Forces

• A force is an interaction that can cause a change in speed, direction or shape. Forces can be classified as Contact or Non-contact.
•  

  Friction - Felt when 2 objects move past each other – always against motion
•  

  Air Resistance - Caused by collision between a moving object and air molecules
•  

  Thrust - The force from an engine
•  

  Gravity - The force of attraction between 2 masses
•  

  Upthrust - Acts to stop objects sinking in a fluid
•  

  Magnetic - Acts between a magnet and magnetic material
•  

  Static - The force caused by positive or negative charge
•  

  Weight - Felt by an object because of gravity (mg)
•  

  Drag - A combination of forces acting against movement
•  

  Tension - The force in stretched objects, acting against the stretch.
•  

  Normal Contact Force (reaction) - Force created by contact with a solid. Always at 90o to the surface
• Types of non-contact forces are gravitational forces, electrostatic/electromagnetic forces and nuclear forces
• Tension - When a material is stretched, the molecular separation increases. This leads to short range attractive forces between the molecules pulling them back together. All objects subjected to a stretching force are in tension. Useful in engineering eg suspension bridge and tow ropes.
• Newton’s first law - This states that an object will remain in a state of rest or continue to move with a constant velocity unless acted upon by a resultant external force. When there are multiple forces acting on a body, with zero resultant force, the body is said to be in equilibrium.
• Free-body force diagrams - All forces can be considered to act through a single point, known as the centre of mass. This means that we can simplify force diagrams to show forces originating from this point.
• Resolving vectors:

  
• Newton's 2nd law of motion - The acceleration of a body is proportional to the resultant force causing its acceleration and is in the same direction. In other words Resultant force = mass x acceleration (ΣF = ma)
• Newton's third law - If body A exerts a force on body B, body B will exert an equal and opposite force on body A.
• The rules for Newton's third law are: Forces are acting on DIFFERENT BODIES, Forces are of the same type, Forces are of equal magnitude, Forces are acting in opposite directions, Forces act in the same line of action
• There are several different types of forces and forces fall into two categories: distant and contact forces
• A body is in equilibrium if, when acted on by a number of forces, the resultant force is zero
• The centre of gravity of an object is the point at which the weight of the object can be said to act
• Forces on free body diagram:

  
• Newton's second law of motion for fixed masses states that the acceleration of a body of constant mass is proportional to the resultant force applied to it and in the direction of the resultant force