1 st Law Law of Inertia

 An object at rest tends to stay at rest and an object in motion tends to stay in motion with a constant velocity and unless acted upon by an unbalanced force.

 Inertia can also be explained as an object’s resistance to change its motion.  The greater the mass of an object, the greater its inertia.

 You placed your book bag in the trunk of your car. Which direction will your book bag slide if you suddenly hit the gas pedal? Why?  Your book bag will slide backward because it was at rest initially and wanted to stay in that position even if the car moves forward.

 You placed a cake you bought on the passenger seat of your car. As you were driving down the street, a dog ran across the street in front of your car which caused you to slam on your breaks. What will happen to the cake? Why?  The cake will slide forward off the seat because it was initially moving forward and wanted to keep moving forward even if the car stopped.

 When all the forces acting upon an object balance each other, the object will be at equilibrium; it will not accelerate.  When an object experiences forces that do not balance each other, the object will accelerate.

 A force is a push or pull upon an object resulting from the object’s interaction with another object.

 Applied force (F app ) – a push or pull on the object by another object.  Force of gravity (F g ) - force with which the earth or other massive body attracts an object towards itself. It is the weight of the object.  Normal force (F N ) = support force exerted upon an object which is in contact with another stable object. Surface force

 Friction force (F f ) - force exerted by a surface as an object moves across it. It opposes the motion of the object.  Air resistance force (F air ) – force which acts upon objects as they travel through the air.  Tension force (F tens ) - force which is transmitted through a string, rope, or wire when it is pulled  Spring force (F spring ) - force exerted by a compressed or stretched spring upon any object

To be done on the white board.

 One Newton is the amount of force required to give a 1-kg mass and acceleration of 1 m/s 2.  Unit of measure: N or kg·m/s 2

 If the forces are acting in the same direction, you need to add the forces together.  If the forces are acting in opposite directions, you need to subtract the lesser force from the greater force.  If the forces are perpendicular to each other, use the Pythagorean Theorem.

 If net force is zero, the object is either at rest or moving at a constant velocity.  If the net force is not zero, the object is acceleration which means velocity is changing.

Practice Problems