What happens with things like liquids "falling" onto objects like sponges? Does the sponge exert an upward force onto the liquid? Also, if Newton's third law is universal, why can people punch through walls? Is this because object have a "maximum" force they can exert?

When liquids fall onto sponges, the sponges do exert upward forces on the liquids. Otherwise, the liquids would continue to fall. When a raindrop hits your hair, you can feel it push on your hair and your hair pushes back, stopping the raindrop's descent. When a person punches through a wall, their hand is moving so quickly that the wall isn't able to stop it in time. The wall exerts a large stopping force on the hand and, in doing so, the wall deforms until it breaks. The hand slows somewhat, but doesn't completely stop. It travels through the wall. Nonetheless, there was never a moment where the force of the hand on the wall wasn't exactly equal to but oppositely directed from the force of the wall on the hand.

In the situation with the falling egg hitting the table, you said that the table pushes on the egg with an upward force of 1000 N while the egg hits the table with a downward force of 1 N. That seems to conflict with Newton's third law. What is going on?

As the egg falls, it is experiencing only one force: a downward weight of 1 N. But when it hits the table, it suddenly experiences a second force: an upward support force of 1000 N. The net force on it is then 999 N, because the upward 1 N force partially cancels the downward 1000 N force. But the force that the egg exerts on the table is not 1 N. It's 1000 N downward. The egg and table push on one another equally hard. The table doesn't move much in response to this large downward force because it's so massive and because it's resting on the floor. But if you were to push you hand under the falling egg, you would feel the egg push hard against your hand as it hit.

On page 22 of the packet you state: "If the sidewalk were exerting too much upward force on the piano, the piano would accelerate upward and move away from the sidewalk." Do you mean to say that if the force the sidewalk was exerting was too strong the piano would float above the sidewalk? And if so, what happened to Newton's 3rd law? Wouldn't there have to be an opposing force to that of the sidewalk that would be equal but in the opposite direction therefore canceling out the force that it is exerting? Wouldn't that disprove Newton's third law? (Since the opposing force, in this case gravity, would not be equal to that of the sidewalk if the sidewalk were able to push the piano away.)

If you put a piano on the sidewalk, the piano will settle into the sidewalk, squeezing the sidewalk's surface until the sidewalk stops it from descending. At that point, the sidewalk will be pushing upward on the piano with a force exactly equal in magnitude to the piano's downward weight. The piano will experience zero net force and will not accelerate. It's stationary and will remain that way.

But if the sidewalk were to exert a little more force on the piano, perhaps because an animal under the sidewalk was pushing the sidewalk upward, the piano would no longer be experiencing zero net force. It would now experience an upward net force and would accelerate upward. The piano would soon rise above the sidewalk. Of course, once it lost contact with the sidewalk, it would begin to fall and would quickly return to the sidewalk.

For an example of this whole effect, put a coin on a book. Hold the book in your hand. The book is now supporting the coin with an upward force exactly equal to the coin's weight. Now hit the book from beneath so that it pushes upward extra hard on the coin. The coin will accelerate upward and leap into the air. As soon as it loses contact with the book, it will begin to fall back down.

Why do you talk about the Big Bang as if its a proven thing. You can't say that energy came from the big bang (and can't be created or destroyed) and be true. It makes me frustrated to be in a classroom where you say things that aren't a fact.

At present, all evidence point to a time somewhere between 10 and 20 billion years ago when the universe had an extremely small size and an extremely high temperature. Direct observation of black body radiation filling the universe indicates a time when the whole universe was hot enough that atoms were unstable and indirect observations of nuclear abundances indicate a time when the whole universe was hot enough that nuclei were unstable. While there are some details to resolve, most cosmologists believe that there was a big bang. Even if you want to doubt the big bang, the conservation of energy isn't in any doubt. The energy that's here now has been around at least 10 billion years and in all likelihood dates back all the way to the very creation of the universe in a big bang.

Since the egg shell broke, did it fail to push equally against the table?

