Newton\'s first law states that an object will move with a constant velocity if
ID: 1422613 • Letter: N
Question
Newton's first law states that an object will move with a constant velocity if nothing acts on it. This seems to contradict our everyday experience that all moving objects come to rest unless something acts on it to keep it going. Does our everyday experience contradict one of Newton's Laws? If it does not, explain the apparent contradiction. If it does, explain why we bother to teach Newton's first law anyway.
Newton's third law says that objects that touch each other exert forces on each other that satisfy:
If object A exerts a force on object B, then object B exerts a force back on object A and the two forces are equal and opposite
Consider the following three situations concerning two identical cars and a much heavier truck.
One car is parked and the other car crashes into it.
One car is parked and the truck crashes into it.
The truck is pushing the car, because the car's engine cannot start. The two are touching and the truck is speeding up.
In which of these situations do you think Newton's 3rd law holds or does not hold? Explain your reasons for saying so.
Newton's second law written in equation form states
a= FNET/m
Your roommate says “That's silly. Everyone knows it takes a force to keep something going at a constant velocity, even when there's no acceleration.” Do you agree with your roommate? If so, explain why physics classes bother to teach the law. If you disagree, how would you try to convince your roommate of the error of his/her ways?
4. Use Newton's Third Law to explain how a goldfish swims.
Explanation / Answer
Newton's first law states that an object will move with a constant velocity if nothing acts on it. This seems to contradict our everyday experience that all moving objects come to rest unless something acts on it to keep it going. Does our everyday experience contradict one of Newton's Laws? If it does not, explain the apparent contradiction. If it does, explain why we bother to teach Newton's first law anyway.
The first law seems to be contradictive with our everyday experience if we expect to see the idealized behavior of the objects in many physics problem to happen in the real world. As a Law, it is true for all the situations. For example, if we roll a ball on a plane surface, we can see that after a given time, the ball stops moving. This happens because there are forces commonly neglected in basic physics problems, such as the friction force of the air around the ball, and the drag force caused by the ball moving through a fluid (the air).
As in this example, what we could think is a contradiction to Newton's first law, is almost always a situation in which we are not taking into account some forces. Mainly the friction (or the loss of energy in form of heat) is one important cause that may make us think an object stopped moved without a force acting on it.
It all depends on the reference frame, and the considerations taken to study a particular situation.
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Newton's third law says that objects that touch each other exert forces on each other that satisfy:
If object A exerts a force on object B, then object B exerts a force back on object A and the two forces are equal and opposite
Consider the following three situations concerning two identical cars and a much heavier truck.
One car is parked and the other car crashes into it.
One car is parked and the truck crashes into it.
The truck is pushing the car, because the car's engine cannot start. The two are touching and the truck is speeding up.
In which of these situations do you think Newton's 3rd law holds or does not hold? Explain your reasons for saying so.
Newton's third law holds for all the three situations.
For the first one: when the second car crashes against the first, the force exerted by the second car is conteracted by many forces, including the friction of the first car's tyres with the ground, the forces required to deflect the materials of both cars, the impact force of the two masses in this non elastic collision, and so on. To sumarize, the cars are not the only bodies involved in this situation, but also the ground and the air are to be taken into account. The second car stops because the parked car exerts a force, because it is in contact with the ground, and there is friction.
It is the same for the second one, even if we perceive that the truck gets less damaged than the car, all the forces involved are counteracted, causing the truck to stop if the engine is not acceleratiog any more.
In the third situation, the engine of the truck generates a torque, wich is ultimately a force, exerted tangentially to a shaft, which is connected to the drivetrain of the truck, and goes through the tyres to the ground. The torque on the tyres is counteracted by the tractive force of the tyres with the ground, so the tyres roll, and the truck moves. The car is exerting a force to the truck, but if the engine develops a torque high enough to overcome that force, the truck will have a positive acceleration, so its speed will grow up.
We can see action - reaction forces at many different levels of detail on all the three situations. It always depends on how accurate we want our description to be.
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Newton's second law written in equation form states
a= FNET/m
Your roommate says “That's silly. Everyone knows it takes a force to keep something going at a constant velocity, even when there's no acceleration.” Do you agree with your roommate? If so, explain why physics classes bother to teach the law. If you disagree, how would you try to convince your roommate of the error of his/her ways?
I would disagree with my roommate. When an object is moving at constant velocity, and a force is applied, it is because that force is counteracting another force (or forces), so the net acceleration is zero.
To convince her, I would help her to determine the forces involved on any constant speed movement of an object, and write the static equillibrium equations for that object, so she can see that the acceleration is zero (or close to zero) only because the sum of the forces involved is zero.
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4.- A goldfish swims because its body (fins and tail) exerts forces on the water, so the water exerts forces on the fish, of the same magnitude and opposite direction. Since the friction and drag force between the fish and the water is much smaller that the forces exerted by the fish, the effect is that the fish moves in the direction opposite to the forces exerted by its tail.