Tag: ramps

If it takes less force to push something up a ramp, why doesn’t it also take les…

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If it takes less force to push something up a ramp, why doesn’t it also take less work?

When you lift an object using a ramp, the uphill force you exert on it is less than its weight but the distance you must travel along the ramp is more than if you simply lifted the object straight up. Since the work you do on the object is the product of the force you exert on it times the distance it travels in the direction of that force, the work isn’t changed by using the ramp. For example, if you lift a cart weighing 15 N straight up for 0.2 meters, you do 3 newton-meter or 3 joules of work on it. To raise that cart that same 0.2 meters upward on the ramp, you’d have to exert a 3 N force on it as you pushed it 1.0 meter along the ramp. The work you’d do to raise the cart by pushing it up the ramp would be 3 joules again. No matter how you raise the cart to the height of 0.2 meters, you’re going to do 3 joules of work on it.

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.