Newton's second law of motion explains that forces acting upon an object in motion are proportionate to the acceleration and weight, or mass, of the object. As for top speed, if this were not the case, Thoroughbred horses would not be ridden by 110-pound jockeys. On a flat or an incline equally, top speed is affected by the ability to overcome gravity and any oncoming force.
Don't Sandbag Your Racecar
Watch a sports car rally and you will notice that, except for spoilers, tires and logos, the cars look similar to ones you see on the street. But looking inside a rally car, you'll find the factory interior package and seats have been removed and replaced with a roll bar and aluminum racing seats. The absence of cushy interiors and air conditioners is typical of racecars that compete in classes, such as stock car races, where all engines have similar horsepower. Racing engineers use sophisticated technology to determine the optimum power-to-weight ratio of a racecar. If you loaded down one car in a particular class with 10 bags of sand, its top speed would be affected, and that car would fall behind the rest of the field.
Lance Armstrong's Huge Thighs
As for top speed, power-to-weight ratio applies differently to cycling because power is provided by the riders. If you watch a competition cycling race, you would likely notice that the riders typically have prominent calf and leg muscles and no visible signs of excess body weight. You would also find that road racers wear aerodynamically designed helmets to reduce the forces of oncoming air. Generally, the differences between a good finish and a poor showing in cycling are strength and endurance levels as they pertain to top speed.
On flat ground in still weather, a rider who is 60 pounds heavier than the overall average of the field would be slower unless he compensated for the additional force of gravity acting upon his body weight. To eliminate his disadvantage, he would have to improve the power-to-weight ratio by increasing strength or losing weight. In addition, he might change the gearing to improve his bike's top-end speed. But unless he compensated for gravity acting upon his body weight, changing the gearing would not allow him to achieve top speed potential.
Going for Gold in the Swimming Pool
To achieve their top speed potential, swimmers must overcome the resistance generated when they swim. Typically, a swimmer with a heavier body that contains body fat has more buoyancy than a swimmer with a slender body. In this comparison, it's likely to assume the slender swimmer would overcome water resistance more efficiently. As for top speed, you might see a winning competition swimmer who is 6 feet tall, but her body is well conditioned. If she gained weight without proportionately improving her strength, increased resistance of the water against her larger body mass would affect her top speed.
How to Sprint to the Front
Training with resistance bands or running while towing weights has shown to improve quickness in many athletes. The annual NFL combine highlights larger players who improved their time in the 40-yard dash by developing their quickness. But some larger players who are coveted by a team because of exceptional athletic ability will be asked to lose weight to improve their top speed.
Strength-to-weight ratio is a factor with sprinters as well. A sprinter might outweigh the lightest sprinter in the competition by 30 pounds and outrun the field. He was able to win because he overcame the natural forces at work by increasing his strength to the point where he could achieve his top speed. Stride length also plays a role, favoring taller athletes. Aside from the effects of age on an athlete's body, gaining weight without increasing strength will affect top speed.