Main types of forces in sports

When you take part in sports, various forces act on your body and any equipment or objects involved. A force is a push or pull that can change the motion, shape, or direction of an object.

Gravity

Gravity is the force that attracts a body towards the centre of the Earth, or towards any other physical body having mass. It acts constantly on all objects, pulling them downwards. In sports, gravity affects how high you can jump, how a ball falls, or how you maintain balance. For example, it pulls your body back to the ground after a leap.

Muscular force

Muscular force is a push or pull applied to an object, provided by muscular contraction. This is a voluntary force you control through your muscles. It allows you to propel yourself or an object, such as kicking a ball or pushing off the ground. Without muscular force, movement in sports would be impossible.

Air resistance

Air resistance is the frictional force that air applies against a moving object. It opposes motion through the air, slowing things down. The faster you move, the greater the air resistance, because you encounter more air molecules per second. In sports, this affects activities like cycling or throwing, where speed and distance are key.

These forces often interact. For instance, to overcome gravity and air resistance, you need to apply sufficient muscular force. When forces balance each other, motion stays constant; when they're unbalanced, acceleration or deceleration occurs.

Ground reaction force and its importance in sprinting

Ground reaction force is the reaction to the force that the body exerts on the ground, based on Newton's Third Law (for every action, there is an equal and opposite reaction). When you push down on the ground, it pushes back with an equal force.

Why ground reaction force matters for sprinters

• It provides the propulsion needed for an explosive start.
• Starting blocks are angled extensions of the ground that allow sprinters to push backwards and downwards, generating a forward reaction force.
• This force helps transition from a stationary position to rapid acceleration, making it essential for gaining speed quickly.

In sprinting, maximising ground reaction force through proper technique can shave seconds off race times.

Using force diagrams in sports

Force diagrams are visual tools that show the forces acting on a performer or object in sport. They help you understand how forces interact and affect motion. Each force is represented as an arrow, where the length shows the magnitude (strength) of the force, and the direction points where the force is applied. Arrows are labelled with the force name and sometimes its size.

Key features of force diagrams

• Balanced forces - When arrows are equal in size but opposite in direction, the object doesn't accelerate.
• Unbalanced forces - When one arrow is longer, the object accelerates in the direction of the larger force.
• Common forces shown - Include gravity (downward arrow), muscular force (in the direction of push/pull), air resistance (opposite to motion), and ground reaction force (upward or forward from contact points).

For example, in a force diagram of a floating canoe, the downward force (gravity acting on the canoe's mass) equals the upward buoyancy force from the water, showing balanced forces that keep it afloat.

How to draw a force diagram

1. Identify the object or performer (e.g., a basketball or a cyclist).
2. Draw arrows from the centre of the object in the direction of each force.
3. Make arrow lengths proportional to force magnitudes.
4. Label each arrow (e.g., "Gravity: 10 N downward").
5. Analyse if forces are balanced or unbalanced to predict motion.

How forces affect performance in different sports

Forces don't just act in isolation – they influence how athletes perform and strategise. By understanding them, you can use some to your advantage while minimising others. This occurs because forces can create balanced or unbalanced situations, leading to constant speed, acceleration, or deceleration.

Forces in basketball

• Gravity pulls the ball downward, so players must judge passing height and use muscular force to counteract it for accurate throws.
• Experienced players adjust automatically for gravity, such as aiming higher for longer shots.
• Air resistance slows the ball, especially on long passes, affecting distance and accuracy.
• Insufficient muscular force results in short passes, while too much can make the ball hard to catch.

Forces in cycling

• At low speeds like 16 km/h, a cyclist applies about 100 N of muscular force to maintain speed against air resistance and gravity.
• Air resistance increases dramatically with speed – if speed doubles, air resistance quadruples, making high speeds like 257 km/h impossible even with 1600 N of force.
• Sprinters might reach 72 km/h, but air resistance limits further acceleration.
• Muscular force must balance resistance for constant speed; when it drops (e.g., fatigue), the bike slows.

Forces in skateboarding

• To start moving, muscular force (pushing off) must exceed resistance forces like friction and air resistance.
• For constant speed, forces balance – muscular force equals total resistance.
• When coasting (no muscular force), unbalanced resistance slows the skateboard down.

Forces in canoeing

• For floating, downward gravity on the canoe's mass balances upward buoyancy from the water.
• Unbalanced forces cause acceleration, such as paddling (muscular force) to move forward against water resistance.

In all these sports, applying muscular force effectively while managing gravity and air resistance leads to better performance.

Phases of sprinting and the forces involved

Sprinting involves distinct phases where forces shift from balanced to unbalanced states. This progression builds speed and then maintains or declines it, with ground reaction force playing a key role throughout.

Starting phase

• The sprinter uses starting blocks to apply muscular force backwards and downwards.
• This generates an equal ground reaction force forwards, enabling an explosive horizontal start.
• Forces are initially balanced but quickly become unbalanced for acceleration.

Acceleration phase

• Muscular force exceeds air resistance and friction, creating unbalanced forces that increase speed.
• Friction between feet and ground is crucial for propulsion – without it, you'd slip.
• Ground reaction force helps push the body forward with each stride.

Maximum speed phase

• Speed reaches an optimum where muscular force balances air resistance.
• Acceleration stops as forces equalise, maintaining top speed.
• Gravity ensures feet return to the ground between strides.

Maintenance/decline phase

• Muscular force decreases due to fatigue, leading to unbalanced forces that slow the sprinter.
• Proper force application creates an optimal stride pattern, with gravity aiding foot placement.
• Air resistance continues to oppose motion, contributing to the decline if not countered.