequently fighting the force of gravity instead of taking advantage of it! Leaning forward with your ankles will relieve stress off the muscles in your legs, and although gravity may not get rid of a very significant burden, you’ll still notice that you will be able to run more efficiently- longer, and faster. The law states that energy expenditure is directly proportional to how much you go up and down. So if you can minimize up and down movement, you can lower your energy expenditure. The more you lean forward when you run, the more it adds a horizontal component to your energy usage.

The pendulum. Wait, what- pendulum? How does a pendulum have anything to do with running? Well let’s consider the properties of a pendulum. People think that the length, the weight, or the initial speed of the pendulum affect the time it takes to complete a period. But really, the length of the pendulum string is the only factor that affects this. This is merely due to the law of physics that conditions that the speed of the swing of the pendulum does not describe the weight of mass at the end of the pendulum. Let’s say you added some weight to the pendulum. What you’ll notice is it will carry further, covering more distance, but it will also result in a longer period because of the additional distance. The same thing happens if you increase the initial speed. To run more competently, you must lessen the amount that you bound up and down when running. So, when you’re running, you want to think of your legs and arms as pendulums. You can use their momentum and force to move you along most efficiently. Shorter legs will make you run faster since your ‘pendulum’ is shorter. So you can bend your legs and lift your knees when you bring them up towards your torso, and you should keep your arms at a ninety degree parallel to your upper body to ensure maximum efficiency. Your arms can pump more quickly, causing you to move faster if you ‘shorten the pendulum’ by bending your elbows.

Friction. What’s friction? Friction is the force that resists the relative motion of things sliding against each other. When dealing with dry friction (between two solid surfaces in contact), you want to know what the coefficient of friction is and how it is used. The coefficient of friction is a scalar value that labels the ratio of the force pressing the two bodies together and the force of friction between them. You use this coefficient when you want to determine the force of friction of certain surfaces either against each other. So, in running, friction actually plays a huge part but is only in effect for a small period of time. During your stride, you’re only in the air for approximately sixty percent of the running cycle, so the friction between the surface and your foot is only actually present for forty percent of the time. So you are propelling forwards because your leg muscles are pushing back on the surface. Now let’s talk about fluid friction. But wait, what’s a fluid? A fluid is anything that takes the shape of its container. So air and liquid are two examples of fluid. The equation of air resistance is Fair = -0.5pv2ACdV. P is the density of the air, v is the velocity, A is the area of the object, Cd is the drag coefficient, and V is the vector that the direction is moving in. So what happens when the air resistance is increased? When air resistance is increased, the same amount of energy is used up. The energy used up to overcome air resistance is directly proportional to the speed. So, when your speed increases, the energy required obviously also increases. In running, sprinting speed, which is approximately 10 mps, the energy cost is 7.8%. For medium speed, approximately 6 mps, the energy cost is 4%. For marathon speed, approximately 5 mps, it’s 2%. So we can now conclude that the force of air is actually a large factor when running, and should definitely be taken into consideration to produce best running results!

People don’t realize how ENORMOUS of a part physics plays in our everyday lives. Running is a leisure activity that takes so many laws of physics into hand; why not learn about these laws and use them to our advantage? So remember; when starting off a run, keep your strides short to save up energy, and later on in your run, resort to cruising speed with longer strides. Always lean forward from your ankles up when running to relieve stress from the muscles in your body, and gravity will help pull you forward and move faster. Bend and lift your arms and knees, as they are your pendulums. And don’t forget, it’s much better to be running on a calm day with no winds.