Background Sir Isaac Newton (1643-1727) an English scientist and mathematician famous for his discovery of the law of gravity also discovered the three laws of motion. He published them in his book Philosophiae Naturalis Principia Mathematica (mathematic principles of natural philosophy) in 1687. Today these laws are known as Newton’s Laws of Motion.

Newton’s 1st Law An object at rest tends to stay at rest and an object in motion tends to stay in motion unless acted upon by an unbalanced force.

What does this mean? An object will “keep doing what it was doing” unless acted on by an unbalanced force. If the object was sitting still, it will remain stationary. If it was moving at a constant velocity, it will keep moving. It takes force to change the motion of an object.

Examples from Real Life A soccer ball is sitting at rest. It takes an unbalanced force of a kick to change its motion. Two teams playing tug of war are both exerting equal force on the rope in opposite directions. This balanced force results in no change of motion.

Law of Inertia Newton’s First Law is also called the Law of Inertia Inertia: the tendency of an object to resist changes in its state of motion.

Law of Inertia Not all objects have the same amount of inertia. The more mass an object has, the more inertia it has (and the harder it is to change its motion).

Law of Inertia

Why do objects want to behave this way? When pulling a tablecloth out from under a glass plate: The plate has a certain amount of inertia and wants to stay still It resists moving with the cloth and stays in the same spot on the table Tablecloth experiment

Why do objects want to behave this way? The car comes to a sudden stop Your body continues to move forward If you do not wear a seatbelt, you may experience harmful injuries.

If objects in motion tend to stay in motion, why don’t moving objects keep moving forever? There’s almost always an unbalanced force acting upon it. A book sliding across a table slows down and stops because of the force of friction. If you throw a ball upwards it will eventually slow down and fall because of the force of gravity.

In outer space, away from gravity and any sources of friction, a rocket ship launched with a certain speed and direction would keep going in that same direction and at that same speed forever.

Newton’s 2nd Law Force equals mass times acceleration. F = ma Acceleration: a measurement of how quickly an object is changing speed.

What does F=ma mean? The force of an object comes from its mass and its acceleration. Something very massive (high mass) that’s changing speed very slowly (low acceleration), like a glacier, can still have great force. Something very small (low mass) that’s changing speed very quickly (high acceleration), like a bullet, can still have a great force.

What is the relationship between acceleration, force, and mass? Acceleration and net force are directly proportional If net force ____ acceleration ____ Acceleration and mass are indirectly proportional If mass____ acceleration ____

How is acceleration calculated? F m a F m Acceleration = Units = m/s2 Net Force = Units = m * a N F a Mass = Units = kg

Example Problems F m a A speed boat pulls a water skier with a mass of 55Kg. The force causes the skier to accelerate at 2m/s2. Calculate the net force required to make the skier accelerate like this. F = m(a) = 2m/s2 (55kg) = 110 N

Example Problems F m a A rock is in free fall with an acceleration of 9.8m/s2 and a net force of 147N. What is the mass of the rock? M = F/a = 147 9.8m/s2 = 15kg

Example Problems F m a What is the acceleration of softball if it has a mass of 0.5 kg and hits the catcher's glove with a net force of 25 N? a = F/m = 25N .5kg = 50m/s2

Any Questions?

Newton’s 3rd Law For every action there is an equal and opposite reaction.

What are the parts of Newton’s third law? Forces always occur in pairs of 2 Action force: The one applied on purpose Reaction force: The resulting force The ball goes forward The girl’s hand stings

What are the parts of Newton’s third law? The 2 forces occur simultaneously. Girl hits ball Ball hits girl’s hand

What are the parts of Newton’s third law? The 2 forces are opposite in direction. The 2 forces are equal in strength. 100N 100N

What are the parts of Newton’s third law? Altogether: If one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction. 100N 100N

Examples of Newton’s 3rd law: When you jump off the back of a boat into water, you will push yourself forward into the water. The same force you used to push forward will push the boat backwards

Examples of Newton’s 3rd law: When you dive off of a diving board, you push down on the springboard. The board springs back and forces you up into the air.

Examples: Label the action and reactions

Why do the action-reaction forces not cancel out? We learned that equal and opposite forces are balanced and do not change motion However, there are equal and opposite forces in Newton’s third law and yet we see motion What is going on???? Reaction on hand Action on ball

Why do the action-reaction forces not cancel out? Action reaction forces do not cancel because they act on 2 different objects Reaction on hand Action on ball 100N Both acting on the ball

Any Questions?