Principals of Movement
Momentum
Impulse Force reception/Absorption
Newtons Laws of Motion
Levers
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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.
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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.
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Force Reception/ Absorption
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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 = Ft
Impulse is equal to Force x time, measured in Newton Seconds.
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Impulse
Depends on:
• The time for which the force
acts
• The size of the force applied
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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)
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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
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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
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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
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The Affect of Collision Time upon the Force
First 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.
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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
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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.
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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
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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.
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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.
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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.
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Newton's Laws of Motion
First 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
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Levers
A 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.
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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.
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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
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Class 2 Lever in the
Body**
Class 3 Lever in the
Body
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•
•
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.
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•
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.
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