EDU2MP Movement Perspectives Principals of Movement Momentum Impulse Force reception/Absorption Newtons Laws of Motion Levers

EDU2MP Movement Perspectives

Principals of Movement

Momentum

Impulse Force reception/Absorption

Newtons Laws of Motion

Levers


Impulse

Impulse = Force * change in Time.

In a collision, the impulse experienced by an object equals the change in momentum of the object. In equation form:F * t = m * change in v.


Momentum Momentum is a vector describing a "quantity of motion" or in

mathematical terms p (momentum) = mass (m) times velocity (v). p=mv

Conservation of Momentum In a closed system, such as when two objects collide, the total

momentum remains the same, though some may transfer from one object to the other. Momentum is always conserved in a closed

system, but most sporting situations in the real world are not a closed system. For example, when a baseball bat hits the ball, the ball will be squished to a certain degree. After few milliseconds, it rebounds back. This contraction and rebound action is causes the release of

heat energy, and some momentum is lost, or transferred elsewhere. Maximizing Momentum

As momentum is the product of mass and the velocity, you can increase momentum by increase either of these elements. In sport,

examples include using a heavier bat or racquet and increasing running speed or hand speed.


Force Reception/ Absorption


Impulse

When a force is applied to an object, the product of the force (F) and the length of time (t) that the force is applied, is called the impulse of the force.

Impulse = FtImpulse is equal to Force x time, measured in Newton Seconds.


Impulse

Depends on:• The time for which the force

acts• The size of the force applied


Reaction - Newton’s Third Law

Newton’s third law of motion states that:

“for every action there is an equal and opposite reaction.”

(Roberts & Falkenburg, 1992)


Momentum

• Refers to the quantity or amount of motion

Momentum = Mass x Velocity

The runner has a mass of 75 kg and is running at 5 m.s-1.

What momentum does he have?

375 kg.m.s-1


Acceleration - Newton’s Second Law Newton’s second law of motion states that:

“When a force acts on an object, the object accelerates in the direction in which the force is acting.”

• Acceleration is the rate of change of velocity and is determined by force.

Acceleration = The final velocity minus the initial velocity divided by time.

A = v – u

t


In a collision, an object experiences a force for a given amount of time which results in its mass undergoing a change in velocity (i.e., which results in a momentum change). There are four physical quantities mentioned in the above statement - force, time, mass, and velocity change. The force multiplied by the time is known as the impulse and the mass multiplied by the velocity change is known as the change in momentum. The impulse experienced by an object is always equal to the change in its momentum. In terms of equations, this was expressed as


The Affect of Collision Time upon the ForceFirst we will examine the importance of the collision time in

affecting the amount of force which an object experiences during a collision. In a previous part of Lesson 1, it was mentioned that force and time are inversely proportional. An object with 100 units of momentum must experience 100 units of impulse in order to be

brought to a stop. Any combination of force and time could be used to produce the 100 units of impulse necessary to stop an

object with 100 units of momentum. This is depicted in the table below.


Force Time Impulse

100 1 100

50 2 100

25 4 100

10 10 100

4 25 100

2 50 100

1 100 100

0.1 1000 100



In racket and bat sports, hitters are often encouraged to follow-through when striking a ball. High speed films of the collisions between bats/rackets and balls have shown that the act of following through serves to increase the time over which a collision occurs. This increase in time must result in a change in some other variable in the impulse-momentum change theorem. Surprisingly, the variable which is dependent upon the time in such a situation is not the force. The force in hitting is dependent upon how hard the hitter swings the bat or racket, not the time of impact. Instead, the follow-through increases the time of collision and subsequently contributes to an increase in the velocity change of the ball. By following through, a hitter can hit the ball in such a way that it leaves the bat or racket with more velocity (i.e., the ball is moving faster). In tennis, baseball, racket ball, etc., giving the ball a high velocity often leads to greater success. Now that's physics in action.


The effect of this strategy is to extend the time over which the collision occurred and so reduce the force. This same strategy is used by lacrosse players when catching the ball. The ball is "cradled" when caught; i.e., the lacrosse player reaches out for the ball and carries it inward toward her body as if she were cradling a baby. The effect of this strategy is to lengthen the time over which the collision occurs and so reduce the force on the lacrosse ball. Now that's physics in action. Throwing eggs


Occasionally when objects collide, they bounce off each other as opposed to sticking to each other and traveling with the same speed after the collision. Bouncing off each other is known as rebounding. Rebounding involves a change in the direction of an object; the before- and after-collision direction is different. Rebounding was pictured and discussed earlier in Lesson 1. At that time, it was said that rebounding situations are characterized by a large velocity change and a large momentum change.


