If the downward motion of lifting a weight transfers energy to you, why does you…

If the downward motion of lifting a weight transfers energy to you, why does your arm get tired?

Your body is unable to store working that’s done on it and also wastes energy even when it is not doing any work. When you lower a weight, the weight does transfer energy to you, but your body turns that energy into thermal energy. You get a little bit hotter. If you were made out of rubber, you might store it as elastic potential energy (like a stretched rubber band). Instead, your muscles don’t save the energy in a useful form. As for getting tired, your muscles turn food energy into thermal energy even when you aren’t doing work. That’s what happens during isometric exercises. There’s nothing you can do about it. It’s like a car, which wastes energy when it’s stopped at a light.

Is it impossible to do work on a ball while carrying it horizontally, or were yo…

Is it impossible to do work on a ball while carrying it horizontally, or were you only referring to the force of gravity in the demonstration? Or must you be “pushing” the ball?

When I carried the ball horizontally at constant velocity, I did no work on the ball. That’s because the force I exerted on the ball was directly upward and the direction the ball moved was exactly horizontal. Since work is force times distance in the direction of that force, the work I did was exactly zero. But when I first started the ball moving horizontally, there was a brief period during which I had to push the ball forward horizontally. That’s when I “got the ball moving.” During that brief period, I did do work on the ball and I gave it kinetic energy. It needed that kinetic energy to move horizontally. When I reached my destination, there was a brief period during which I had to pull the ball backward horizontally. That’s when I “stopped the ball from moving.” During that brief period, I did negative work on the ball and removed its kinetic energy.

What forces are involved when a football player who is running is tackled by ano…

What forces are involved when a football player who is running is tackled by another player?

If the two players collide hard, they will both exert enormous forces on one another. The player running toward the right will experience a force to the left and will accelerate toward the left (slowing down). The player running toward the left will experience a force to the right and will accelerate toward the right (slowing down). The forces involved would cause bruises if they weren’t wearing pads. The pads reduce the magnitudes of the forces on their skin by prolonging the accelerations (smaller forces exerted for longer times). If one player simply trips up the other player, then the player who falls will still come to a stop. However, that player will be experiencing most of the stopping force from the ground by way of sliding friction.

What happens with things like liquids “falling” onto objects like sponges? Doe…

What happens with things like liquids “falling” onto objects like sponges? Does the sponge exert an upward force onto the liquid?

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 falling egg hits a table and breaks, did it fail to push equally on the t…

When a falling egg hits a table and breaks, did it fail to push equally on 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.

How do you push a shopping cart and have the cart exert the same force on you, i…

How do you push a shopping cart and have the cart exert the same force on you, if you are still traveling forward? Friction? Air Resistance?

When you push a shopping cart straight forward down an aisle, you are pushing it forward and it is pushing you backward. If nothing else were pushing on the two of you, the cart would accelerate forward and you would accelerate backward. But the cart is experiencing friction and air resistance, both of which tend to slow it down. They are pushing the cart backward (in the direction opposite its motion). So you must keep pushing it forward to ensure that it experiences zero net force and continues at constant forward velocity. As for you, you need a force to keep yourself heading forward; otherwise the cart’s backward force on you would slow you down. So you push backward on the ground with the soles of your shoes. In return, the ground pushes on you (using friction) and propels you forward. As a result, you also experience zero net force and move forward at constant velocity.

When a person bumps into something or has something dropped on them and a bruise…

When a person bumps into something or has something dropped on them and a bruise forms, does it form because of the object hitting the person or from the person exerting a force on the object to keep that object from pass through their skin?

The bruise forms because of the force exerted on the person by the object. When an object hits you, it’s obvious that the object pushes on you. But the object also pushes on you when you hit it. In fact, it’s a matter of perspective which is hitting which. To a person standing next to you when you’re hit by a ball, the ball hit you. To a person running along with the ball, you hit the ball. In each case, the ball pushes on you and gives you a bruise. You also push on the ball, causing it to accelerate away from you.

How does a surface know how hard it must push upward on an object to support tha…

How does a surface know how hard it must push upward on an object to support that object?

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.

Thus, if the sidewalk pushed upward too hard, the piano would rise upward and leave the sidewalk’s surface and if the sidewalk pushed upward too weakly, the piano would sink downward and enter the sidewalk’s surface. A balance is quickly reached where the sidewalk pushes upward just enough to keep the piano from accelerate either up or down.