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4.17: Acceleration Due to Gravity

Difficulty Level: At Grade Created by: CK-12
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It’s a good thing this mountain climber’s safety gear is working, because it’s a long way down to the ground! If he were to fall, he’d be moving really fast by the time he got there. The higher any object starts falling from above Earth’s surface, the faster it’s traveling by the time it reaches the ground. Do you know why? The reason is gravity.  

Speeding up While Falling Down

Gravity is a force that pulls objects down toward the ground. When objects fall to the ground, gravity causes them to accelerate. Acceleration is a change in velocity, and velocity, in turn, is a measure of the speed and direction of motion. Gravity causes an object to fall toward the ground at a faster and faster velocity the longer the object falls. In fact, its velocity increases by 9.8 m/s2, so by 1 second after an object starts falling, its velocity is 9.8 m/s. By 2 seconds after it starts falling, its velocity is 19.6 m/s (9.8 m/s + 9.8 m/s), and so on. The acceleration of a falling object due to gravity is illustrated in the Fiugre below . For a more detailed explanation of acceleration due to gravity, watch this video:

http://www.youtube.com/watch?v=HRUhRjqzzDM

You can compare the acceleration due to gravity on Earth, the moon, and Mars with the interactive animation called “Freefall” at this URL:

http://jersey.uoregon.edu/vlab/

Q : In this diagram, the boy drops the object at time t= 0 s. By t = 1 s, the object is falling at a velocity of 9.8 m/s. What is its velocity by t = 5 s? What will its velocity be at t = 6 s if it keeps falling?

A : Its velocity at t = 5 s is 49.0 m/s, and at t = 6 s, it will be 58.8 m/s (49.0 m/s + 9.8 m/s).

Mass and Acceleration Due to Gravity

What if you were to drop a bowling ball and a soccer ball at the same time from the same distance above the ground? The bowling ball has greater mass than the basketball, so the pull of gravity on it is greater. Would it fall to the ground faster? No, the bowling ball and basketball would reach the ground at the same time. The reason? The more massive bowling ball is also harder to move because of its greater mass, so it ends up moving at the same acceleration as the soccer ball. This is true of all falling objects. They all accelerate at the same rate due to gravity, unless air resistance affects one object more than another. For example, a falling leaf is slowed down by air resistance more than a falling acorn because of the leaf’s greater surface area. You can simulate the effect of air resistance on acceleration due to gravity by doing the interactive animation at this URL: http://www.science-animations.com/support-files/freefall.swf

Q : If a leaf and an acorn were to fall to the ground in the absence of air (that is, in a vacuum), how would this affect their acceleration due to gravity?

A : They would both accelerate at the same rate and reach the ground at the same time.

Summary

  • When gravity pulls objects toward the ground, it always causes them to accelerate at a rate of 9.8 m/s2.
  • Regardless of differences in mass, all objects accelerate at the same rate due to gravity unless air resistance affects one more than another.

Vocabulary

  • acceleration : Measure of the change in velocity of a moving object.
  • gravity : As traditionally defined, force of attraction between things that have mass.

Practice

Watch the video at the following URL. Based on the teens interviewed in the video, identify two common misconceptions about gravity, mass, and acceleration due to gravity. Then explain why each misconception is false.

http://www.dnatube.com/video/11676/Misconceptions-About-Falling-Objects

Review

  1. Why do objects fall faster the longer they fall toward Earth?
  2. What is the rate of acceleration due to gravity?
  3. How does mass affect acceleration due to gravity?

Vocabulary

acceleration

acceleration

Measure of the change in velocity of a moving object.
gravity

gravity

As traditionally defined, force of attraction between things that have mass.

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Description

Difficulty Level:

At Grade

Grades:

7 , 8

Date Created:

Nov 01, 2012

Last Modified:

Nov 28, 2014
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