Biomechanics- Linear motion Flashcards
Formulas
What are the formulas for
Velocity =
Acceleration =
Momentum =
Velocity = displacement ÷ time
Acceleration = (final velocity – initial velocity) ÷ time
Momentum = mass x velocity
What are the formulas for
Force =
Impulse =
Angular momentum =
Force = mass x acceleration
Impulse = force x time
Angular momentum = moment of inertia x angular velocity
Explain What is meant by scalar quantity (how something scales)
When measurements are only described in terms of size or magnitude – mass, distance and speed.
Explain What is meant by scalar quantity (how something scales)
Vector quantity = When measurements are described in terms of magnitude (size) and direction – weight, acceleration, displacement, velocity and momentum
Define Mass and Weight
Mass is a quantitative measure of inertia (in KG’s).
Weight is the gravitational force that the earth exerts on the object (in Newtons).
Define Speed and Displacement.
Speed is distance divided by time (distance per unit of time).
Displacement is distance in metres (or equivalent) from the starting to finishing point.
Define Acceleration and Momentum
Acceleration is the rate at which an object changes its speed. It is calculated using the equation: acceleration = change in speed/time taken (e.g. miles per hour).
Momentum = mass x velocity (if the mass is in kilograms and the velocity in metres per second then the momentum is in kilogram metres/second (kg m/s)).
Explain the difference between distance and displacement
Both used to describe the extent of a body’s motion- tell us how far a body has travelled.
Distance is the length of the path a body follows when moving from one position to another.** E.g. **200 m runner completing race has run a distance of 200m.
Displacement is the length of a straight line joining the start and finish points. e.g. the length of the path the athlete follow is 200m- but their displacement will be the straight line measurement from the start to the finish.
Try explaining what is happening in this distance-time graph at each point
A-B= Speed is decreasing
B-C= Speed is decreasing
C-D=No movement
D-E= Speed is increasing
E-F= Speed is constant
Define Speed and Velocity
Speed and velocity are measurements of how fast a body moves.
Speed is the rate of change of distance.
Speed = Distance covered (m)
Time taken (s)
Velocity refers to how fast a body travels in a certain direction.
It is the rate of change of displacement.
Velocity = Displacement (m)
Time taken (s)
C
Q1. Identify how long Calre maintained her highest speed [1 mark]
Q2. Calculate the average speed Clare was travelling over the 100 metres. [total 2 marks]
- 4 seconds
- Average speed = 100/20 = 5m/s (metres per second/ms-1).
Cyclists need to be able to accelerate at the start of a race. The figure below shows a velocity–time graph of a cyclist at the start of a road race.
Q1. Identify the two points in the graph above between which the cyclist was accelerating at the greatest rate.
Between point __________ and point __________
(Total 1 mark)
Q2. Calculate the cyclist’s acceleration between points B and C in the graph above.
Give the correct units in your answer.
Cyclist’s acceleration _______ Units _______
(Total 2 marks)
Q1. A-B
Q2. Cyclist’s acceleration = 2 (1)
Units = ms-2/m/s2/metres per second squared/metres per second per second (1)
The start of a 100 m sprint race requires athletes to accelerate as quickly as they can out of the blocks.
Acceleration is a vector quantity.
Define what is meant by a vector quantity.
[total 1 mark]
A vector quantity has both magnitude/size and direction. (1)
D
The start of a 100 m sprint race requires athletes to accelerate as quickly as they can out of the blocks.
The table below shows the velocity of a sprinter measured at 10 m intervals, as well as the split times for each 10 m segment of the race.
Distance at end of 10 m split (m)
10 20 30 40 50
Split time (s)
2 1.5 0.9 0.8 0.8
**
Velocity at the end of the split (m/s)**
4 10 11 12 12
Q1. Calculate the sprinter’s acceleration between 10 and 20 m using the data in the table above.
