*balanced or unbalanced

• Forces


  an object not moving doesn't mean forces aren't acting to push, pull or twist it.
• Contact Force

  a rock's force of gravity is pulling it toward the centre of the earth (weight) but the rock doesn't actually move toward the centre of the earth because of the ground pushing up the rock.

  these two forces are balanced meaning they are equal in size and opposite in direction.

  this is called contact force.
• More than two forces acting

  when wind is blowing, the rock will be pushed from side to side but won't move sideways because the pushing force from the wind is balanced by friction between the rock and ground
• Forces acting on a Rock


  the longer the angles, the bigger the force

  
• Forcing action on a Rock when the wind is blowing


  when drawing a forces diagram with balanced forces, make sure the arrows are the same length and point in opposite directions

  
• Starting to Move

  the rock will move in the direction of the larger sideways force because the sideways forces are not balanced and the forces acting up and down are still balanced
• The forces are no longer balanced and the rock will start to move


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• Slowing Down


  unbalanced or unequal forces can also make moving objects slow down

  eg; when an object is falling quickly, the parachute causes a force of air resistance that is larger than the weight of the object
• How a parachute works

  When it first opens, the forces are unbalanced, which is what makes the object slow down

  When the object slows, the air resistance decreases, so the force become balanced again and the object falls at a constant speed due to gravity.

  
• Changing Direction


  unbalanced forces can also make objects change direction, eg; a tennis ball

  when it contacts the tennis racket, the ball pushes on the tennis racket. the hitting force must be larger than the force of the ball to make the ball go back in the opposite direction

  
• Gravity with objects in space

  The force of gravity is a constant, unbalanced force.

  When an object moves in a circle, its direction is always changing and a constant unbalanced force is needed to keep an object moving in a circle.
• Summary

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