P8 - Forces in balance

• Displacement


  Distance in a certain direction
• Vector quantity


  Physical quantity that has both a size and a direction
• Scalar quantity


  Physical quantity that has size, but no specific direction
• Magnitude of a vector quantity is its size
• A vector quantity has magnitude and direction, a scalar quantity has magnitude only
• A vector quantity can be represented by an arrow, the direction of the arrow shows the direction of the vector quantity, and the length of the arrow represents the magnitude of the vector quantity

  
• When more than one force acts on an object, the forces on the object sometimes need to be shown on a scale diagram

  
• Scale diagram


  A diagram where a unit of distance represents a unit of the physical quantity
• Force diagrams usually show more than one force, and the forces on a diagram should always be clearly labelled to identify the force

  
• Force diagrams do not always need to be scale diagrams, but if a diagram is a scale diagram, the scale should be shown

  
• Contact force


  Force that occurs when two objects must touch each other to interact
• Non-contact force


  Force that does not require the objects to touch, e.g. magnetic, electrostatic, gravitational
• Newton's third law states that when two objects interact, they exert equal and opposite forces on each other
• The unit of force is the newton (N)
• The force of friction of the road on the tyre is in the forward direction, and the force of the friction of the tyre on the road is in the reverse direction. These two forces are equal and opposite to each other.
• Resultant force


  A single force that has the same effect as all the forces acting on an object
• If the resultant force on an object is zero, the forces acting on the object are balanced
• If the resultant force on an object is not zero, the forces acting on the object are not balanced, and the movement of the object depends on the size and direction of the resultant force

  
• If an object is acted on by two unequal forces acting in opposite directions, the resultant force is equal to the difference between the two forces, and in the direction of the larger force
• A free-body force diagram shows all the forces acting on an object
• The movement of the object depends on the size and direction of the resultant force
• Resultant force


  The difference between the two forces acting on an object in opposite directions
• If an object is acted on by two unequal forces acting in opposite directions, the resultant force is equal to the difference between the two forces and in the direction of the larger force
• Free-body force diagram


  • Shows the forces acting on an object without any other objects or other forces shown. Each force is shown by a vector, which is an arrow pointing in the direction of the force.

  
• The moment of a force is a measure of the turning effect of the force on an object
• Moment of a force

  M = F x d, where F is the force in newtons and d is the perpendicular distance from the line of action of the force to the pivot in metres
• Lever


  A force multiplier that can be used to raise one edge of a heavy object. The weight of the object is the load, and the force the person applies to the lever is the effort. The point about which the lever turns is the pivot.
• Gears are like levers because they can multiply the effect of a turning force
• Low gear


  A small gear wheel driven by the engine shaft is used to turn a large gear wheel on the output shaft, so the output shaft turns slower than the engine shaft. This gives a high turning effect.

  
• High gear


  A large gear wheel driven by the engine shaft is used to turn a small gear wheel on the output shaft, so the output shaft turns faster than the engine shaft. This gives a lower turning effect but a higher speed.

  
• Changing gears


  When a low gear is chosen: a small gear wheel driven by the engine shaft is used to turn a large gear wheel on the output shaft. So the output shaft turns slower than the engine shaft. The turning effect of the output shaft is greater than the turning effect of the engine shaft. Low gear gives low speed and a high turning effect.

  
• Changing gears


  When a high gear is chosen: a large gear wheel driven by the engine shaft is used to turn a small gear wheel on the output shaft. So the output shaft turns faster than the engine shaft, so the car can move at a higher speed. But the force of the smaller gear wheel acts nearer to its shaft than the force of the larger gear wheel acting on its shaft. So the turning effect of the output shaft is less than the turning effect of the engine shaft. High gear gives high speed and a low turning effect.

  
• The moment of the force on a gear wheel of radius 40mm should always be less than 4.8Nm.
• Anticlockwise moments


  The sum of the clockwise moments about a point
• All the forces acting on an object that don't pass through a fixed point can turn an object about that point
• Moment


  The direction of the force and the position of the fixed point determines whether the moment acts clockwise or anticlockwise
• Calculating the force needed to stop an object turning

  Know all the forces that don't act through the pivot and their perpendicular distances from the line of action to the pivot

  
• The parallelogram of forces is a geometrical method to find the resultant of two forces

  
• Parallelogram of forces


  • Adjacent sides represent the two forces and the angle between them is the same as the angle between the forces
  • The resultant force is the diagonal of the parallelogram from the origin of the two forces

  
• The resultant force is equal and opposite in direction to the vector representing the weight in the experiment with weights and pulleys