chapter 3 Flashcards
(22 cards)
biomechanics
angular and linear kinematics:
- distance and displacement
- speed and velocity
- acceleration
- projectile motion
angular and linear kinetics:
- newtons laws
- inertia
- mass
- force
- momentum
- impulse
equilibrium and human movement:
- levers
- stability and balance
Kinetics
- Kinetics is the study of the forces that cause motion and also the forces resulting from
motion. - Forces cause changes in motion and a change in motion cannot occur without force.
- Forces cannot be created unless two or more bodies interact.
Kinematics
- is the study of movement with reference to time, distance, displacement and
velocity. - It is the description of motion.
- Force
- Force is defined as a push or a pull.
- It is an effect on one body that results from the
interaction of a second body and can cause it to accelerate (speed up), decelerate (slow down) and/or change direction.
Forces can affect objects in two ways:
* Change the shape of an object (stretching, squashing or twisting)
* Move the object (however, they need to sufficient enough to overcome the inertia
of the body).
Force can be calculated by multiplying the mass of the object by the acceleration:
Force (N) = Mass x Acceleration
Types of Force:
Friction
- Friction occurs when two surfaces come in contact with each other.
- Friction opposes the motion of an object.
- In sporting situations, there are times when it is beneficial to decrease the friction between two surfaces, such as in downhill skiing.
- In other examples, it may be important to increase the friction between the surfaces, such as a gymnast applying chalk to their hands prior to an uneven bar routine.
Types of Force:
Air and water resistance (Drag)
- When an object or body moves through air or
water it will experience a drag force. - Drag forces oppose the direction of motion of
the object, slowing it down. Drag is affected by a number of factors including air density, cross-sectional area of the body and the speed the object is travelling. - In sports that require high speed, drag is often minimised through technique (body position) and clothing (fabric and design).
Types of Force:
gravitational force
- On Earth the gravitational force is the force (gravity) that causes objects to fall downwards – towards the centre of the Earth.
- The acceleration due to gravity is equal to .
- All objects thrown or projected into the air are affected by the force of gravity.
- Mass and Weight
- Mass is the quantity of matter found within a particular body.
- It is measured in kilograms (kg).
- There is a direct relationship between mass and inertia (mass = inertia).
- For example, an object with a large mass will require a large amount of inertia to move it.
- Similarly, a moving object with a large mass will require a large amount of inertia to stop it.
- Weight is directly proportional to the mass of an object.
- It can be calculated by multiplying mass by acceleration due to gravity.
- Because weight is a force, the units used are newtons.
- Inertia
- Inertia is the resistance of a body to a change in its state of motion, whether that state is at rest (static inertia) or moving with a constant velocity (dynamic inertia).
- An example of static inertia include the reluctance of a heavy piece of equipment to be moved, or the reluctance of a stationary wrestler to be shifted from their position in a contest.
- An example of dynamic inertia is the reluctance of a runner travelling at speed to
be stopped (change of direction and velocity). - It is much harder to move or change the state of motion of an object that has greater inertia.
- The amount of inertia an object has is directly related to its mass.
- The greater the mass of an object, the greater its inertia, and the greater the force needed to change is state of motion.
- If the force applied to an object is not greater than the inertia of the object,
there will be no change in the object’s motion.
- Momentum
part 1
- Momentum is considered a cornerstone of physics and has been defined as the ability of an object to continue moving because of its mass and velocity.
- Momentum is the product of mass (kg) x velocity (m/s).
Momentum (p) = mass (m) x velocity (v) - If an object has zero velocity (not moving), then it has zero momentum.
- If two objects have the same mass, then the object with the greater velocity will have the greater momentum.
- Similarly, if two objects have the same velocity, then the object with the greater mass will have the greater momentum.
- Objects with a greater momentum are more difficult to stop. For example, catching a medicine ball is more difficult to catching a tennis ball.
- Momentum
part 2
- Momentum plays a key role when objects collide, making it very important in sport.
- Every time two objects collide, there is a change in momentum of each object as a result of a change in the object’s velocity.
- When two objects collide, both will usually continue moving in the direction of the object with the greatest momentum.
- For example, if two netball players moving with similar velocities collide, both players will continue moving in the direction of the player with the greater mass.
- Impulse
- Impulse is the change of momentum in an object. To change the momentum of an object, a force must be applied over a period of time.
- Impulse = force applied x time
- The greatest changes in momentum (whether speeding up or slowing down) will occur when maximum forces are applied for as long as possible.
- Force Reception The same principle of impulse applies for receiving forces.
For catching a fast moving ball, the best result will be achieved if the force of the ball can be received over a longer time by ‘giving with the ball’ as it is caught. - Gymnasts are encouraged to adopt a motor bike landing and give with the knees to decrease the force of the ground, stick the landing and reduce the risk of injury.
- Impulse remains constant but by increasing the time by giving, force is decreased.
