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Chapter 3: Forces

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**Section 1 (part 1) - Introduction to Forces and Newton’s 2nd Law Of Motion**

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**Warm-up: How did we previously define the term “force”?**

Describe the relationship between forces and motion.

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**Learning Goals: Distinguish between the different types of forces.**

Define and apply Newton’s 2nd Law of Motion.

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**Types of Forces There are actually many types of forces including:**

Applied force Frictional force Gravitational force Normal force Thrust Drag Lift

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**Applied Force Objects are often touching A push or a pull on anything**

Contact force Example: A person picking up a box applies a force the the box in order to pick it up.

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**Frictional Force Works opposite of an object’s motion**

Takes away energy Works to slow down or stop objects Contact force Example: As cart moves along the road, friction slows the cart down.

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**Gravitational Force Pulls toward the center of the earth**

Non-contact force Relatively constant on Earth, regardless of location and size of the object Example: A stone thrown in the air will be pulled back down to the Earth’s surface.

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Normal Force Opposes gravity Contact force Usually upward Does not cause acceleration Usually equal and opposite to the applied force Example: The book sitting on the table has a normal force holding it up.

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**Thrust Rockets, planes, boats**

Something pushes backwards causing forward motion Gas, water, air pushed by engines, propellers, or explosions Example: An airplanes thrust is provided by its engines.

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**Drag Air resistance Type of friction Opposes the motion of an object**

Moving through water can also cause drag on a boat Example: As the airplane moves through the air the force of drag pushes back on the airplane.

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**Lift Opposes gravity Lift is usually “upward”**

Causes airplanes, hot air balloons to go up but not forward Example: The force of lift pushes the airplane up and thrust pushes it forward.

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**Newton’s Laws of Motion**

Sir Isaac Newton published his three laws of motion in his book Principia in 1687. Laws describe the effects of forces on the motion of objects.

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**Newton’s 1st Law of Motion (aka Inertia)**

An object moving at a constant velocity keeps moving at that velocity unless an unbalanced net force acts upon it. An object at rest will stay at rest unless an unbalanced force acts upon it.

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**Newton’s 2nd Law of Motion**

Newton’s 2nd Law of Motion: the acceleration of an object is in the same direction as the net force on the object. Also states that acceleration is equal to the net force divided by mass

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**Newton’s 2nd Law of Motion**

An object will have greater acceleration if a greater force is applied to it. Tossing vs. Throwing

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**Newton’s 2nd Law of Motion**

The mass of the object also affects acceleration. A softball’s mass is about 0.20 kg while a baseball’s mass is about 0.14 kg. If you throw both with the same force, the baseball has greater acceleration because it has less mass.

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**Using Newton’s 2nd Law F = ma Force = mass / acceleration**

F = force in Newtons (N) m = mass in kilograms (kg) a = acceleration in meters per second squared (m/s2)

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Formula Sheet F = ma m = F/a a = F/m

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Example Problem #1 Engineers must determine the net force needed for a rocket to achieve an acceleration of 70 m/s2. If the mass of the rocket is 45,000 kg, how much net force must the rocket develop?

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Example Problem #2 What is the mass of a truck if it produces a force of 14,000 N while accelerating at a rate of 5 m/s2 ?

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Example Problem #3 If the mass of a helicopter is 4,500 kg, and the net force on it is 18,000 N, what is the helicopter’s acceleration?

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