Tag: ramps

You said that if I push on a friend they will push back (even if they are asleep…

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You said that if I push on a friend they will push back (even if they are asleep). But if I push hard enough, they will fall to the ground, whereas I will not. Therefore, I don’t see how the reaction is equal. Can you please explain this? – JK

Newton’s third law only observes that the forces two objects exert on one another are equal in amount but opposite in direction. The law doesn’t make any statement about the consequences of those forces on the objects involved. Moreover, it doesn’t say that those forces are the only forces on the objects. When you push on an awake friend, your friend will obtain additional forces from the ground or a nearby wall, and will manage to avoid falling over. Even though you push your friend away from you, your friend will see to it that the ground pushes them toward you. As a result, they will probably stay in one place. But when your friend is asleep, they won’t be able obtain the additional forces necessary to compensate for the force you exert on them and they may accelerate away from you or fall over.

Without gravity in space, what would happen to the recoil if a gun were shot off…

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Without gravity in space, what would happen to the recoil if a gun were shot off? — DZ, Illinois

Even in the depths of space, so far from any planet that gravity is virtually absent, a gun will have its normal recoil. When you push on something, it pushes back on you just as hard as you push on it. That rule, known as Newton’s third law of motion, is as true in empty space as it is on earth. Thus when the gun pushes the bullet forward, the bullet pushes the gun backward equally hard and you feel the gun itself jump backward as result. This recoil effect is the very basis for rocket propulsion—the rocket pushes its exhaust backward and the exhaust pushes the rocket forward. That’s why rockets can work outside the earth’s atmosphere and away from any celestial objects—the rocket only has to push on its exhaust in order to obtain a push forward.

If Newton’s third law is true – then how can you move anything? If it exerts the…

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If Newton’s third law is true – then how can you move anything? If it exerts the exact same amount of force on you that you exert on it, wouldn’t the net force be zero and the object wouldn’t move?

The total force on the two of you (the object you’re pushing on and you yourself) would be zero, but the object would be experiencing a force and you would be experiencing a force. As a result, the object accelerates in one direction and you accelerate in the other! To see this, imaging standing on a frozen pond with a friend. If the two of you push on one another, you will both experience forces. You will push your friend away from you and your friend will push you in the opposite direction. You will both accelerate and begin to drift apart. Each of you individually will experience a net force. (It’s true that the two of you together will experience zero net force, which means that as a combined object, you won’t accelerate. The way this appears is that your overall center of mass won’t accelerate. It will remain in the middle of the pond even as the two of you travel apart toward opposite sides of the pond.)

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

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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…

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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…

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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…

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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…

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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.

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

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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.

When you drop a glass on a hard floor, why does it sometimes break and sometimes…

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When you drop a glass on a hard floor, why does it sometimes break and sometimes not?

When the glass hits the floor, the floor exerts all of its force on the part of the glass that actually touches the floor. That small part of the glass accelerates upward quickly and comes to rest. The remainder of the glass isn’t supported by the floor and continues downward. However the glass is relatively rigid and parts of it begin to exert forces on one another in order to stop the whole glass from bending. These internal forces can be enormous and they can rip the glass apart. Glass is a remarkable material; it never dents, it only breaks. As the glass tries to come to a stop, the internal forces may bend it significantly. It will either tolerate those bends and later return to its original shape or it will tear into pieces. Which of the two will occur depends critically on the precise locations and amounts of the forces. If the forces act on a defect on the glass’s surface, it will crack and tear and the glass is history. If the forces all act on strong parts of the glass, it may survive without damage.