Newton's third law of motion is naturally applied to collisions between two objects. In a collision between two objects, both objects experience forces which are equal in magnitude and opposite in direction. Such forces often cause one object to speed up (gain momentum) and the other object to slow down (lose momentum). According to Newton's third law, the forces on the two objects are equal in magnitude.


Consider the collision between the club head and the golf ball in the sport of golf. When the club head of a moving golf club collides with a golf ball at rest upon a tee, the force experienced by the club head is equal to the force experienced by the golf ball. Most observers of this collision have difficulty with this concept because they perceive the high speed given to the ball as the result of the collision. They are not observing unequal forces upon the ball and club head, but rather unequal accelerations. Both club head and ball experience equal forces, yet the ball experiences a greater acceleration due to its smaller mass. In a collision, there is a force on both objects which causes an acceleration of both objects. The forces are equal in magnitude and opposite in direction, yet the least massive object receives the greatest acceleration.





Newton's Laws of MotionFirst Law: Every body continues in its state of rest or motion in a straight

line unless compelled to change that state by external forces exerted upon it.

Second Law: 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 Third Law: To every action there is an equal and opposite reaction OR for every force that is exerted by one body on another there is an equal and

opposite force exerted by the second body on the first


LeversA lever is a rigid structure, hinged at one point and to which forces are

applied at two other points. The hinge or pivot point is known as the fulcrum. One of the forces that act on the lever is known as the weight that opposes movement and the other is the force that causes movement. For

more details see the page on Levers.

For your arm, leg or any body part to move the appropriate muscles and bones must work together as a series of levers. A

lever comprises of three components -Fulcrum or pivot - the point about which the lever rotates

Load - the force applied by the lever system Effort - the force applied by the user of the lever system

The way in which a lever will operate is dependent on the type of lever.


Classification of Levers

•Class 1 - The fulcrum lies between the effort and the load •Class 2 - The fulcrum is at one end, the effort at the other end and the load lies between the effort and the fulcrum •Class 3 - The fulcrum is at one end, the load at the other end and the effort lies between the load and the fulcrum

Class 1 Lever

Class 2 Lever

Class 3 Lever

Class 3 is the most common class of lever to be found in the human body.


Examples in strength training

Class 1 - Seated dumbbell triceps extension

•Class 2 - Standing heel lift** •Class 3 - Seated biceps curl

Class 1 Lever in the

Body


Body**


Body



• LAWS OF MOTION• Understanding the relationship between force and motion owes much to the work of an English scientist, Sir Isaac Newton. He is best

remembered for his three laws of motion.

Newton's First Law of Motion It is important to know the definition for each of the three laws of motion and more important, know how to apply the laws in practical situations. Newton's first law of motion states:

"All bodies continue in a state of rest or uniform motion in a straight line unless acted upon by some external force."

What are the applications of this law? A sprinter, for example, will not move from the blocks until his legs exert force against them. The high jumper will not take off from his approach run unless a force is applied to change direction.

Newton's Second Law of Motion - Law of Acceleration

"The acceleration of a body is proportional to the force causing it and takes place in the direction the force acts."

More force means more acceleration. A sprinter's acceleration from the blocks is proportional to the force exerted against the blocks. The greater the force exerted, the greater will be the acceleration away from the blocks. In the throwing events, the larger the force exerted on an implement, the greater will be the acceleration and consequently, distance thrown. Once an implement has been released there are no forces which can act to accelerate it. The same is true in the jumping events. The greater the force the athlete exerts at take-off the greater the acceleration and height or distance achieved. Once the athlete has left the ground nothing he does will accelerate the body. When maximal forces are needed the muscles contract to generate this force and this is why injuries are more likely to occur in the acceleration or deceleration phases of a movement.

Newton's Third Law of Motion - Law of Reaction

"To every action there is an equal and opposite reaction."

A runner exerts a force against the ground. This creates an equal and opposite reaction force which moves the body over the ground.


• The law of reaction also applies to movements that occur in the air. In these situations the equal and opposite reaction is shown in movements of other parts of the body. A long jumper, for example, will bring the arms and trunk forward in preparation for landing. The equal and opposite reaction is movement of the legs into

a good position for landing.



Documents

EDU2MP Movement Perspectives Principals of Movement Momentum Impulse Force reception/Absorption Newtons Laws of Motion Levers