They describe the motion of objects in terms of  their MASS and  the FORCES acting on them Mass – the amount of matter that an object contains a measure of the INERTIA of an object or its resistance to a change in motion. [metric unit of mass is gram, kilogram, centigram...] 1

 Force –a PUSH or a PULL– is always acting on an object even if the object is at rest or moving. [metric unit of force is Newton]  It can cause an object to start moving, stop moving or change direction. 2

Balanced Force Equal forces in opposite directions produce no change in motion

Unbalanced Forces Unequal opposing forces produce an unbalanced force causing a change in motion

To move an object at rest or change the motion of a moving object [i.e. accelerate], the forces acting on it must be  The object will then accelerate in the direction of the greater force UNBALANCED Net force is 0, so no motion Net force to the right +=+= + = 5

The forces are BALANCED 6

Fg and the push from the table balance each other. However, there is no force present to balance the force of friction. As the book moves to the right, friction acts to the left to slow the book down. This is an unbalanced force. 7

Equilibrium is a state of no change