Force

Starter

• Match up each Newton's Law
• Match up each equation
• Match up the correct key term with their definition

Force

Force & its effects

Force is a push or pull that alters, or tends to alter, the state of motion of a body

It is measured in Newtons (N)

There are 2 types of force:

Internal force is generated by the contraction of skeletal muscles.

A 100m sprinter must contract the rectus femoris to extend the knee and gastrocnemius to plantar flex the ankle to generate the force required to drive away from the block

External force comes from outside the body and acts upon it.

We consider the forces of weight, reaction,

friction, and air resistance

To create motion, force is essential

Force can have 5 effects

Use the objects and instructions in front of you to work out the 5 effects and their sporting examples

Apply it!

Fill in the blanks of the following paragraph:

Force has ....... effects on a body used in sport. A football at rest on the penalty spot must have an .............. force applied to create .............. . When a tennis player applies a large force to the tennis ball during a serve, it will ..............at a great rate towards the service box. When a netballer receives a chest pass, they cushion the ball to .............. it. A rugby player applies opposing forces to the ground when performing a dodge to .............. .............. . When a trampolinist lands a seat drop, the force applied to the bed makes it .............. .............. .

Force can be generated but also have no effect on the motion of a body

How can this be?

Think about a plank and what internal forces are acting upon the body...

When performing a plank, the rectus abdominis isometrically contracts generating muscular force; however, a static position is maintained

Force is essential to motion

When we understand the internal and external forces that can act on a body, we can start to consider which forces to maximise and which to minimise to optimise performance

Net Force

Can you think of any examples which you apply to your sport?

Vertical Forces

So...

Net Force: the sum of all forces

acting on a body, also termed resultant

force. It is the overall force acting on a

body when all individual forces have been considered

Weight is the gravitational pull that the earth exerts on a body

Balanced forces: these occur when 2 or more

forces acting on a body are equal in size and

opposite in direction. The body will remain at rest

or in motion with constant velocity because

Net force = 0

Unbalanced forces: these occur when 2 forces

are unequal in size and opposite in direction.

A net force will be present and the body

will change its state of motion, either

accelerating or decelerating

Reaction is the equal & opposite force exerted by a body in response to the action force placed upon it

Can you think of any examples in sport?

Weight and reaction are both measured in Newtons (N)

Weight

Weight force is always present & acts downwards from the body's centre of mass

It can be shown on a diagram by a vertical arrow extending from the centre of mass downwards

Weight (N) = mass (kg) x acceleration due to gravity (m/s/s)

Reaction

Reaction force is a result of

Newton's 3rd law of motion & is

always present when 2 bodies are in contact

Normal reaction can be shown on a diagram by a vertical arrow extending upwards from the point of contact with

the surface

Examples

Miss Carter's Examples:

Paralympic sprinter Jonnie Peacock has a mass of 73kg. Acceleration due to gravity can be assumed to be 10m/s/s. Therefore, Jonnie's weight would be 730N (W = 73kg x 10m/s/s)

Gymnast Max Whitelock has a mass of 56kg & a weight of 560N (W = 56kg x 10m/s/s) that acts

downwards from his centre of mass. When

performing a handstand, reaction force is equal

in size & opposite in direction to weight, which

acts upwards from the point of contact.

These forces are balanced & net force is

0; therefore the body remains at

rest

Horizontal Forces

Free Body Diagrams

A picture paints a thousand words

Friction is the force that opposes the motion of 2 surfaces in contact & is measured in Newtons (N)

A free body diagram clearly shows where the force originates, the size of the force and the direction the force is acting in

This allows us to consider the net force acting on the body and therefore the resultant motion that occurs

A sprint cyclist's tyres would tend to slip backwards as they rotate; friction opposes this and acts forwards

Keep it simple!

Draw stick men/women to represent an athlete and identify all the forces acting upon them at the moment described in the question

Apply it

Example

Friction

Marathon runner traveling at a constant velocity.

W = R, F = AR, balanced forces, net force = 0, runner is in constant forward velocity

Example

To apply these horizontal forces:

Sprinter Adam Gemili wears spikes on a rubberised track to generate a large frictional force from the starting blocks. His low, tucked body is clad in smooth Lycra and moves relatively slowly through the drive phase, which minimises air resistance. In this phase of the 100m friction far exceeds the opposing forces of air resistance. the forces are unbalanced, there is a positive net force, and therefore the body accelerates in a forward direction

Long-jumper accelerating forwards and upwards at

take-off. R > W, F > AR, unbalanced forces, positive

net force, long-jumper experiences forward and

vertical acceleration

Activity

Label and annotate all of the athletes

Friction can be shown on a diagram by a horizontal arrow extending (usually) in the same direction as motion from the point of contact parallel to the sliding surface

Experiment

Friction is affected by several factors:

Use the experiment instruction sheets to conduct the friction experiment and work out the 4 ways friction can be affected

Air Resistance

Homework: extend your knowledge box in booklet

The force that opposes the motion of a body traveling through the air

It is a form of fluid friction

Measured in Newtons (N)

It can be shown on a diagram by a horizontal arrow extending against the direction of motion from the centre of

mass

Air resistance

is affected by several factors:

Velocity:

By increasing velocity, air resistance increases; e.g. the greater the velocity of the sprint cyclist around the velodrome track, the greater the force of air resistance opposing the motion

Shape:

The more aerodynamic the shape

the lower the air restistance. Many

sports use a tear-drop or aerofoil shape to minimise air resistance; e.g. the shape of a cyclist's helmet.

This concept is known as streamlining - the creation of smooth air flow around an

aerodynamic shape to minimise air

resistance

Frontal

Cross-Section

By decreasing the frontal cross-sectional area, air resistance decreases; e.g. the low, crouch point of giant slalom skiers in the straight

Smoothness of surface

By increasing the smoothness of the surface, air resistance decreases; for example the smooth Lycra suits of sprinters, cyclists and skiers