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The Laws of Motion Chapter 4

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Gravity Section 2

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**At this moment you are exerting an attractive force on everything around you**

This force is called gravity Anything with mass is attracted by gravity One of the four basic forces Electromagnetic force Strong nuclear force Weak nuclear force Gravity What is Gravity?

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GRAVITY is an attractive force between two objects that depends on the masses of objects and the distance between them. Increases with mass Increases as objects move closer Only Earth is close enough and large enough that you can feel its attraction What is Gravity?

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**The Law of Universal Gravitation**

Scientists wanted to be able to calculate the gravitational force between two objects Isaac Newton published the Law of Universal Gravitation in 1687. F = G m1 m2 F = gravitational force d2 G = Universal gravitation constant m1 = mass of object 1 m2= mass of object 2 d = distance between the two objects. The Law of Universal Gravitation

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**The Range of Gravity m2= mass of object 2**

Attraction rapidly decreases as two masses move away from each other. How rapidly? Let’s look at the equation… F = G m1 m2 F = gravitational force d2 G = Universal gravitation constant m1 = mass of object 1 m2= mass of object 2 d2 = distance between the two objects. The Range of Gravity

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**Earth’s Gravitational Acceleration**

If you dropped a bowling ball and a marble at the same time, which would hit the ground first? What if there were no air resistance? Acceleration due to gravity (g) is 9.8 m/s2 Earth’s gravitational force can be calculated: F = mg F= force (N) m= mass (kg) g = gravitational acceleration Earth’s Gravitational Acceleration

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**Gravitational force is acting on you whether you are falling or standing on the ground.**

Gravitational force acting on an object is weight. W = mg W = weight (N) m = mass (kg) g = gravitational acceleration Weight

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**Weight and Mass Weight and mass are not the same**

Weight is a force Mass is the amount of matter in an object Weight can change with gravitational force Mass will not change Weight and Mass

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**Weightlessness and Freefall**

Notice that object in space shuttles always seem to be floating They aren’t floating They are falling They are all falling at the same acceleration Let’s consider you on a scale in an elevator: What happens when you go up? What happens when you go down? Weightlessness and Freefall

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**Thrown objects do not always travel in straight lines**

Anything thrown or shot through the air are called projectiles. Gravity causes projectiles to follow a curved path Projectile Motion

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**Horizontal and Vertical Motions**

A thrown object has both a horizontal and vertical velocity The horizontal velocity (resulting from the applied force by the thrower) will be constant in the forward direction But an object thrown is being pulled by gravity. The object will have a vertical velocity resulting from earth’s gravitational pull The ball appears to curve The two motions are independent of each other Horizontal and Vertical Motions

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**Horizontal and Vertical Distance**

If I throw a ball and then drop a ball of the same mass at the same time, which will hit the ground first? They will hit at the same time!! Horizontal and Vertical Distance

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**When an object goes around a curve, it accelerates toward the center of the curve.**

Acceleration toward the center of a curve is called centripetal acceleration The net force exerted toward the center of the curve is the centripetal force Centripetal Force

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**Centripetal Force and Traction**

When a car rounds a corner, a force must act to keep the car moving in a curved path The centripetal force is the friction force (traction) between the tires and the road The road is slippery and the force is small, the force may not be large enough to keep the car following the curve Centripetal Force and Traction

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