3.1a Newton's Laws and Forces

Key Terms

• Net Force - The overall force acting on a body, when all individual forces have been considered

• Balanced Force - When two or more forces acting in opposite directions are equal. The net force is 0

• Unbalanced Force - When two or more forces acting in opposite directions are not equal. The net force is either positive or negative

• Vertical Forces - Weight and reaction

• Horizontal Forces - Friction and air resistance

• Weight - the force on a body, exerted by the earth's attraction

  -  drawn from the centre of mass down

• Reaction - opposes weight, like in Newton's third law

• drawn upwards from every point in contact with the floor

• if weight > reaction, movement = downwards

• if weight < reaction, movement = upwards

• Air Resistance - the force acting in the opposite direction of the motion of a body travelling through the air

  • arrow is drawn from the centre of mass, backwards

• Friction - the force acting in the opposite direction to the movement of one surface over another

• the arrow is drawn forwards from all points in contact with the floor

• if friction > air resistance, movement = accelerating

• if friction < air resistance, movement = decelerating

• Momentum - the quantity of motion possessed by a moving body

  • Mass X Velocity

• Acceleration - the rate of change of velocity

• Force - a push or pull that alters the state of motion of a body

• Inertia - resistance of a body to a change in motion - will remain stationary or travelling at a consistent velocity

• Velocity - the rate of change of displacement

Newton's Laws Of Motion

Newton's First Law - The Law of Inertia

• An object will remain at a state of rest or uniform velocity unless acted upon by an external or unbalanced force

• A sprinter will remain in the starting blocks until a force acts upon it to overcome its inertia

• The greater the mass of the sprinter, the greater the force needed to accelerate out the blocks

Newton's Second Law - The Law of Acceleration

• A body's rate of change in motion is proportional to the size of the force applied, and acts in the same direction of the force applied

• A sprinter in the starting blocks will accelerate out of the blocks at a greater rate if they push off with a greater force (due to the greater change of momentum)

Newton's Third Law - The Law of Reaction

• For every action there is an equal and opposite reaction force

• The sprinter will apply the downwards and backwards force in the blocks, and the blocks will provide an equal and opposite upwards and forwards reaction force

Forces

• Separated into internal and external

• Internal = contraction of the skeletal muscle - the quadriceps will contract to cause the knee to extend

• External = outside the body - weight, friction, reaction, and air resistance

Force has 5 effects on the body:

• Create motion

  • a ball will remain at rest until a force is applied

  • a sprinter will remain still until a force is applied to cause them to move

• Accelerate a body

  • the greater the force of the foot kicking the ball, the greater the acceleration of the ball

  • the greater the force from a javelin thrower's shoulder, the greater the acceleration of the javelin

• Decelerate a body

  • as a ball moves through the air, air resistance will act the opposite way and slow it

  • air resistance will slow a javelin and decrease the distance it travels

• Force can change the direction of a body

  • when a goalkeeper's hand touches the ball, the force will cause the direction of the ball to change

• Force can change the shape of a body

  • a ball touches a net, and the force from the ball will change the shape of the net

A force can be generated and have no effect

Net Force

• the sum of all forces acting on a body - resultant force

• net force 0 = no change in motion - balanced forces

• if net force is present, the forces are unbalanced so there is a change in motion

  • two forces opposite in direction and unequal in size

Vertical Forces

Weight

• gravitational pull that the earth exerts on a body

• Weight = mass x acceleration due to gravity

• measured in Newtons

Reaction

• the equal and opposite force exerted in a body in response to the action force placed upon it

• result of Newton’s third law and is always present when two bodies are in contact

• measured in Newtons

Horizontal Forces

Friction

• opposes the motion of two surfaces in contact

• measured in Newtons

Air Resistance

Calculations

Velocity - Velocity = displacement/ time taken

• velocity = m/s

• displacement = m

• time = s

Momentum - Momentum = mass X velocity

• momentum = kgm/s

• mass = kg

• velocity = m/s

Acceleration - Acceleration = (final velocity - initial velocity)/ time taken

• acceleration = m/s/s

• change in velocity = m/s

• time = s

Force - Force = mass X acceleration

• force = N

• mass = kg

• acceleration = m/s/s

Free Body Diagrams

Weight = from CoM vertically downwards

Reaction = from point of contact directly upwards

Friction = from point of contact extending horizontally in the direction of motion

Air Resistance = from CoM horizontally in the direction opposing motion