# Forces in Physical Activity

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• 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
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## 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)
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## 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
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## Weight Force

VERTICAL Force

• GRAVITATIONAL pull that the Earth EXERTS on a body (Newtons)
• Acts DOWNWARDS from COM

Weight = Mass x Gravity

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## Reaction Force

VERTICAL Force

• EQUAL and OPPOSITE force exerted by a body in RESPONSE to ACTION Force
• Acts UPWARDS from point of CONTACT
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## 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
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## 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
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## 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
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## 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
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## 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

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## 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

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## 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
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## 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)
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## 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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