Forces

• Vector 


  Magnitude and direction
• Scalar


  Just magnitude
• Examples of Vectors Vs scalars

  • Speed is a scalar whereas velocity is a vector
  • distance is a scalar whereas displacemnt is a vector
  • time is a scalar acceleration is a vector
  • mass is a scalar and force is a vector
  • Energy is a scalar and momentum is a vector
• Imaginr a car travelling round a roundabout at constant speed. While its speed is constant, its direction is constantly changing - so its velocity is constantly changing therefore it is accelerating
• Force


  Push or pull that acts on an object due to the interaction with another object. All forces between objects are either contact or non-contact
• Representing a vector quantity

  The direction of the arrow shows direction of the Vector

  Length represents the magnitude of the vector quantity
• Scale diagrams 


  When more than one force acts on an object, the forces on the object sometimes need to be shown on a sclae diagram
• Newtons third law of motion

  Newtons third law of motion states that when two objects interact with each other, they exert equal and opposite forces on each other (every action has its equal opposite reaction)
• Driving force 


  The driving force on a car is the force that makes it move. This is sometimes called the engine force or the motive force. This force pushes the car forward because there is friction between the ground and the tyre of each drive wheel. Friction acts where the tyre is in contact with the ground
• When a car moves forward 


  The force of the fiction of the road on the tyre is in the forward direction

  The force of the friction of the tyre on the road is in the reverse direction
• Newtons first law of motion

  If forces acting on each other are balanced the resultant force on the object is zero, and

  If the object is at rest, it stays stationary

  If the object is moving it keeps moving with the same speed and in the same direction
• Unbalanced forces 


  When the resultant force on an object is not zero, the forces acting on the object are not balanced. The movement of the object depends on the size and direction of the resultant force
• If an object is acred on by two unequal forces acting in opposite directions, the resultant force is

  Equal to the difference between the two forces

  In the direction of the larger force
• Example of a balanced force

  When a heavy crate is pushed along a rough floor at a constant speed without changing its direction, the push force on it is equal in size, and acting in the opposite direction, to the friction of the floor on the crate. Newtons first law states that the crate will continue moving with the same speed, and in the same direction
• Unbalanced forces 


  When a jet plane is taking off, the thrust force of its engines is greater than the force of air resistance (or drag) on it. The resultant force on the plane is the difference between the thrust force and the force of air resistance acting on it. The resultant force is therefore greater than zero
• Force diagrams 


  When an object is acted on by more than one force, you can draw a free-body force diagram to work out the resultant force on the object. A freebody force diagram shows the forces acing on an object without any other objects or other forces shown. Each force is represented on the diagram by a vector

  
• Centre of mass 


  The centre of mass of an object is the point at which its mass can be thought of as being concentrated
• Suspended equilibrium 


  If you suspend an object and then release it, it will sooner or later come to rest with its centre of mass directly below the point of suspension
• The centre of mass of a symmetrical object 


  For a flat object that is symmetrical, its centre of mass is along the axis of symmetry

  If the object has more than one axis of symmetry, its centre of mass is where the axis of symmetry meet
• Parallelogram of forces

  Take the 2 forces and draw a parallelogram and take two lines parallel to the forces then draw the resultant force in

  
• Key conditions for an object to be at equllibrium 


  The resultant force on the object is zero

  The forces acting on object have no overall turning effect
• To work out whether or not an object is in equilibrium

  If the lines of action of the forces are parallel, the sum of the forces in on direction must be equal to the sum of the forces in hte opposite direction. This means that resultant force on the object is zero

  If the lines of action of the forces are not parallel, the forces can be resolved into two components along the same perpendicualr lines. The component along each line must balance out if the resultant force is zero
• Distance time graphs 


  Stationary objects are - a horizontal line

  Moving at a constant speed, is a straight line that slopes upward
• The gradient of a distance-time graph represents its speed
• An object moving round in a circle has a direction of motion that changes continuously as it goes round. So its velocity is not constant even if its speed is constant
• An object that travels at constant velocity travels at a constant speed without changing it direction. So it travels in a straight line in a given direction. the word displacement is used instead of direction
• The acceleration of an object is its change of velocity per second
• A motion sensor linked to a computer cna be used to measure velocity changes
• the gradient of the line on a velocity-time graph represents acceleration
• if a velocity-time graph is a horizontal line, the acceleration is zero
• A positive gradient on a velocity-time graph represents acceleration and a negative gradient represents deceleration
• The area under a velocity-time graph represents the distance travelled
• The speed of an object moving at constant speed is given by the gradient of the line on its distance-time graph
• The speed, at any instant of time of an object moving at a changing speed is given by the gradient to the tangent of the line
• Newton's second law of motion

  Says that acceleration of an object is

  Proportional to the resultant force on the object

  Inversely proportional to the mass of the object
• The inertia of an object is its tendency to stay at rest or in uniform motion
• The weight of an object is the force acting on the object due to gravity. It mass is the quantity of matter in the object
• An object acted on only by gravity accelerates at about 10 m/s²
• The terminal velocity of an object is the velocity it eventually reaches when it is falling. The weight of the object is then equal to the frictional force on the object

  When an object is moving at terminal velocity, the resultant force on it is zero
• Friction and air resistance oppose the driving force of a vehicle

  The stopping distance of a vehicle depends on the thinking distance and the breaking distance

  High speed, poor weather conditions and poor vehicle maintenance all increase the breaking distance. Poor reaction time (due to tiredness, alcohol, drugs or using a mobile phone) and high speed both increase thinking distance