SEHS Biomechanics

1. SEHS 4.3..The Fundamentals of
Biomechanics III

2. Define Newton’s three laws of motion
• Newton's first law states that a body at rest will
remain at rest, and a body in motion will remain in
motion with a constant velocity, unless acted upon
by a force. This law is also called the law of inertia
• Newton's second law states that a force acting on a
body is equal to the acceleration of that body times
its mass. Expressed mathematically, F = ma, where F
is the force in Newtons, m is the mass of the body
in kilograms, and a is the acceleration in meters per
second per second.

3. Cont’d
• Newton's third law states that for every action
there is an equal and opposite reaction. Thus, if
one body exerts a force F on a second body, the
first body also undergoes a force of the same
strength but in the opposite direction.
• Might want to know this one too…. the law of
conservation of energy states that the total energy
of an isolated system cannot change—it is said to
be conserved over time. Energy can be neither
created nor destroyed, but can change form; for
instance, chemical energy can be converted to
kinetic energy.

4. Newton’s First Law
An object at rest remains at rest and an
object in motion remains in motion with
the same speed and direction unless
acted on by a force.

5. aka – The Law of Inertia
Inertia is the tendency of an object to
resist a change in its motion. A soccer
ball will remain resting on the grass until
a force acts on it (a kick).

6. Once it is kicked, the soccer ball’s inertia will
keep it going because the ball RESISTS
changing its motion.
If the ball doesn’t hit
anything, the forces
of gravity and friction
will eventually stop the
ball.
On Earth, gravity and friction are
unbalanced forces that often change an
object’s motion.

7. •Inertia explains
many common events,
such as why you move
forward in your seat
when a car stops
suddenly.
•When the car stops,
inertia keeps you
moving forward.
•A force, such as the
pull of a seat belt, is
required to change
your motion.

8. Things tend to keep on doing whatever they’re doing until
something else acts on them.
Objects with a lot of mass have a lot of inertia.
It’s hard to change the motion of objects
with lots of inertia…
…But easy to
change the motion
of objects with
little inertia.

9. FORCE – TIME GRAPHS

10. NEWTON’S 2ND LAW OF MOTION

11. The Second Law of Motion
• Suppose you are baby-sitting two children
who love wagon rides. Their favorite part is
when you accelerate quickly. When you get
tired and sit in the wagon, one of the children
pulls you. He soon finds he cannot accelerate
the wagon nearly as fast as you can.
• How is the wagon’s acceleration related to the
force pulling it?

12. How is the wagon’s acceleration
related to the force pulling it?
•According to Newton’s second law of
motion, acceleration depends on the
object’s mass and on the net force
acting on the object.

13. Newton’s Second Law
An unbalanced force causes an object to
accelerate. The acceleration of the object
is equal to the net force acting on it
divided by the object’s mass.

14. Newton’s Second Law
When a pitcher throws a baseball, the harder he
throws, the more the ball accelerates.
The mass of the ball stays the same, but the force
increases.

16. What is Acceleration? The speeding up, slowing down, or change in
direction of an object. Acceleration is affected by the forces applied to
objects as well as the mass of the objects in question.
Acceleration = Force / Mass
(If you double the mass of an object you cut
the acceleration in half)
Objects with lots of inertia (ability to resist a
change in motion) have a large mass and objects
with little inertia have a smaller mass

17. •Look at the pictures
on the right.
•Which vehicle do
you think would
require a greater
force to push?
•Why do you think
this?

18. Weight v. Mass
Weight = the force of gravity acting on an
object. You stand on a scale, gravity pulls
you down, and the needle measures your
weight
Mass = how much matter makes up an object.
***A person will have the same mass no matter
where in the universe it is measured.
However, a person’s weight will be different
depending on the force of gravity where it is
being measured. ***

19. MOMENTUM AND IMPULSE
Newton’s Second Law

20. STARTER: Pairs discussion
• What makes an object hard to stop?
• Is it harder to stop a bullet, or a truck travelling along the
highway?
• Are they both as difficult to stop as each other?

21. Learning Objectives
• Define linear momentum and impulse
• Explain the relationship between linear
momentum and linear impulse
• Analyze force-time graphs

22. Momentum
• The bullet is hard to stop because it is travelling very fast,
whereas the truck ishard to stop because it has a very large
mass.

23. Momentum
• It makes sense to assume that a bullet travelling twice as fast
would be twice as hard to stop, and a truck twice the mass would
also be twice as hard to stop.