Give the correct units in your answer
Sprinter’s acceleration _______________ Units _______________
(Total 2 marks)
* 4 (1)
* m/s2/ms−2/metres per second squared/metres per second per second. (1)
Define acceleration
Acceleration is the rate of change of velocity.
When velocity increases, positive acceleration occurs.
When velocity decreases, negative acceleration takes place.
Measured in metres per second m/s2
Define momentum
Momentum is the product of the mass and velocity of an object.
Momentum (kgm/s) = Mass (kg) x velocity (m/s)
Explain how internal and external forces affect a performer during linear motion.
[total 3 marks]
A force changes a body’s state of motion; e.g. returning a serve in tennis- a force has changed the direction of the ball. (Hitting it back)
Internal force- skeletal muscles contract- e.g. concentric contraction of quadriceps extending the knee when jumping
External force- Force from outside of the body e.g. friction, air resistance, weight, gravity
Forces acting on a performer during linear motion can be classed as
Horizontal= friction/air resistance or vertical = gravity/weight
Explain the forces that are acting on this performer [total 3 marks]
- Gravity pulls the performer down.
- Air resistance acts against a performer.
- Muscular force pushes the performer forwards.
- Weight acts downwards through gravitational pull.
- Reaction Force (not mentioned in spec)= Newton’s 3rd law considered (for every action there is an equal and opposite reaction)
- Frictional force acts at the point of contact with the ground.
Explain the two types of frictional force
Static = force exerted on one surface by another when there is no motion between the two surfaces. E.g. before an object begins to slide.
Sliding = When dry friction acts between two surfaces that are moving relative to one another.
Friction acts in opposition to motion- arrow is drawn in the opposite direction to slipping (usually in the same direction as motion)
However, in skiing the friction arrow opposes motion as the slipping occurs in a forward direction.
Factors that affect friction:
- surface characteristics of 2 bodies - Temperature of 2 bodies - Mass of sliding objects
Explain How air resitance affects a performer
Air resistance opposes the motion of a body travelling through the air and depends on;
Velocity of moving body- faster they are moving, greater air resistance
Cross-sectional area of body- larger, greater resistance (crouching over handlebars-Tour de France)
Shape and surface of body- streamlined, less resistance (swimmers shaving body hair to create smooth surface- swim hat)
In water, air resistance is frequently referred to as “drag”. Compare running in water to land, there is a much greater drag force in water due to greater density.
What is meant by net force
This is a resultant force acting on body when all other forces have been considered.
Often discussed as balanced vs unbalanced forces
Balanced= 2 or more forces acting on a body that are equal in size but opposite direction. (When standing- weight equal to reaction force but opposite in direction)
Unbalanced= Force acting on body is larger than force acting in opposite direction.(jumping in air- performer accelerates upwards as reaction force is bigger than weight force)
Explain “impulse”
Impulse is the time it takes a force to be applied to an object/body.
Impulse = Force x Time
Measured during sprinting – force platforms on track measuring horizontal forces
Impulse is mainly concerned with single events. e.g footfall (running)
Involves Horizontal forces only
Two parts to a footfall
- Landing: always first and negative (braking)
- Push off: Always 2nd and positive
Displayed as Force.time graph
A
B
Identify and explain two external forces acting on the athlete in the photograph.
Force 1
Force 2
(Total 4 marks)
* Air resistance (1) this will be acting in the opposite direction of travel to the jumper / will depend on the velocity of the jumper / the cross sectional area of the jumper / the streamlining / shape of the jumper (1)
* Weight / Gravity (1) this will be pulling the jumper downwards towards the sand pit / the greater the mass of the jumper the greater the weight force will be pulling the jumper down (1).
Accept other appropriate explanations of the external forces acting on the athlete.
During sporting events performers have to apply force to execute skills correctly.
The graphs below show three impulse graphs of a sprinter at different stages of a 100 metre race.
Identify which impulse graph represents the start, middle and end of the race.
