7. COLLISIONS OF THE HUMAN BODY Flashcards
1
Q
- What is linear momentum?
A
- it is the mass of the object times by the velocity of the object
- the symbol for linear momentum is p
2
Q
- What is the formula for linear momentum?
A
p = m x v
= mass x velocity
3
Q
- How does Newton’s Second Law relate to linear momentum?
A
- Newton’s Second Law states:
F= m x a - acceleration can be calculated by:
a = ▵v / ▵t - Newton’s Second Law can be rewritten as:
F = m (▵v) / ▵t
F = ▵ (mv) / ▵t
F= ▵p / ▵ t
▵p = F x ▵t
4
Q
- What is Impulse?
A
- it is the product of the net force and the contact time
- it is the change in momentum
5
Q
- How is Impulse calculated?
A
▵p = F x ▵ t
6
Q
- What is Newton’s Second Law in terms of momentum?
A
- the net force acting on an object is equal to the rate of change of momentum
7
Q
- What does Newton’s Third Law state?
A
- there is an equal and opposite force for every force
- forces occur in force pairs
(action-reaction pairs) - these force pairs are always equal in magnitude to each other
- the always act in the opposite direction of one another
- these forces never act on the same object
8
Q
- How can Newton’s Third Law be related to momentum?
A
- when the momentum of one object changes,
the momentum of another object will be
changing by exactly the opposite amount - Newton’s Third Law requires that the momentum be conserved
9
Q
- What is the Law of Conservation of linear momentum?
A
- the total linear momentum of an isolated system remains constant
- this linear momentum is conserved
10
Q
- What is the formula for the Conservation of Linear Momentum?
A
p final = p initial
11
Q
- What tends to happen to human beings when we collide with objects?
A
- we break down
12
Q
- What happens if an object is travelling at a constant acceleration for a given time?
A
- the velocity changes
13
Q
- What formula accurately shows how to work out final and initial velocity?
A
- vf = vi + at
- final velocity= initial velocity + (acceleration x time)
14
Q
- What is an equation to work out the distance travelled by an object?
A
- ▵x = (vi.t) + (½. a. t²)
- distance = (initial velocity x time)
+ (½ x acceleration x time²)
15
Q
- What is a simplified formula for finding the acceleration of an object during a collision?
A
- a = vf-vi / total time of the collision
16
Q
- How would you work out the force felt by the object during the collision?
A
- F = m x a
17
Q
- Which kinds of situations of motion are totally inelastic?
A
- when the object decelerates to zero
18
Q
- What are sticky or totally inelastic collisions?
A
- this is when two objects collide
- they then stick together after the collision
- they move off together
- they share a common velocity after the collision
19
Q
- What is an inelastic collision?
A
- this is a collision where only momentum is conserved
- object deformation or heat is produced
- there is a loss of kinetic energy
20
Q
- What is an elastic collision?
A
- a collision where both kinetic energy and momentum are conserved
- the total kinetic energy is unchanged by the collision
21
Q
- What happens in a totally elastic or partially elastic collision?
A
- the objects bounce back
22
Q
- During which collision is there the greatest loss of kinetic energy?
A
- when the two objects stick together after the collision
- this is called a completely inelastic collision
23
Q
- What does a compressive force have the greatest effects on?
A
- the thinnest bones
- these absorb the impact
24
Q
- What is the equation for pressure?
A
P = F/ A
= the force per unit area
25
25. What is the unit for pressure?
- Pascals (Pa)
- we usually use MPa for pressure equations of the body
- this is the pressure times by 10⁸
26
26. Upon compression, at approximately which pressure do bones typically break?
- 170 MPa
27
27. What is the measurement of the pressure at which bones break called?
- the damage threshold
- it can also be known as the ultimate compressive stress
28
28. What is another form of writing the Pressure equation (in terms of collisions)?
- P = m x vi / A x t
= mass x initial velocity / area x duration of the collision
29
29. How can the effect of the pressure on the body be reduced?
- the duration (t) of the collision can be increased
- the area of contact (A) can be increased
30
30. What are three ways to increase the collision time?
(duration of the collision)
1. Better padding
- use better padding at the site of the collision
- EG: wear better sneakers
2. Increase the duration of the impact
- adjust your body during the actual impact
- EG: letting your knees bend during the impact after you
have jumped
3. Rolling over
EG: this can be done at the end of a parachute jump
: this increases the impact area (A)
jump
31
31. What determines the survivability of crashes of the human body?
- what part of the body makes contact
- what type of surface it hits
- the deceleration time
32
32. During whole body impacts, what determines your fate?
- the part of the body which makes contact:
- the back (supine position)
- the front (prone)
- the side (lateral)
33
33. What exactly happens during an injury to the head?
- In direct trauma:
- the head hits an object
- the skull can fracture
- the brain hits the interior of the skull:
- this leads to bruising on the brain (a contusion)
- or leads to brain bleeding (a haemorrhage)
- In addition to the primary impact (the coup):
- the brain can bounce off the skull
- it can hit the back of the skull (the countrecoup)
34
34. What is indirect trauma to the brain?
- this is when the brain is subject to large accelerations
(even without impact)
- this can lead to severe injury
35
35. What happens in Inertia injuries?
- the nerve cells axons are damage by stretching
- EG: Shaken Baby Syndrome
: Whiplash
- this is an effect of rotational accelerations and decelerations
36
36. How is the gravity of inertia injuries qualitatively described?
- it is described by the Abbreviated Injury Scale (AIS)
37
37. What determines the severity and the likely fatality of head injuries?
- guidelines
- these guidelines are determined from the records of past accidents
38
38. What is the name of the guideline to asses severity and likely fatality of head injuries?
- the Gadd Severity Index (GSI)
39
39. What does your foot experience every time is touches the ground?
- a collision
40
40. What is the magnitude of the forces on the foot during a very slow step?
- the forces slowly build up from 0 to mbg
- mbg= the mass of the body x the gravitational
acceleration (9.81)
41
41. What is the magnitude of the forces on the foot during running?
- the foot reaches a peak of approximately 2.0 - 3.6 mbg
42
42. Which parts of the leg feel the initial impact of the collision with the foot and the ground?
- the foot
- the lower leg
- the thigh
43
43. What is the formula we use when two objects collide and then move off together afterwards?
(ma.va)i + (mb.vb)i = (ma + mb)vf
- if the object is stationary:
- then vi= zero
44
44. What is the formula we use when two objects collide and rebound?
(ma.va)i + (mb.vb)i = (ma.va)f + (mb.vb)f
- remember to put positive and negative signs for objects going in different directions
45
45. What is the formula we use when an object is dropped vertically on a moving object?
(ma.va)i + (mb.vb)i + (ma + mb)vf
- the vi of the dropped object is zero
46
46. What is the formula we use if there is an explosion?
(ma+mb) vi = (ma.va)f + (mb.vb)f
- any stationary object has a vi of zero
- take opposite directions into account
47
47. What formula do we use when two objects are connected to each other by springs?
(ma + mb) vi = (ma.va)f + (mb.vb)f
- the initial velocity is usually zero
- take into consideration the opposite directions
48
48. What formula do we use when a bullet is being shot out of a gun?
(mg + mb) vi = (mg.vg)f + (mb.vb)f
- initial velocity is usually zero
- take into consideration the opposite directions
49
49. Read through summary.
Does everything make sense?
- yes
50
50. Read through this summary.
Does everything make sense?
- yes
51
51. Read through this summary.
Does everything make sense?
- yes
52
52. Read through this summary.
Does everything make sense?
- yes
53
53. Read through this summary.
Does everything make sense?
- yes
