# Newton’s Second Law of Motion

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Newton’s Second Law of Motion
the acceleration of an object is dependent upon two variables - the net force acting upon the object and the mass of the object. Law of Acceleration, Force. and Mass

Acceleration, Force, and Mass
Newton determined that Acceleration is affected by Force directly and Mass inversely. So…. As mass increases (while under the same amount of force) acceleration would decrease. As force increases (while applied to the same mass) acceleration would also increase.

Check for Understanding
How can both of these objects under free fall conditions accelerate at the same rate? The elephant has more force applied to it. The boy has more force applied to him. The forces on the objects are the same.

Newton’s Third Law of Motion
Law of Action-Reaction

Newton’s Third Law of Motion
If one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object “For every action there is an equal but opposite reaction”

Action – Reaction Pairs
Paddle pushes on water, water back on paddle, person on kayak, kayak on person. Water on kayak, kayak on water. Rider on horse, horse on rider.

Do Action – Reaction Forces “Balance”?
Action – Reaction forces do not “balance” out because they are acting on 2 different objects – therefore, can cause a change in motion!

Detecting Motion You can’t always detect motion when paired forces are in action. If one of the pairs is much more massive, you will only see the less massive object move Example – When you push down on the Earth, you won’t see the Earth move, only you jumping in the air

Objects motion changes in the same direction as the force.
Unbalanced Objects motion changes in the same direction as the force. Fnet≠0 a≠0 Balanced Object remains at rest or uniform motion. Fnet=0 a=0

Summation Equations Forces are often represented using Free Body Diagrams so we can quickly determine if there is an unbalanced force present. Net force is the summation of all of the forces acting on an object. We often draw these forces using a Free Body Diagram. They take into account the magnitude and direction of the forces acting on an object. Free body diagrams “free” us from having to look at EVERY SINGLE interaction. We can focus on a single object and what is happening to it. Essentially within each Force vector is an action reaction pair.

Balanced or Unbalanced?
Use a summation equation to determine if forces are balanced or unbalanced. Fnet=f1+f2+f3+f4…. Would this change if I said that the box is accelerating to the right?

Check for Understanding
Write the summation equation for each of the following. Which has the greatest net force? Include summation equation written in variables. Then numbers. Which F is providing the unbalanced force if there is one.

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