Forces

• Vector quantity


  A physical quantity that has both magnitude and direction

  
• Scalar quantity


  A physical quantity that has magnitude only
• Resultant vector


  The single vector which has the same effect as the original vectors acting together
• Force


  An interaction between two objects which can cause a change in the state of motion of the objects
• Weight (Fg)


  The gravitational force the Earth exerts on any object on or near its surface
• Normal force (FN)


  The perpendicular force exerted by a surface on an object in contact with it
• Frictional force due to a surface (Ff)


  The force that opposes the motion of an object
• The order in which vectors are added does not affect the magnitude or direction of the resultant
• • The maximum resultant occurs when the vectors act in the same direction (0° to each other)
  • The minimum resultant occurs when the vectors act in opposite directions (180° to each other)
• Perpendicular components should not appear on a free-body diagram
• Resolving a vector into perpendicular components


  Vector F is resolved into component Fx and component Fy
• Types of forces


  • Contact forces: Frictional, Tensional, Normal reaction, Compression, Applied, Thrust
  • Non-contact forces: Gravitational, Electrostatic, Magnetic

  
• Newton's Third Law


  • Every force (the action force) has a "counter" force - the reaction force
  • Action-reaction pairs: equal in magnitude, act in opposite directions, act on different objects, occur simultaneously, act along the same line
• Weight (Fg)


  • The gravitational force the Earth exerts on any object on or near its surface
  • Calculated as Fg = m x g, where g is the gravitational field strength (approx 9.8 N/kg near Earth's surface)
• Normal force (FN)


  • The force exerted perpendicularly by a surface on an object in contact with it
  • Changes depending on the forces acting on the object

  
• Normal force scenarios


  • Object standing still on a flat surface
  • Object stationary on a surface with another vertical force
  • Object stationary against a wall
  • Object with an applied force on it
• Frictional force (due to a surface), Ff


  The force that opposes the motion of an object in contact with a surface and it acts parallel to the surface the object is in contact with
• Frictional force


  • When we increase the applied force, the static frictional force will also increase until it reaches a maximum value (Ffsmax). Past this point the object starts to move
  • When object is not yet moving Ffs = Fapplied up to Ffsmax
  • When object is moving Ffs = μfFN
  • Kinetic friction is less than static friction as the surfaces interact less when they are moving
  • The magnitude of the frictional force depends on the surfaces and the magnitude of the normal force on the object
• Ffsmax


  The maximum static friction force is proportional to the magnitude of the normal force
• To find components of weight we use trigonometry


  1. Fg⊥ = m.g.cosθ

  2. Fg// = m.g.sinθ
• The perpendicular force (on to the slope) is less than the weight if θ>0 and is equal in magnitude to the Normal force on the object from the slope
• The parallel force (down slope) is less than the weight if θ<90o and will try to cause the object to move down the plane. If the object remains still due to friction only then the magnitude of the friction is the same as this force
• At the moment it starts to move Ffsmax = Fg//
• What is the definition of weight (Fg)?

  The gravitational force the Earth exerts on any object on or near its surface.

  
• What is the normal force (FN)?

  The perpendicular force exerted by a surface on an object in contact with it.

  
• What is the frictional force due to a surface (Ff)?

  The force that opposes the motion of an object and acts parallel to the surface with which the object is in contact.

  
• What is inertia?


  The property of an object that causes it to resist a change in its state of rest or uniform motion.

  
• What does Newton's first law state?

  An object continues in a state of rest or uniform velocity unless it is acted upon by a net force.

  
• What does Newton's second law describe?

  When a net force is applied to an object of mass, it accelerates in the direction of the net force.

  
• What is the formula for Newton's second law?

  Fnet = ma
• What is one Newton defined as?

  The amount of force required to give a 1 kg mass an acceleration of 1 m/s².

  
• What are the two parts of Newton's first law?

  • Predicts the behavior of stationary objects.
  • Predicts the behavior of moving objects.

  
• How does inertia relate to the mass of an object?


  The mass of an object is dependent on its inertia.

  
• Give examples of real-life situations that demonstrate Newton's first law and inertia.


  • Getting tomato sauce out of a nearly empty bottle.
  • An object sliding on a seat when the car turns or stops.
  • Jerking in a bus or train when it starts or stops suddenly.
  • Tow ropes snapping when jerked.
• What should you consider when tailoring your answer to a question about Newton's first law?

  You should consider the external force applied, the object's current state, and the final effect on the object.

  
• What does Newton's third law state?

  When object A exerts a force on object B, object B simultaneously exerts an oppositely directed force of equal magnitude on object A.

  
• What are the properties of action-reaction pairs according to Newton's third law?

  • Equal in magnitude.
  • Act in opposite directions.
  • Act on different objects.
  • Occur simultaneously.
  • Act along the same line.
• What does Newton's second law imply about acceleration?

  The acceleration is directly proportional to the net force and inversely proportional to the mass.

  
• What is the unit of force in Newton's second law?

  kg·m·s⁻² or N.

  
• How do you calculate the net force acting on an object?

  Fnet = Σ Forces in the plane of motion.