Newton’s Laws of Motion

Sir Isaac Newton ( )  an English scientist and mathematician famous for his discovery of the law of gravity also discovered the three laws of motion.  studied Galileo’s work,expanded on it and wrote PhilosophiaeNaturalis Principia Mathematica, short version – Principia.  in Principia, he defines mass and force and introduced his laws of motion

1 st Law of Motion  An object at rest remains at rest, and an object in motion remains in motion at constant speed and in a straight line unless acted on by an unbalanced force. AKA Law of Inertia

What does it all mean????  if an object is still, it will remain still until an unbalanced force pushes or pulls on it  if an object is already moving, it will continue moving until an unbalanced force slows, stops, or changes its direction  unbalanced force – force that results when the net force acting on an object in not equal to zero

Examples:  a soccer ball sitting still needs an unbalanced force, like a kick, to make it move  Two teams are playing tug of war. They are both exerting equal force on the rope in opposite directions. This balanced force results in no change of motion.

Inertia-tendency of an object to resist a change in it motion

Examples:  car suddenly stops but you continue forward..  this is why seat belts are sooo important  An adult and a small child are swinging in separate swings. Which person do you want to push or help to stop on those swings?  The child  The adult Why????????

2nd Law of Motion  The acceleration of an object depends on the mass of the object and the amount of force applied. Force = mass x acceleration F = ma

What does it mean?????  Force = mass x acceleration  F = ma  acceleration = Force/mass  a = F/m  mass = Force/acceleration  m = F/a

Now lets practice….  An automobile was a mass of 1,000 kilograms accelerates when the traffic light turns green. If the net force on the car is 4,000 Newtons, what is the car’s acceleration?  First, WHAT information is give??  Mass, m=1000 kg  Force, F = 4000 N (in a forward direction)  a = F/m  a = 4000 N/1000 kg  a = 4 m/s 2

More practice…….  An automobile with a mass of 1200 kg accelerates at a rate of 3.0 m/s 2 in a forward direction. What is the net force acting on the automobile?  A net force of 15 N is exerted on an encyclopedia to cause it to accelerate at a rate of 5 m/s 2. Determine the mass of the encyclopedia.

3 rd Law of Motion  Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first. In other words…. For every action there is an equal and opposite reaction.

What does it mean???  if a force is exerted, another force occurs that is equal in size and opposite in direction – no motion occurs