24. Momentum
Momentum is a measure of the “oomph”(quantity of
motion) that an object has due to its motion. The
more mass an object has and the more speed it has
the more momentum it has.
The formula for momentum is _______________
(p is momentum, m is mass, and v is velocity)

25. Momentum is a conserved quantity.
- The momentum of a system will not change
unless an outside impulse (strike with time ) is
applied to it. (Newton’s 1st Law)
- If the system remains isolated, its total
momentum will not change.
- That does not mean that individual parts of a
system cannot interact with each other and
exchange momentums.
The unit of momentum is a kg•m/s

26. Impulse
The only way to change momentum is through
impulse.
Impulse is an outside force applied for a
specific time.
The harder you push and the longer you push
the more the momentum will be changed.

27. How hard is it to stop a moving object?
Tostop an object, we have to apply a force
over a period of time.
This is called Impulse
Impulse = F * Δt
J = impulse (N∙s)
F = force (N)
Δt = time elapsed (s)
Impulse is expressed as N.s (The Newton Second)
J

28. FORCE TIME GRAPHS

29. Impulse and Force-time graphs
HorizontalForce
(N)
F = ma
+force +acceleration
F = ma
-force - acceleration
Time (s)
At each instant in time during a contact, a force acts to produce an acceleration.
The Impulse is the net effect of all those instantaneous forces.
In other words, it is the average force multiplied by the total time over which the
forces have acted.

30. Running Contact
• During a single running contact, your body
undergoes both positive and negativeforces that
produce positive and negative accelerations.
• A force acting for a period of time produces
an impulse.
• If the positive and negative impulses cancel each
other out (equal areas), then the net impulse is
zero and the runner is moving at a constant speed.

31. Force Time Graph Question

32. Newton’s Third Law
Whenever one object exerts a force on a
second object, the second object
exerts an equal and opposite force on
the first object. These are known as:
Action/Reaction Forces.

33. GROUP THOUGHT
Push the block down into the water
THINK!
What do you see happening?
Why is it happening?

34. Newton’s 3rd law
If a body A exerts a force on body
B, body B will exert an equal but
opposite force on body A.
Hand exerts force on
table
A CT IO N
Together
these arrows
are known as a
FORCE PAIR
Table exerts force on
hand
REACTION

35. • You constantly use action-
reaction force pairs as you
move about.
• When you jump, you push
down on the ground.
• The ground then pushes up
on you. It is thisupward
force that pushes
you into the air. This is unbelievably cool!

36. • When you walk
forward, you
push backward
on the ground.
• Your shoe pushes
Earth backward,
and Earth pushes
your shoe
forward.

37. • Do the action/reaction forces
cancel each other out?
• NO!!!
• Look at the volleyball player on
the left
• She exerts an upward force on
the ball.
• In return, the ball exerts an
equal but opposite downward
reaction force back on her
wrists.
• The action and reaction
forces act on different
objects.

38. • On the other hand, the volleyball players are both
exerting a force on the same object – the
volleyball.
• When they hit the ball from opposite directions,
each of their hands exerts a force on the ball
equal in strength but opposite in direction.
• The forces on the volleyball are balanced and the
ball does not move either to the left or to the
right.

39. Action-reaction pairs
explain how a gymnast
can flip over a vaulting
horse, how a kayaker can
move through the water,
and how a dog can leap
off the ground.
•In a similar
way, a
kayaker
moves
forward by
exerting an
action force
on the
water with a
paddle.
•The water
pushes back
on the
paddle with
an equal
reaction
force that
propels the
kayak
forward.

40. DrawtheFORCE PAIRSonto
the diagramsbelow
A: Cat on the table B: Pencil on the paper
J r
C: Board on thewall D: Fuelon therocket

41. A:Cat on the table B: Pencilonthe paper
Table onthe cat Paper onthe pencil
C:Board on the wall D: Fuelon the rocket
VVallon the board Kocketonthe 1ue1
,,.
f,
,_

42. GROUP THOUGHT
• Why use starting blocks in a sprint?

43. Law of Conservation of Momentum
• Complete on worksheets

44. Explain how Newton’s three laws of motion
apply to sporting activities
• 1st law--Basically, if an object is in motion, it keeps going
unless something stops it. What are examples of outside
forces that affect inertia? Most anything in the real world--
gravity, the surface of the playing field, a defensive player,
or the braking action of an athlete's body to stop.

45. Cont’d
• 2nd Law -- If a baseball player hits a ball with double
the force or with a bat of double the mass, the rate
at which the ball will accelerate (speed up) will be
doubled. Football players can slow down, stop, or
reverse the direction of other players depending
upon how much force they can generate and in
which direction.

46. Cont’d
• A swimmer propels herself through the water because the
water offers enough counterforce to oppose the action of
her hands pushing, allowing her to move. An athlete can
jump higher off a solid surface because it opposes his body
with as much force as he is able to generate, in contrast to
sand or other unstable surface.