Download 3 - Newton`s Laws

The following force–time
graphs were obtained during
the various stages of a
runner’s 100-metre sprint.
Using Figure 6, identify which
graph is associated with each
of the following phases of a
100-metre sprint, giving
reasons for your answers:
(i) early in the sprint;
(ii) during the middle part of
the sprint; and
(iii) towards the end of the
sprint.
(6 marks)
A 200 metre runner must exert a large force in a short
period of time to generate an impulse.
Sketch and label a graph to show the impulse generated
during the acceleration phase of a 200 metre race. (3
marks)
A. X Axis – (time)/milliseconds/seconds
B. Y Axis – (force)/Newton’s
C. Shape of graph – negative and positive components of
force shown with negative first
D. Positive impulse clearly larger than negative impulse
E. Positive and negative (force) labelled
Biomechanics
• Mechanics of movement:
– vectors and scalars – velocity, acceleration and
momentum/impulse in sprinting
– Newton’s Laws applied to movements –
application of forces in sporting activities
– projectile motion – factors affecting distance,
vector components of parabolic flight
– angular motion – conservation of angular
momentum during flight, moment of inertia
and its relationship with angular velocity.
Newton’s Laws
•Newton’s three laws
• Forces acting on athletes
• How they can be applied to sporting
situations
Newton’s 1st Law:
‘Every body at rest, or moving with
constant velocity in a straight line, will
continue in that state unless
compelled to change by an external
force exerted upon it.’
• The motion of an object will not change without some
force acting on it
• A stationary object will remain stationary unless
something causes it to move
• A moving object will continue at a constant speed
unless something speeds it up or slows it down.
Remember Inertia!
• Inertia is the reluctance of a body to move or change
its state of motion
• Objects will stay in their state of inertia (i.e.
stationary or moving at a constant velocity) unless a
force overcomes that inertia.
Applying Newton's 1st Law:
• The athlete will remain stationary in the blocks unless
they exert a force to cause them to move (Newton’s 1st
Law)
• The force must be large enough to overcome their
inertia.
•Apply Newton’s First Law to three other sporting
examples
Newton’s 2nd Law:
‘The acceleration of a body is
proportional to the force causing it,
and the acceleration takes place in the
direction in which that force acts.’
This is sometimes expressed by the equation F=ma
where:
F = Force
m = Mass
a = Acceleration
This assumes that in most situations, the mass remains
constant.
Applying Newton's 2nd Law:
• The athlete will accelerate away from the blocks in
proportion to the force their muscles are applying, and in
the direction that their muscles are applying the force
(Newton’s 2nd Law)
• When the muscles have reached their maximum force
generation the athlete will stop accelerating and should
continue at a constant velocity (Newton’s 1st Law)
Apply Newton’s Second Law to three other sporting
examples
Newton’s 3rd Law:
‘When one object exerts a force on a
second object, there is a force equal
in magnitude but opposite in direction
exerted by the second object on the
first.’
Or in other words:
For every action, there is an equal and opposite reaction.
Applying Newton's 3rd Law:
• As the athlete pushes down onto the ground, there is
an equal and opposite force pushing the athlete from the
ground
• This is called the ground reaction force
•Apply Newton’s Third Law to three other sporting
examples
Newton’s 3rd Law
Newton’s 3 Laws of Motion
Force – Recap from Lesson 1
Force is the ‘push or pull’ exerted on an object or body,
which may either cause motion of a stationary body or a
speeding up, slowing down or change of direction of a
moving body.
Force can be generated internally by Muscle contraction or
externally by Gravity, friction, water and air resistance
Without such forces, movement would not be possible, and
when such forces are understood and adapted to the same
aim, optimal performance can be achieved.
Force is a Vector quantity – what does this mean?
It has magnitude and direction.
Forces
A force is a push or a pull that can change the…
• …velocity (therefore causing acceleration or
deceleration),
• direction…
• or shape…
…of an object.
There are always many different forces acting on an athlete.
