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Newton’s Second and Third LawsPowerPoint Presentation

Newton’s Second and Third Laws

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### Newton’s Second and Third Laws

Chapter 4 Section 3

Newton’s First Law

- From Newton’s 1st Law of Motion an object with balanced external forces acting on it is in a state of equilibrium.
- ΣF = 0
- No acceleration

- If the Forces are not balanced then there is a change in the motion of the object.
- ΣF ≠ 0
- Acceleration occurs

Acceleration and Force

- Acceleration is directly Proportional to the Force
- Acceleration ~ Force

- If the Force is increased, then the acceleration must increase by the same ratio as long as mass is held constant.

Force and Acceleration

- Acceleration is always in the direction of the net force.

Acceleration and Mass

- Acceleration is inversely proportional to the mass of the object.
- Acceleration ~ 1 / Mass

- If the mass increases, then the acceleration decreases as long as the force remains constant.
- If the mass is doubled, then the acceleration is cut in half.

Force, Mass and Acceleration

- The acceleration is directly proportional to the Force divided by the Mass
- Acceleration ~ Force / Mass

- This is where Newton’s 2nd Law is created from.

Newton’s 2nd Law of Motion

- Newton’s Second Law – The acceleration of an object is directly proportional to the net external force acting on the object and is inversely proportional to the mass of the object.
- ΣF = ma

Equation Variables and Units

- Newton’s Second Law variables
- Σ: Greek Letter Sigma meaning “The sum of”
- F: Force (Newton – N)
- m: Mass (Kilograms – kg)
- a: Acceleration (meters per second² - m/s²)

What is a Newton?

- A Newton is the amount of force needed to move a 1 kilogram mass at an acceleration of 1 meter per second squared.
F = ma

N = kg • m/s²

N=kgm/s²

Example Problem

- What force is needed to move a 3.2kg book across a table with an acceleration of 2.1 m/s² to the right?
- Answer: 6.7 N to the right

Solving Problems With Multiple Forces

- It is often easier to break the Newton’s 2nd Law into components.
- The sum of the forces in the x-direction equals the mass multiplied by the acceleration in the x-direction.
- ΣFx = max

- The sum of the forces in the y-direction equals the mass multiplied by the acceleration in the y-direction.
- ΣFy = may

- The sum of the forces in the x-direction equals the mass multiplied by the acceleration in the x-direction.

Net External Force equals Zero

- If the net external force is zero, then the acceleration is equal to zero regardless of how much mass is present.
- ΣF = ma
- ΣF = m • 0m/s²
- ΣF = 0

Newton’s 3rd Law

- Newton’s Third Law – If two bodies interact, the magnitude of the force exerted on object 1 by object 2 is equal to the magnitude of the force simultaneously exerted on object 2 by object 1, and these two forces are opposite in direction.
- For every action, there is an equal and opposite reaction.

Forces Always Exist in Pairs

- Forces always exist in pairs, therefore there can not be a single isolated force.
- If you push on a wall with 100N, the wall presses back on you with 100N.
- Equal and opposite, as long as there is no acceleration.

- If you push on a wall with 100N, the wall presses back on you with 100N.
- If Earth is pulling you down with a force equal to your weight, what is the second force?

Action-Reaction Pair

- Action-Reaction Pair – A pair of simultaneous equal but opposite forces resulting from the interaction of two objects.
- The action and reaction occur at the same exact time.

Field Forces

- Field Forces also exist in pairs as well.
- Field forces such as gravity and electromagnetism.

- If you drop a ball the earth pulls down on the ball, but the ball pulls up on the earth by the same amount.
- But why doesn’t the earth move and the ball does?

Example Problems #1

- The net external force on the propeller of a 0.75kg model airplane is 17N forward. What is the acceleration of the airplane?

Example Problem #1 Answer

- 23m/s² forward

Example Problem #2

- A ball pushed with a force of 13.5N accelerates at 6.5m/s² to the right. What is the mass of the ball?

Example Problem #3

- Two people push on a box resting on a frictionless floor. One person pushes to the left with a force of 17N and the other person pushed with a force of 37N to the right. If the mass of the box is 10kg, what is the acceleration of the box?

Example Problem #3 Answer

- 2 m/s2

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