Core 2: The Body in Motion

1. Core 2: The Body in Motion Critical Question 3: How do Biomechanical Principles Influence Movement?

2. What does the syllabus say? Students Learn about: Students Learn to: apply principles of motion to enhance performance through participation in practical workshops apply principles of balance and stability to enhance performance through participation in practical workshops apply principles of fluid mechanics to enhance performance through participation in practical workshops   describe how principles of fluid mechanics have influenced changes in movement and performance, eg technique modification, clothing/suits, equipment/apparatus apply principles of force to enhance performance through participation in practical workshops. • motion • the application of linear motion, velocity, speed, acceleration, momentum in movement and performance contexts • balance and stability • centre of gravity • line of gravity • base of support • fluid mechanics • flotation, centre of buoyancy • fluid resistance • force • how the body applies force • how the body absorbs force • applying force to an object.

3. Fluid Mechanics - Flotation • When a body is immersed in water, the water exerts an upwards force on the body that we call flotation or buoyancy. • Archimedes’ Principle the size of the buoyant force is equal to the weight of water displaced by the body. This means that to maximise the buoyant force, as much of the body as possible should be kept below water. • Example – floating on backs - there is a strong temptation to try to lift your face clear of the water, but this reduces the volume of your body underwater. Therefore, the volume of water displaced is reduced and, consequently, the buoyant force becomes less. Keeping as much of your body under water as possible will maximise the amount of water displaced, thus increasing the buoyant force and making it easier to float.

5. Centre of Buoyancy • Centre of buoyancy the point in the body where the amount of volume under the water is equally distributed on either side. The centre of buoyancy tends to be higher in your body than the centre of gravity, because of the effects of dense legs at one end and low-density lungs towards the other end. • When a body is suspended in water, it will rotate until the centre of buoyancy and the centre of gravity are aligned, one above the other.

7. Fluid Resistance • When we move through a fluid (air or water), we have to push that fluid aside as we move through it. This creates resistance on the body that tends to slow our movements. This is often called a drag force. • Fluid resistance is caused by three different effects: surface drag, form drag and wave drag.

8. Factors that influence Drag • Fluid density- water is denser than air therefore forward motion in this fluid is more difficult • Shape- streamlined bodies/objects tapered to the tail experience less turbulence and less resistance • Surface- a smooth surface causes less turbulence • Size of frontal area- if area making initial contact with fluid is large, resistance is increased.

9. Types of Drag • Surface drag caused by friction between the surface of an object and the fluid surrounding it. A larger surface area or a rougher surface will increase the amount of surface drag present. • Form drag determined by the size and shape of an object. Objects with lower form drag have a narrower profile and a tapered shape at the back. • Wave drag produced when an object moves near the boundary between two different types of fluid (e.g.water and air).

12. The amount of form drag can change depending on how the fluid flows past the object. Sometimes a slightly rougher surface on an object can reduce form drag by helping the fluid to stay close to the object as it flows around

13. The effect of smoothness on fluid resistance can be confusing to us because roughening an object will always increase the amount of surface drag, • but can sometimes reduce the amount of form drag, if it substantially reduces the amount of cavitation. • Dimples reduce the amount of air resistance on a golf ball only because the resulting reduction in form drag is greater than the • increase in surface drag. Cavitation: resistance caused by an air pocket behind the object being sucked along in its wake. This same effect also occurs in water, where fluid is dragged along behind an object.

14. Force • A force pushes or pulls an object, tending to make it change its position. • Forces are often applied through direct contact, when one object pushes against another. • Forces are measured using units called Newtons (N). 10 N = 1 kilogram.

15. How the Body Applies Force • Contract muscles or build up momentum within the body. • Example - hitting a hockey ball. • Whenever two surfaces rub past one another, they produce friction. • Example - Basketball shoes produce high friction on a wooden floor, but not so much on grass. In contrast, football boots produce relatively high friction on grass, but can be very slippery when walking on concrete.

16. Activity • Calculate your weight in Newtons. • Do you think you could apply more force to a volleyball by throwing it, or by performing a serve? Hint: which action feels like there is more force applied to your hand?

17. How the Body Absorbs Force • When a moving object, such as a ball, hits your body, it applies a force to your body. The amount of force applied depends on the momentum and time. • Example – When catching a cricket ball, players keep their fingers, wrists and arms relatively loose before impact so that the hands move backwards at impact, increasing the duration of impact. This can greatly reduce the force of impact compared to a stiff-armed catch, reducing the force against the hand and reducing the likelihood of the ball bouncing straight back out of the hands.

18. Applying Force to an Object • When a force is applied to an object, movement will result—unless there is some other force also present to prevent movement.

19. Newton’s First Law of Motion • Inertia • A stationary object will remain stationary until a force causes it to accelerate. Similarly, once it is moving, it will tend to keep moving in a straight line unless a force causes it to slow down or change direction. • The amount of acceleration experienced in response to an applied force is determined by the amount of force applied, and by the mass of the object.

20. Newton’s Second Law of Motion • If twice as much force is applied, then twice as much acceleration will result. • Mass has an inverse relationship with acceleration; if the amount of mass is doubled, the acceleration will be halved. • Objects with a large mass have a large amount of inertia, meaning they are difficult to accelerate; while objects with a small mass need only a small force to be accelerated. • a = F / m

21. Newtons Third Law of Motion • When one object exerts a force on a second object, the second object must also exert an equal-sized force back onto the first object, in the opposite direction. • Example – When we catch a ball by applying a force to slow it down, the ball will exert an equal and opposite force back onto our hands—this is a force equal in size, but opposite in direction. The force on the ball causes it to slow down; while the force on our hands may cause pain and will certainly cause our hands to accelerate in the direction of the ball.