These include Vertical Forces:
• Weight
• Ground Reaction Force / Buoyancy in water
and Horizontal Forces:
• Action Force (or driving force)
• Reaction Force (from any object that has been struck)
• Drag – which can include:
• Friction
• Air resistance
• Water resistance
Sketch the diagram and label the forces being shown by
the arrows:
Recapping Resultant Forces
The motion of an object depends on the sum of all the
forces acting upon it at any one time (i.e. the resultant
forces).
a)
d)
b)
e)
c)
f)
Net Forces
The motion of an object depends on the sum of all the
forces acting upon it at any one time (i.e. the resultant or
net forces).
If two forces opposing each other are equal (and
opposite) we can say that they are balanced.
This means the net force is zero.
The object will not change its motion.
If the two forces are not opposing each other are
unbalanced, then there will be a net force that will change
the velocity, direction or shape of the object.
The ball will increase in velocity (accelerate) upwards
Drag
Any object in motion, unless in a vacuum, will experience drag
(friction, air resistance and / or water resistance).
This always acts in the opposite direction to motion, and slows
down the object.
If the action / driving force on an object and the combined
forces of friction, air resistance and/or water resistance are
equal, the net force would be zero.
The object’s motion would not change – it would remain stationary
or moving at a constant speed.
If the action force is larger than drag, the object will accelerate.
If the action force is smaller than drag, the object will
decelerate.
Friction
Caused by two objects rubbing against each other.
Factors affecting friction include:
• Roughness of the surfaces
• Surface area
• Size of the down force / weight
• Warmth of the surfaces
Air Resistance
Air resistance is the force of air pushing against a moving
object. It is a form of fluid friction due to contact
between the object and the air.
Factors affecting air resistance are:
• Velocity of the object
• Frontal cross sectional area
• Shape and surface characteristics
Water resistance is the same as air resistance, except it
is due to water molecules pushing against a moving object.
Free Body Diagrams
• Choose a sporting event where a number
of forces are in action and label a
sketch diagram of that event showing
the forces and their magnitude acting
upon that body.
Apply it…
Ice hockey is often regarded as a very physical game, with many
collisions occurring during normal play. A stationary ice-hockey puck
was struck with an ice-hockey stick and travelled across the ice until
it struck and rebounded from a wall.
The diagram shows the changes in horizontal linear velocity
experienced by the puck. Assume that air resistance and friction on
the ice are negligible.
Describe and explain the horizontal motion of the puck associated
with each of the periods of time identified as P, Q, R, S and T.
(7 marks)
P
1. Puck stationary/at rest on the ice/zero horizontal linear velocity;
2. No/negligible net external forces acting;
Sub max 1 mark
Q
3. Time when stick in contact with puck/force applied by stick;
4. Puck changing velocity/accelerating/positive linear velocity;
5. In direction of force applied by stick.
Sub max 2 marks
R
6. Puck travelling with constant (horizontal) velocity;
7. No/negligible net external forces acting/friction free;
Sub max 1 mark
S
8. Time when puck hits wall;
9. Puck decelerates . (caused by force applied by wall);
10. Eventually travels in opposite direction/negative horizontal linear
velocity/ rebounds off the wall/travels towards the performer.
Sub max 2 marks
T
11. Puck moving across ice with constant negative horizontal linear velocity;
12. No/negligible net external forces acting;
13. Reduced velocity (due to energy absorbed by impact).
Sub max 1 mark
(Do not credit speed)
7 marks
Apply It…
In the Kirrin cycling event, a cyclist has to travel at many
different speeds, from slow to sprinting.
Explain, in terms of forces, how a cyclist is able to change
and maintain their speed during the course of the race.
(6 marks)
• Weight and ground reaction force are equal
• Action / driving force is cyclists legs on pedals
• Opposing force is friction and air resistance (AR).
• Friction does not change much during the race.
• To accelerate action force must be greater than AR.
• As the cyclist speeds up, (AR) increases
• When the action force and AR are equal, speed is
constant
• To increase to a higher velocity, a greater action force
must be applied and maintained.
• To decelerate, the action force is reduced (by pedalling
less hard) so it is less than AR
1. The final stage of an endurance race often involves a
sprint finish.
Using Newton’s Second Law of Motion, explain how an
athlete is able to accelerate towards the finish line.
(3 marks)
2. Using ‘Newton’s First and Second Laws of Motion’,
explain how the swimmer dives off the starting blocks.
(4 marks)