Newton’s First Law: The Law of Inertia
- Newton’s First Law states that ‘An object will remain at rest, or in it’s current state, unless acted upon by an external force.’
- An object will only change their state if there is an external (unbalanced) force put on the object.
- (Generally objects want to continue doing whatever it is they are doing).
- If an object is heavy (has a greater inertia) then a greater force is required.
Newton’s 2 nd Law: The Law of Acceleration
- Newton’s 2 nd Law states: ‘The rate of acceleration of an object is dependent upon the net force acting on that object and the mass of the object.
Force (Newton) = mass (kg) x acceleration ()
EXAMPLE OF NEWTONS 2ND LAW:
why an under 10 cricket player should use a light weight bat, compared to a heavy bat.
Define: The rate of acceleration of an object is dependent on the net force acting on the
object and the mass of the object.
Equation: Force = ma
Explanation: The young cricket player needs a light enough bat in order to produce sufficient acceleration on the bat, in order to gain/develop the necessary force for success (to hit the ball).
Often Newton’s 2 nd Law is applied to sprinters and cyclists.
F = ma or Acceleration = force/mass
If the force remains constant, someone with a heavier mass will accelerate slower than someone with a lighter mass.
Newton’s 3 rd Law: The Law of Action / Reaction
Newton’s 3 rd Law states: For every action there is an equal and opposite reaction.
- When two objects come in contact, they apply equal forces on each other, but in different directions.
- The greater the force applied, the greater the force applied back.
In a sporting context:
* The harder we push into the ground, the higher off the ground we will go. This is
because the force we push into the ground will lead to an equal and opposite force
pushing us into the air.
* So if a gymnast wants to do a somersault they are likely to get more air time
(reaction force) if they push harder into the ground (action force).
Angular Motion
-Angular motion is movement around an axis (internal or external) and is seen in any
movement where rotation occurs.
- It is caused by eccentric forces. An eccentric force is a force that does not pass through the centre of gravity of the body/object on which it acts.
Torque
- Torque is the amount of rotation of an object.
- It can be calculated by multiplying force by lever arm (or where the force is applied in
relation to the axis). - More rotation can be applied to an object with greater force and/or by applying the force further away from its axis point.
- Curving soccer shots, or bending them, is an example of angular motion. It is done by
kicking the ball off-centre, putting a spin on it and sending it into a curve.
Angular Momentum
- Angular momentum is defined as the quantity of angular motion of an object or in other words the quantity of rotation of an object around an axis.
- Angular momentum is the product of moment of inertia and the angular velocity of an object rotating around an axis.
- Angular momentum is involved in performing a somersault in gymnastics or diving.
- It is also involved in a cricket or baseball player swinging their bat.
Moment of Inertia
- Moment of inertia refers to the reluctance of an object to rotate. It is defined as the
resistance of an object to changes in its angular motion. - The radius refers to how far the mass is distributed from the axis of rotation.
The moment of inertia is dependent on both:
* The mass of the rotating object or body
* The distance the weight is distributed from the axis of rotation
Moment of Inertia
part 2
- Angular momentum is greater if the mass is larger and the further the bass is distributed from the axis.
- When rotating a cricket bat, for example, angular momentum can be increased by increasing the mass of the bat, as well as the length of the bat (that increases
the distance the weight is distributed from the axis). - The greater the angular momentum,
the greater the transfer of momentum to the ball.
-Angular momentum is greater if the mass is larger and the further the bass is distributed from the axis.
- When rotating a cricket bat, for example, angular momentum can be increased by increasing the mass of the bat, as well as the length of the bat (that increases
the distance the weight is distributed from the axis).
- The greater the angular momentum,
the greater the transfer of momentum to the ball.
- Modified equipment is another way of reducing the moment of inertia (both the mass and radius of rotation are reduced), promoting the ability to move the equipment with more ease.
- However, this also reduces the overall angular momentum. For example, using a
smaller tennis racquet or cricket bat.
Moment of Inertia
In the sporting context:
- Many athletes control the distribution of mass of the human body in order to
increase or decrease their rate of rotation. - For example, springboard divers bring
their arms inwards when performing a twist. - This decreases the moment of inertia,
as the mass is now concentrated closer to the radius of rotation. - This results in an increase in the rate of rotation (angular velocity).
- However, when a springboard diver pulls their arms out prior to entering the water, they increase their moment of inertia and therefore decrease their angular velocity.
- This enables the diver to slow down and enter the water cleanly.
Conservation of Angular Momentum
- Angular momentum is conserved when the body is in flight.
- This means that the total angular momentum remains constant throughout the whole movement.
- This is as a result of a trade-off between angular velocity and moment of inertia.
- For example, as an ice skater brings her arms into her chest, the moment of inertia
decreases and the angular velocity increases. - However, angular momentum remains
constant. - On the other hand, if the ice skate was to pull their arms out, the moment of inertia would increase and the angular velocity decrease.
- Angular momentum would still remain
constant.
