Forces in Physical Activity
- Created by: T - raffe
- Created on: 07-12-16 10:24
Net Force
- SUM of all FORCES acting on a body
- OVERALL force acting on a body
Net Force = Mass x Acceleration
NO change in MOTION - Net Force = 0
If Net Force is present:
- Creates MOTION
- Causes moving body to ACCELERATE, DECCELERATE or CHANGE DIRECTION
- Changes SHAPE
Balanced Forces
- When two or more forces acting on a body are EQUAL in SIZE and occur in OPPOSITE DIRECTIONS
- Net Force = 0 as there is NO change in MOTION
If Net Force = 0:
- Body will remain STATIONARY (E.g. Rugby scrum)
- Body will ove at CONSTANT VELOCITY (E.g. Marathon runner)
Unbalanced Forces
- When two or more forces are UNEQUAL in SIZE and occur in OPPOSITE DIRECTIONS
- Force acting in one direction is GREATER in size than froce in OPPOSITE direction
- Net Force will be PRESENT = CHANGE in MOTION
Weight Force
VERTICAL Force
- GRAVITATIONAL pull that the Earth EXERTS on a body (Newtons)
- Acts DOWNWARDS from COM
Weight = Mass x Gravity
Reaction Force
VERTICAL Force
- EQUAL and OPPOSITE force exerted by a body in RESPONSE to ACTION Force
- Acts UPWARDS from point of CONTACT
Friction
HORIZONTAL Force
- OPPOSES the MOVEMENT of one surafce over another
- Acts PARALLEL to the point of CONTACT
Affected by:
- SURFACE characteristics - Rough = High Friction
- TEMP - High temp = High Friction
Air Resistance
HORIZONTAL Force
- OPPOSES MOTION
- Acts in OPPOSITE DIRECTION from COM
Affected by:
- VELOCITY - High Velocity = High AR
- Frontal cross-sectional AREA - High Area = HIgh AR
- SURFACE - Rough = High AR
Free Body Diagram
Clearly labelled sketch showing all of the forces acting on a body.
Include:
- Point of APPLICATION
- Direction of MOTION
- SIZE of Force
Impulse
- PRODUCT of a force MULTIPLIED by the TIME
- Measured in Ns (Newton seconds)
Impulse = Force x Time
or
Impulse = Change in Momentum
How to increase an Impulse:
- Increase the SIZE of the Force
- Increase the TIME over which the force is applied
By increasing an impulse you increase:
- MOMENTUM of an object
- VELOCITY of an object
- Increase the DISTANCE an object travels
- Increases the CONTROL over an object
Take off phase of Vertical Jump (Force/Time Graph)
A-B
- Impulse = 0
- Net Force = 0
- BALANCED Forces (W=R)
= NO ACCELERATION & jumper remains STATIONARY
B-C
- Impulse = -ve
- Net Force present
- UNBALANCED Forces (W>R)
= Jumper BENDS knees to dip DOWNWARDS. Causes DOWNWARDS ACCELERATION
Take off phase of Vertical Jump (Force/Time Graph)
C-D
- Impulse = +ve
- Net Force present
- UNBALANCED Forces (R>W)
= UPWARDS ACCELERATION (large action force = large reaction force)
D-E
- Impulse = +ve (BUT reducing in size)
- Net Force present
- UNBALANCED Forces (R>W)
= Jumper EXTENDS legs their ACTION force DECREASES as does the REACTION force
100m Sprinter Foot Plant (Force/Time Graph)
Start = ACCELERATION
1. NEGATIVE impulse
- FORCE applied as foot makes contact with ground acts OPPOSITE to MOTION
- Forward momentum DECREASES
2. POSITIVE impulse
- FORCE applied as body moves over supporting foot acts in SAME direction as MOTION
- Forward momentum INCREASES
Overall:
- Positive impulse > Negative impulse
- ACCELERATION
100m Sprinter Foot Plant (Force/Time Graph)
Middle = MAX speed
1. NEGATIVE impulse
- FORCE applied as the foot makes contact with ground acts OPPOSITE to MOTION
- Forward momentum DECREASES
2. POSITIVE impulse
- FORCE applied as body moves over supporting foot acts in SAME direction as MOTION
- Forward momentum INCREASES
Overall:
- Positive impulse = Negative impulse
- Net force = 0
- CONSTANT speed (no acceleration)
100m Sprinter Foot Plant (Force/Time Graph)
End = DECELERATION
1. NEGATIVE impulse
- FORCE applied as foot makes contact with ground acts OPPOSITE to MOTION
- Forward momentum DECREASES
2. POSITIVE impulse
- FORCE applied as body moves over supporting foot acts in SAME direction as MOTION
- Forward momentum INCREASES
Overall:
- Negative impulse > Positive impulse
- DECELERATION (Due to fatigue or AR)
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