No. It pushed hard against the table and the table pushed hard against it. The forces exerted were exactly equal but in exactly the opposite directions. Each object experienced a strong push from the other object. But as they say, "whether the rock hits the pitcher or the pitcher hits the rock, it's bound to bad for the pitcher." The egg couldn't take the push and it broke.

With the falling egg, if it weighs 1 N, is it exerting a 1 N force on the table, or 1000 N, like the table exerts up? How does Newton's third law fit in?

During the impact, the two objects are exerting 1000 N forces on one another, as required by Newton's third law. The 1 N weight is experienced only by the egg, throughout the activity, and it has very little effect during the collision.

If the space shuttle falls continuously, does it keep accelerating infinitely? 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!

How can there be no acceleration up. If you shoot a bullet up in the air, the moment that the bullet is shot, there must be acceleration upwards. The bullet went from 0 m/s to whatever, so there must be acceleration up. Yes or no? Also, does the bullet go from 0 to maximum speed instantly. If not, then isn't it accelerating upward?

When you shoot a bullet upward, is does accelerate as long as it's in the gun. The burning gases push upward on the bullet and it accelerates upward. But as soon as it leaves the gun, it's a falling object, with the only force on it being gravity (we'll ignore air resistance, which is also present). When I state that there is no acceleration upward, I am focusing on the time when the object is falling, which is after the event that started it on its upward path. As for whether the bullet reaches maximum speed instantly, the answer is no. It accelerates gradually, like everything else. However, the forces that push on the bullet are extremely large and it accelerates extremely rapidly. It goes from 0 to maximum speed in about a thousandth of a second.

When you drop an egg into a pile of feathers, why doesn't that break the egg? Isn't the pile of feathers exerting the same force up (about 1000 N) as the table was?

The egg doesn't break because the feathers exert a much smaller force on the egg than the table did. The feathers can move so when the egg first hits them, the feathers don't have to stop the egg so quickly. To keep the egg from penetrating into the table, the table had to stop the egg's descent in about a thousandth of a second. That required a huge upward force on the egg of perhaps 1000 N. It was this large upward force, exerted on one small point of the egg, that broke the egg. But when the egg hits the feathers, the feathers can stop the egg's descent leisurely in about a tenth of a second. They only have to push upward on the egg with a smaller force of perhaps 10 N. This modest force, exerted on many points of the egg, shouldn't break the egg. During this tenth of a second, the feathers and the egg will both move downward and the egg will come to a stop well below the place at which it first touched the feathers.

In "magnitude" the quantity is of what: the object or the force on it?

Magnitude is simply the amount part of a vector quantity, as opposed to the direction part of a vector. You can talk about the magnitude of a force or the magnitude of a velocity or the magnitude of an acceleration. In each case, the magnitude is a number and a unit. For example, if you weight 500 N (500 newtons), then the magnitude of your weight (a vector) is 500 newtons and the direction of your weight is downward.

How can the ground exert a force on a car's tires? Where does the upward force come from and why don't we experience any horizontal forces?

The horizontal force on the car's tires is due to friction, a topic that we will discuss in the section on wheels. Without friction, the car's tires would only experience an upward force. While the car wouldn't fall into the earth, it couldn't propel itself forward. When you stand on level ground, the support force on your feet is strictly upward. If it weren't for friction, you wouldn't be able to get anywhere either. But when you try to slide you feet sideways along the ground, it resists your motion. You and the ground push against one another through this frictional effect and you both accelerate. The ground and earth accelerate relatively little because they have enormous masses. You accelerate much more easily and are soon moving horizontally at a good speed.

Why would slowing down be acceleration instead of deceleration? Is it just because the velocity is changing?

Deceleration is just a special case of acceleration. It's acceleration in the direction opposite your velocity. When you decelerate in a car by stepping on the brake, the car accelerates backwards and its velocity goes from being forward and large to forward and small and then finally to being zero.

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 extremely symmetrical. It rises to a maximum height in a parabolic arc and then returns to the ground as the continuation of that same parabolic arc. 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 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.