Forces Flashcards
What is a force
“…a push or pull that alters the state of motion of a body”
Force (N) = mass (kg) x acceleration (m/s/s) [f = ma]
Measured in (N)
What are internal and external forces
Internal forces are generated by the contraction of a skeletal muscle, for example; the force of the bicep brachii during the flexion of the elbow is caused by a concentric contraction. Internal forces can be understood via the lever systems
An external force comes from outside the body and is a force that acts upon the body. For example; weight, reaction, friction or air resistance.
What are the 5 effects of force on a body?
- Force can create motion – the football will remain on the penalty spot until a force is applied (NL1)
- Force can accelerate a body – the greater the force applied by the footballer’s foot to the ball, the greater the acceleration towards the goal
- Force can decelerate a body – when the football is travelling towards the goal, the force of air resistance will act in the opposite direction and cause the ball to slow down
- Force can change the direction of a body – as the goal keeper dives to save a high corner shot, he will apply a force from his hands to the ball, changing its direction and pushing the ball away from the goal
- Force can change the shape of a body – if the goal keeper fails to make the save, the force of the ball coming into contact wit the net will make the net change shape
What is a net force
The net force is the overall force acting upon a body when all the individual forces have been considered or the sum of all individual forces
What is a positive and negative net force
A positive net force will accelerate a body: Within the example of a sprinter the horizontal force of the sprinter towards the finish line (push) is greater than the horizontal force backwards acting on the athlete as well as the vertical forces.
A negative net force decelerate a body: A shuttlecock will decrease rapidly as air resistance acts in the opposite direction to motion and is greater than the acting force.
What is weight force
Gravitational pull that the earth exerts on a body and is measured in Newtons (N) - The force acts downwards from the centre of mass of the body.
The greater the mass/weight of the body, the greater the weight force pulling the body down closer to the earth
Acts downwards on the body from the centre of mass (make sure you start the line from the centre of mass – the centre of mass isn’t always on the inside of the body!)
Weight (N)= mass (kg) x acceleration due to gravity (m/s/s)
What is reaction force
Equal and opposite force exerted by a body in response to the action force placed upon it and is measured in Newtons (N).
Reaction is a result of Newtons Third Law of motion and is
always present when two bodies are in contact
Reaction forces are present at all points of contact
Normal reaction is shown by a vertical arrow extending upwards from the point of contact with the surface
What is friction
Friction is the force that opposes the motion of two surfaces in contact.
Measured in Newtons.
In certain sports friction is extremely important to the success of the performance.
Factors which affect friction are:
Roughness of the surfaces in contact
Temperature of the surfaces in contact
Size of normal reaction
What is air resistance
Air resistance is the force that opposes the motion of a body travelling through the air.
Also measured in Newtons.
It is affected by several factors:
Velocity of the body
Shape of the body
Frontal cross-sectional area
Smoothness of the surface
How does velocity affect air resistance
Increasing the velocity will increase the air resistance
The quicker an athlete is moving the greater the air resistance
E.g. a sprint cyclist will have a greater force of air resistance opposing their motion
This is why an aerodynamic body position and equipment is so vital within sprints over longer distances
How does shape affect air resistance
The more aerodynamic the shape the lower the air resistance
A race car will travel faster at the same speed than a lorry
A higher massed athlete has more air resistance than an athlete of lower mass
Many sports use a tear-drop or aerofoil shape to minimise air resistance – a cyclists helmet (known as streamlining)
Streamlining the creation of smooth air flow around an aerodynamic shape to minimise air resistance
How does smoothness affect air resistance
By increasing the smoothness of the surface, air resistance will decrease
Hence why skiers, cyclists and sprinters all wear lycra suits
How does frontal cross sectional area affect air resistance
To decrease air resistance you need to decrease the frontal cross-sectional area (a person stood standing face on will have more air resistance than a person stood to the side)
A lower body position will also assist in the decreasing of the frontal cross-sectional area
E.g. the crouched position of a slalom skiers in the straights
Free body diagrams
When we combine all vertical and horizontal forces on a body we need to consider the state of motion.
Free body diagrams allow us to do this.
The size and direction of the arrows must reflect the size and direction of the force acting on the body.
The arrows showing the size and direction of AR and W should originate from the ‘centre of mass’ also known as the ‘centre of ‘gravity’.
