Category: falling balls

The physics and science of falling balls

Why do you feel no acceleration in free fall, even though you are accelerating?

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Why do you feel no acceleration in free fall, even though you are accelerating?

This wonderful question has many answers. The first, and most direct, is that you do feel the acceleration. You feel an upward fictitious force (not a real force at all, but an effect of inertia) that exactly balances your downward weight. The feeling you experiences is “weightlessness.” That’s why your stomach feels so funny. You’re used to having it pulled downward by gravity but the effect of your fall is to make it feel weightless.

If you had an object in an empty sphere with a radius of a few miles, surrounded…

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If you had an object in an empty sphere with a radius of a few miles, surrounded by equally distributed and very concentrated mass, what effects of gravity would the object feel?

As long as the mass isn’t so concentrated that the laws of general relativity become important, the object won’t feel any gravity at all. The forces from opposite sides of the surrounding mass will cancel exactly. For example, if you were at the center of the earth in a large spherical opening, you would be perfectly weightless. The force from the north side of the earth would balance the force from the south side. This effect is quite remarkable and depends on the fact that gravity becomes weaker as the inverse square of the distance separating two objects. That way, even if you aren’t in the exact center of the earth, the forces still cancel.

Why does a ball fall 4.9 meters during its first second of falling?

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Why does a ball fall 4.9 meters during its first second of falling?

As a simple argument for that result, think about the ball’s speed as it falls: it starts from rest and, over the course of 1 second, it acquires a downward speed of 9.8 m/s. Its average speed during that first second is half of 9.8 m/s or 4.9 m/s. And that is just how far the ball falls in that first second: 4.9 m. By holding the ball 4.9 m above the floor, you can arranged for it to hit one second after you drop it.

What is deceleration?

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Are you accelerating when your speed decreases?

Yes! If you are walking east and you come to a stop, it is because you accelerated to the west! By "deceleration" we mean acceleration in the direction opposite our direction of motion. Thus in a car, when you stomp on the brake and decelerate, you are actually accelerating toward the rear of the car (in the direction opposite its direction of motion).

If you jump off of a diving board, are you exerting force on the board or is it …

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If you jump off of a diving board, are you exerting force on the board or is it exerting force on you?

Actually, as you stand on the end of the board or as you push off from its end, you are pushing on the board and it is pushing back on you. The forces you exert on one another are exactly equal in amount but opposite in direction. That observation is called Newton’s third law of motion and is the real meaning behind the phrase “for every action there is a reaction.”

Why does an object accelerate when it changes direction?

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Why does an object accelerate when it changes direction?

What you mean by “changes direction” is that the direction part of its velocity changes. For example, instead of heading east at 10 m/s (or 10 miles-per-hour, if that feels more comfortable), it heads north at 10 m/s (or 10 miles-per-hour). This change in direction involves acceleration. The car must accelerate toward the west in order to stop heading east, and it must accelerate toward the north in order to begin moving north. Actually, it probably does both at once, accelerating toward the northwest and shifting its direction of motion from eastward to northward.

As the Space Shuttle falls, does it accelerate forever and does it go faster and…

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As the Space Shuttle falls, does it accelerate forever and does it go faster and faster?

Yes to the first part, no to the second part. Remember that acceleration can change the direction of velocity without changing the magnitude of velocity (the speed of the object). When the space shuttle accelerates, its speed doesn’t change, only its direction of travel. Although it accelerates endlessly, it never goes faster or slower. Actually, if the shuttle’s orbit isn’t circular, its speed does increase and decrease slightly as it orbits the earth in an ellipse, but that’s an unimportant detail. For a circular orbit, the shuttle’s speed is constant but its velocity (speed and direction) is not constant!

If you shot a gun and dropped a bullet at the same time, how could they land at …

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If you shot a gun and dropped a bullet at the same time, how could they land at the same time? Wouldn’t the acceleration behind the bullet keep it in the air longer?

If you shot the bullet horizontally, it really would hit the ground at the same time as the bullet you simply dropped. During the firing, the bullet would accelerate like crazy, but only horizontally. It would leave the gun with a velocity that was only in the horizontal direction. With no forces pushing on it horizontally after that (we’ll neglect air resistance), the bullet will make steady progress downfield. But at the same time, it will begin to fall. The vertical component of its velocity will gradually increase in the downward direction as it falls. Like the dropped bullet, it will drift downward faster and faster and the two will hit the ground together.

Why is 45° above horizontal the ideal angle to throw something the greatest …

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Why is 45° above horizontal the ideal angle to throw something the greatest distance if gravity is acting on the vertical direction but not the horizontal?

The 45° angle is ideal because it gives the ball a reasonable upward component of velocity and also a reasonable downfield component of velocity. The upward component is important because it determines how long the ball will stay off the ground. The downfield component is important because it determines how quickly the ball will travel downfield. If you use too much of the ball’s velocity to send it upward, it will stay off the ground a long time but will travel downfield too slowly to take advantage of that time. If you use too much of the ball’s velocity to send it downfield, it will cover the horizontal distances quickly but will stay of the ground for too short a time to travel very far. Thus an equal balance between the two (achieved at 45°) leads to the best distance. Note that this discussion is only true in the absence of air resistance.