Forces and motion

• Speed


  A scalar quantity that only has a magnitude (size)
• Velocity


  A vector quantity that has both a magnitude (size) and a direction
• When an object changes its velocity, it is accelerating
• Acceleration is a change in a vector quantity (velocity), so acceleration is also a vector
• Forces


  Vector quantities - it is important to know the direction in which a force is acting, as well as how big it is
• Forces can be represented by diagrams, where the size of the force is represented by the length of the arrows
• Thrust


  The upwards force from a rocket's engines
• Resultant force


  The overall force on an object, calculated by adding or subtracting the individual forces
• If the resultant of all the forces on an object is zero, we say the forces are balanced. If there is a non-zero resultant force on an object, the forces are unbalanced.
• Centripetal force


  The force that causes an object moving in a circle to change direction, acting towards the centre of the circle
• Mass


  The quantity of matter in an object, only changes if the object itself changes
• Weight


  A measure of the pull of gravity on an object, depends on the strength of gravity
• On Earth the gravitational field strength has a value of about 10 newtons per kilogram (N/kg). This means that each kilogram is pulled down with a force of 10 N.
• The weight of an object can be calculated using the equation: weight = mass x gravitational field strength
• On Earth, a falling object has a force of air resistance on it as well as its weight
• Acceleration


  A measure of how much an object's velocity changes in a certain time
• The acceleration in the direction of a resultant force depends on the size of the force and the mass of the object
• Inertial mass


  The force on an object divided by the acceleration that force produces
• Calculating an object's inertial mass from values of force and acceleration gives the same mass value as that found by measuring the force of gravity on it
• Newton's Third Law is about the forces on two different objects when they interact with each other. There is a pair of forces acting on the two interactive objects, called action-reaction forces. The two forces are always the same size and in opposite directions.
• Action-reaction forces are not the same as balanced forces. In both cases the sizes of the forces are equal and act in opposite directions, but action-reaction forces act on different objects, while balanced forces all act on the same object.
• Action-reaction forces


  • They are the same size and in opposite directions
  • They are the same type of force
• Action-reaction forces


  • Rope and dog exerting pulling forces on each other
  • Two gravitational forces
• Equilibrium situation


  Nothing is moving
• Force in the rope pulling on the dog


  Dog is also pulling on the rope
• Vertical forces in photo A

  • Weight of the dog pushing down on the ground
  • Force from the ground pushing up on the dog
• Action-reaction forces


  They act on different objects
• Balanced forces


  They all act on the same object
• Balanced forces acting on the dog

  • Force of ground on dog
  • Force of gravity on dog (weight)
  • Force from rope on dog
  • Force of friction on dog
• Newton's Third Law
• Collisions


  Action-reaction forces occur when things collide
• Ball bouncing off footballer's head


  Head exerts a force on the ball, ball also exerts a force on head
• Action and reaction forces during a collision are the same size, but they do not necessarily have the same effects on the two objects because the objects have different masses
• Player's head and the ball both change velocity during the collision


  Effects on the two objects are different
• Momentum is a measure of the tendency of an object to keep moving or how hard it is to stop it moving
• Momentum


  Depends on an object's mass and velocity
• Force, mass and acceleration

  F = ma
• Momentum and acceleration


  Force = change in momentum / time
• The largest ships are oil tankers and it can take several miles for a moving oil tanker to come to a stop
• Conservation of momentum


  The total momentum of both objects is the same before the collision as it is after the collision, as long as there are no external forces acting