5. Recall of Newton’s Laws of Motion:
Newton’s First Law
Newton's first law states ‘that every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force.’ This is normally taken as the definition of inertia. The key point here is that if there is no net force acting on an object (if all the external forces cancel each other out) then the object will maintain a constant velocity. If that velocity is zero, then the object remains at rest. If an external force is applied, the velocity will change because of the force.
Newton’s Second Law
Newton’s second law of motion (known as the law of acceleration) states that ‘the rate of change of momentum of a body is proportional to the force causing it and the change takes place in the direction in which the force acts
Newton’s Third Law
The third law states ‘for every force exerted by one body on another, there is an equal and opposite force exerted by the second body on the first’ (e.g. when a sprinter pushes against the starting block the force coming from the blocks is equal and opposite).
Newton’s third law states that ‘for every force, there is an equal and opposite force exerted’ when a sprinter leaves the blocks they push against them. That force is the reversed equally (equal and opposite) pushing the sprinter down the lane.
The law of inertia states ‘that every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force.’ The rugby player that was tackled whilst would have continued in the same direction or uniform motion that he was heading however that state was changed by an external force- somebody tackling him whilst running.
The law of acceleration states that ‘the rate of momentum of a object is proportional to the force causing it and the change takes place in the direction in which the force acts’ for example when the ice hockey puck is struck by another player it will change direction.
Mechanical Principles
Materials undergo mechanical stress, such as tension and compression. Tension is when an object is being pulled from both ends, for example if a person was to hang from a bar as if doing chin ups their arms would be stretched from bother ends which would put them under high tension. Compression is the opposite of tension. The body can experience 2.5 times its own weight during running, this can increase to 5 times body weight from volleyball smash or a basketball lay up. The spine endures high compression loads in certain sports. The forces involved in the spine during the delivery stride of a bowler in cricket. Another type of load that the body can be subjected to is torsion. Torsion is a twisting action. An example would be the forces in the tibia if a person tried to pivot on one foot which would cause a twisting action in the lower leg.
Extreme mechanical stress can cause injuries either acute or chronic. The injuries cause by theses forces can range from anything to a contusion to a fracture. A common type of fracture is compression fractures which is caused by a significant trauma. This could not only lead to fractures by also soft tissue damage to the ligaments and tendons. Chronic injuries may also be causes of injuries induced by shock and impacts. For example inappropriate footwear or the wrong type of surface can be a cause of overuse injuries. In order to reduce the chance of overuse injuries the shock needs to be absorbed other than by the body. For example running on soft surfaces or having footwear with good shock absorption will produce fewer injuries.
Many sports involve projectiles. Projectiles are considered to be any object which has no external forces acting upon it other than gravity. An object that is said to be a projectile is if experiences no air resistance, as this would be an external force. However all objects are looked upon as projectiles. The shape of the flight path of a projectile would be symmetrical if we discount air resistance and lift. Air resistance, which causes a drag force, and aerodynamic factors, that cause lift, will act to shorten or lengthen the flight path of a projectile. An object moving through air or water will experience a drag force. The greater the velocity of the moving object, the greater the drag force, for example a cyclist will experience a greater drag force than a runner. This is why lip streaming has such a vital part in cycling. A cyclist can save up to 30% of their energy by cycling close behind their competitor or team mate. The drag force depends in the size and the shape of the object. A downhill skier in a tuck position wearing a body suit will generate less drag force than a recreational skier in an upright position wearing salopettes and a ski jacket. The final factor that affects the drag forces is the density of the medium- water creates more drag force that air. The flow of air or water around a projectile depends upon the shape of the projectile. A smooth symmetrical shape will have a symmetrical around it. However many objects in sports are not symmetrical .for example aerofoil has an asymmetrical shape. Air travels faster over the top of an aerofoil than underneath it. This means that the air pressure is lower above the aerofoil than below it. This is called Bernoulli’s principle. The difference in pressure causes creates an upward force that causes the aerofoil to lift. Some sports exhibit lift, for example javelin and discus. In some cases an aerofoil is inverted and used to create a download force. Spoilers in racing cars force the car on to the ground, which makes them less likely to skid, especially when coming round corners.
Spin also influences trajectory projectile. If an object is spinning, it surface interacts with eh passing air molecules to a greater extent that if it was not spinning. This can cause the air to slow down or speed up, depending on the direction of the spin. The spinning object will cause unequal pressure and hence forces will start to act upon the item similar to the force on the aerofoil. This force may cause a lift just like in the case of a golf ball, which is normally hit with backspin. However, it could be used for a swerve or topspin in order to cause a volleyball to dip. The surface material of the object also plays an important part in trajectory. An example is golf balls. A golf ball is covered in dimples increasing the amount of lift force on the ball which in turn will make it go further, which will make it get closer to the hole.
The force friction applies when any two surfaces move against each other. Friction acts in the direction opposite to the direction of motion. The magnitude of the frictional force depends in the contact force and the nature of the contacting surfaces. The larger the contact force the larger the friction force.
The reason racing cars can create large down forces with the use if a spoiler is to stop them sliding off the track. Different surfaces have different coefficient of friction. For example, rubber has a high coefficient of friction when interacting with most materials. This means it will not slide very well. The outer soles of running shoes are made of rubber to provide better grip. However, too much friction can lead to ankle and knee injuries. Synovial joints have an extremely low coefficient of friction, which reduces wear and tear on joints.
In sport, movement is rarely linear for example shot put throw. However the same concepts and principle used to describe linear motion can be used to describe angular or circular motion. The major difference in measuring angular or circular motion is the basic unit measurement. With linear motion, meters are used to measure distance and displacement, and all subsequent units are derived from that (e.g. ms¯¹, ms¯²). With angular motion, changes in position are measured by angles in units of degrees (º). The position of a particular object at any movement in time is its angular position. If angular position changes the difference between the starting position and the final position is called the angular displacement. The rate of change of angular displacement is termed angular velocity. It is calculated in a similar way to linear velocity but instead of displacement divided by time its angular displacement divided by time. Thus, the average angular velocity of a rotating object is defined as the angular displacement divided by the time taken to turn through this angle.
Given that inertia (i.e. objects are reluctant to move angularly) a force is needed to produce angular motion. For example a tennis player putting a topspin on a ball must provide a force toward the top of the ball. This force allows the ball to spin in an angular motion. The turning effect produced by a force called torque. A torque is equal to the product of the force and the distance between this force and the axis of the rotation. This distance is referred to as a moment arm because a torque is sometimes termed a moment of force. The units of measurement for torque are Nm. Torque is vector quantities, they are direction specific. In the case of the player who has just hit the topspin on the right side of the court, if the view was of the player that just hit the topspin the direction of the force would be anticlockwise. If the same player put a clockwise force on the ball, this would be backspin.