For each graph justify your reason. [total 3 marks]
A. (start of the race) Graph C – large positive impulse indicating acceleration.
B. (middle of the race) Graph A – positive and negative impulse equal showing constant speed.
C. (end of the race) Graph B – large negative impulse indicting deceleration.
D. If all three graphs in correct order (C, A, B) but explanation incorrect.
As a sprinter accelerates along the track at the start of a 100m race, an impulse is generated.
(a) Define impulse. State the units of measurement.
(2)
(b) Sketch and label a graph to show the impulse generated at the start of a 100m race.
(3)
(Total 5 marks)
Award one mark for each of the following points:
* A measure of force applied over time / Impulse = force x time (1)
* Measured in Newton seconds / Ns (1)
Do not accept newtons per second / NS-1 / N/S
Accept any other appropriate definition of impulse. Answers must refer to force and time.
(b) [AO2 = 3]
Award one mark for each of the following points:
* X axis: time s / seconds, Y axis: force N / Newtons (1)
* Negative and positive components of force shown with negative first (1)
* Positive impulse larger than negative impulse (1)
Analyse the graph and suggest what impulse would be produced by the sprinter when:
* they leave the blocks to start the race
* they are running (between 40m and 80m)
* they finish the race (between 100m and 110m).
Justify your answers.
(total 15 marks)
AO1 − Knowledge of impulses
E.g. an impulse is the change of momentum of an object when the object is acted upon by a force for an interval of time. It is caused by a force during a specific time interval. Impulse = force × time and is sometimes referred to as force vs time graphs.
* X axis − (time)/milliseconds/seconds
* Y axis − (force)/Newton’s
* shape of impulse graphs − negative and positive components of force shown with negative first.
AO2 general points
General points:
* axes should be appropriately labelled
* differentiation between negative and positive impulse should be obvious
* mass of the runner would remain constant
* net impulse is combination of negative and positive impulses.
Leaving the blocks:
* net impulse is positive
* the performer is accelerating.
Between 40–80 m:
* positive and negative impulses relatively equal
* net impulse of zero / negligible.
After crossing the line:
* net impulse is negative
* deceleration.
**
AO3 − Justification of answers**
E.g. leaving the blocks − The positive aspect of the graph is bigger than the negative. When leaving the blocks the runner would hope to gain a positive impulse to accelerate. The greatest proportion of the graph is above the line and therefore shows the positive force in Newton’s law. The take-off / drive toe / ball of foot would create force for a relatively short amount of time as quick feet movement would be key to gain the acceleration. The breaking impulse (negative) is less than propulsive impulse (positive) / movement / contact with ground to limit breaking impulse. There must be appropriate ground reaction force to generate acceleration.
Between 40–80 m − There is a relatively stable nature of the middle of the race necessitates a relatively equal impulse. The proportions of the graph are fairly equal and there is a balancing of breaking impulse (negative) with propulsive impulse (positive) to maintain speed being generated. The importance of the middle section is to maintain position or to ensure speed is maintained to match or better opposition. There is no acceleration / deceleration / running at constant velocity / speed.
After the finish line − The negative aspect of the graph is bigger than the positive. After crossing the line, the runner would hope to gain a negative impulse to decelerate. The greatest proportion of the graph is below the line and therefore shows the negative force in Newton’s law. The landing foot is accentuated onto the ground for a longer time than during the race. The breaking impulse (negative) is more than the propulsive impulse (positive) / movement / contact with ground to increase breaking impulse. The importance of appropriate ground reaction force is to generate deceleration.
Accept any other relevant justification of the impulses that would be produced at the various stages of the 100m race.
Heptathletes are required to complete the 200 metre sprint.
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.
(Total 3 marks)
X Axis – (time) / milliseconds / seconds
Y Axis – (force) / Newton’s
Axis must be labelled with correct units to be credited. Accept with just units
Shape of graph – negative and positive components of force shown with negative firstPositive impulse clearly larger than negative impulsePositive and negative (force) labelled
Only awarded if negative impulse is first in the diagram
A reminder about Veloicy graphs
A reminder about Veloicy graphs
