i- Forces and Motion

Force. (vector)

A force is a push or a pull, exerted by one object on another.

In SI force measured in or kg.m/s2

It has a direction and a magnitude so it is vector

Force (F) = mass (m) x acceleration (a)

Force depends mass and

  • Acceleration is directly proportional to force.
  • Acceleration is inversely proportional to mass.

The three laws of motion

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The resultant of two or more forces acting along the same line.

 Case 1 (the resultant force = zero)

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Balanced forces have the same effect as No force at all which means No acceleration is done.

1)   At rest….If an object is at rest
This object will never move unless an external force acting on it.

The total sum of the force in opposite directions will be = zero.

2)   At Motion with constant speed…..

This object will never stop, decrease its velocity or increase it velocity unless an external force acting on it (e.g. friction with air or ground, gravitational force or breaking system or increasing the thrust force.)

The total sum of the force in opposite directions will be = zero.

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The resultant of two or more forces acting along the same line.

 Case 2 (the resultant force not equal to zero = unbalanced forces)

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Unbalanced forces cause the body to be

1)   At acceleration.

2)   At deceleration.

Unbalanced force = Net Force = mass x acceleration

Fnet = m x a

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  • Finding Net Force A net force is the combination of all of the forces acting on an object.
  • If two forces are in the same direction, you add the forces to calculate the net force.
  • If the forces are in opposite directions, you must subtract the forces to find the net force.



  • If the net force on an object is 0 N, the forces on the object are said to be balanced.
  • But if the net force is not equal to 0 N, the forces on the objects are unbalanced.
  • Whether the forces are balanced or unbalanced determines whether the motion of an object changes.

Balanced Forces: No Change in Motion If the forces on an object are balanced, the motion of the object will not change.

  • The object will not increase or decrease in speed, or change direction.
  • If an object is standing still, it will remain so.
  • Increasing Speed with Net Force If the forces on an object are unbalanced, the motion of the object will change.
  • A net force on a nonmoving object will cause the object to move in the direction of the net force.
  • Decreasing Speed with Net Force A net force can also slow down an object that is already moving.

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