Category: falling balls

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 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 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.

Does a bullet go from 0 to maximum speed instantly?

The bullet accelerates gradually, like everything else. However, the forces that push on the bullet when the gun is fired are extremely large and it accelerates extremely rapidly. It goes from 0 to maximum speed in about a thousandth of a second.

In what sense is the Space Shuttle falling toward the earth?

When the space shuttle circles the earth, it’s experiencing only one force: the force of gravity. As a result, it’s perpetually accelerating toward the earth’s center. If it weren’t moving initially, it would begin to descend faster and faster until…splat. But it is moving sideways initially at an enormous speed. While it accelerates downward, that acceleration merely deflects its sideways velocity slightly downward. Instead of heading off into space, it heads a little downward. But it never hits the earth’s surface. Instead, it arcs past the horizon and keeps accelerating toward the center of the earth. In short, it orbits the earth—constantly accelerating toward the earth but never getting there.

Why is force = mass * acceleration an exact relationship (i.e. why not force = 2 * mass * acceleration)?

The answer to this puzzle lies in the definition of force. How would you measure the amount of a force? Well, you would push on something with a known mass and see how much it accelerates! Thus this relationship (Newton’s second law) actually establishes the scale for measuring forces. If your second relationship were chosen as the standard, then all the forces in the universe would simply be redefined up by a factor of two! This redefinition wouldn’t harm anything but then Newton’s second law would have a clunky numerical constant in it. Naturally, the 2 is omitted in the official law.

Does air resistance affect a horizontally thrown ball?

Yes. A ball thrown horizontally gradually loses its downfield component of velocity. For that reason, you must throw a ball somewhat below the 45° angle from horizontal in order to make it travel as far as possible. Actually, the air has even more complicated effects on spinning balls.

Is it possible for a ball to fall to earth at a different angle from the one at which it rose?

If the ground is level and there were no air resistance, the answer would be no. The flight of the ball is perfectly symmetric. It rises to a maximum height in a parabolic arc and then returns to the ground as the continuation of that same parabolic arc.

However, if the ground isn’t level, then the angle it hits the ground at might be different. For example, if you toss a ball almost horizontally off a cliff, it will hit the ground almost vertically. Horizontal and vertical are two very different directions.

Air resistance also tends to slow a ball’s motion and it’s particularly effective at stopping the downfield component of its velocity. Gravity makes sure that the ball descends quickly, but there is no force to keep the ball moving downfield against air resistance. The result is that balls tend to drop more sharply toward the ground. When you hit a baseball into the outfield, it may leave your bat at a shallow angle but it will drop pretty vertically toward the person catching it.

Finally, if the ball is spinning, it can obtain special forces from the air called lift forces. These forces can deflect its path in complicated ways and are responsible for curve balls in baseball, slices and hooks in golf, and topspin effects in tennis.

Why on Pg. 6, 2nd full paragraph, it says the car is accelerating if the slope of the road changes but in the “not accelerating” list it says a bicycle going up a hill is not accelerating. Aren’t those the same situation?

Here is why the two situations are different:

In the first case, the car is traveling on a road with a changing slope. Because the road’s slope changes, the car’s direction of travel must change. Since velocity includes direction of travel, the car’s velocity must change. In short, the car must accelerate. Picture a hill that gradually becomes steeper and steeper—the car’s velocity changes from almost horizontal to almost vertical as the slope changes.

In the second case, the bicycle is climbing a smooth, straight hill at a steady speed. Since the hill is smooth and straight, its slope is not changing. Since the bicycle experiences no change in its direction of travel or its speed, it is traveling at a constant velocity and is not accelerating.

Doesn’t weight have resistance to acceleration?

No, weight measures a different characteristic of an object. Mass measures inertia (or equivalently resistance to acceleration). But weight is just the force that gravity exerts on an object. While an object that has great weight also has great mass and is therefore hard to accelerate, it’s not the weight that’s the problem. To illustrate this, imagine taking a golf ball to the surface of a neutron star, where it would weigh millions of pounds because of the incredibly intense gravity. That golf ball would still accelerate easily because its mass would be unchanged. Only its weight would be affected by the local gravity. Similarly, taking that golf ball to deep space would reduce its weight almost to zero, yet its mass would remain the same as always.