Basic Inertia/Newton's Laws Questions

[Gilbert189]

Hello,
I am taking physics (freshman, high school) and I am very confused on the current topic. We are learning about Newton's first, second and third laws. Our teacher speaks very fast and I cannot seem to understand many topics that he talks about.

Questions I am confused on:
1. A plane is moving in a constant velocity. The air drag of the plane is 1800N. What is the net force?
2. My second question is an attached file.

If you have the time and you are solving this, please write out each step carefully when solving. I am completely lost. If you have the time, THANK YOU SO MUCH!

Thanks,
Gilbert

 

[Ray Vickson]

Hello,
I am taking physics (freshman, high school) and I am very confused on the current topic. We are learning about Newton's first, second and third laws. Our teacher speaks very fast and I cannot seem to understand many topics that he talks about.

Questions I am confused on:
1. A plane is moving in a constant velocity. The air drag of the plane is 1800N. What is the net force?
2. My second question is an attached file.

If you have the time and you are solving this, please write out each step carefully when solving. I am completely lost. If you have the time, THANK YOU SO MUCH!

Thanks,
Gilbert

We are not allowed to solve your problems for you---it is against PF rules. So, you need to do some work yourself on problems 1 and 2, and show us your work.

 

[Wanting to Learn]

A plane is moving in a constant velocity. ... What is the net force?

If it is moving at a constant velocity, what is the acceleration?

 

[CWatters]

Remember that in the equation F=ma that F is the net or total force.

 

[Gilbert189]

We are not allowed to solve your problems for you---it is against PF rules. So, you need to do some work yourself on problems 1 and 2, and show us your work.

Well... I am stuck on them. I will try my best I guess.

1. So for the first one I know that Net force is equal to all the forces acting on it. So Drag would be in the left direction if the plane was going right. Since it's a constant velocity, that means they balance out (I think!). So is it 1800N to the left?

2. I really don't know where to start for this one... All I know is that A=F/M and the force is 10 N to the right.

 

[Gilbert189]

If it is moving at a constant velocity, what is the acceleration?

The acceleration would be zero.

 

[haruspex]

Since it's a constant velocity, that means they balance out

Right, but what exactly is balancing out, and how does that relate to net force?

A=F/M

More precisely, F_net=ma.

 

[Gilbert189]

Right, but what exactly is balancing out, and how does that relate to net force?

More precisely, F_net=ma.

If the object is moving forward at 1800N and it is in constant velocity, that means that it should be in equilibrium, so that means there would be a 1800N thrust? I am so confused. If the plane is moving forward and isn't going anywhere, how is it moving? Or am I doing something wrong...

 

[haruspex]

that means there would be a 1800N thrust?

Yes. So what is the net force?

isn't going anywhere

It is going somewhere, at constant velocity.

 

[Gilbert189]

Yes. So what is the net force?

It is going somewhere, at constant velocity.

So the net force is 1800N East?

 

[haruspex]

So the net force is 1800N East?

What does 'net force' mean? What does the equation I gave in post #7 say?
What are the forces acting on the plane?

 

[Gilbert189]

What does 'net force' mean? What does the equation I gave in post #7 say?
What are the forces acting on the plane?

Net force is the sum of all the forces acting on an object.
The equation for net force is mass times acceleration.
I guess the grav. force and the lift balance the plane vertically, and we are given that there is 1800N drag.

 

[haruspex]

Net force is the sum of all the forces acting on an object.

Yes.

The equation for net force is mass times acceleration.

Right, and we know the acceleration is zero, so...?

I guess the grav. force and the lift balance the plane vertically, and we are given that there is 1800N drag.

You missed one, mentioned in post #54.

 

[Gilbert189]

Yes.

Right, and we know the acceleration is zero, so...?

You missed one, mentioned in post #54.

So if a is 0 then F=0, cause if you multiply something by 0 then it will be 0.

So in that case the plane is in dynamic equilibrium, so the thrust has to be 1800N?

So the net force is 0?

 

[haruspex]

So if a is 0 then F=0, cause if you multiply something by 0 then it will be 0.

So in that case the plane is in dynamic equilibrium, so the thrust has to be 1800N?

So the net force is 0?

Right.

 

[Gilbert189]

Could anyone help with the second question?

 

[haruspex]

Could anyone help with the second question?

Do you know how to draw Free Body Diagrams?

Do you understand what is meant by a system?
You are free to choose what assemblage of bodies constitutes a system. Typically you want them such that every mass in the system has the same acceleration. You can then ignore forces between parts of the system and just look at external forces which act on the system as a whole, and apply F_net=ma to this.
In problem 2, you can treat each block as a system and draw the FBD for it. There is an alternative. Either will work.

 

[CWatters]

+1

Imagine you are towing a car. If you want to calculate the tension in the tow rope using f=ma would you need to know the mass of every nut and bolt that makes up the car individually?

 

[Gilbert189]

Do you know how to draw Free Body Diagrams?

Do you understand what is meant by a system?
You are free to choose what assemblage of bodies constitutes a system. Typically you want them such that every mass in the system has the same acceleration. You can then ignore forces between parts of the system and just look at external forces which act on the system as a whole, and apply F_net=ma to this.
In problem 2, you can treat each block as a system and draw the FBD for it. There is an alternative. Either will work.

freebody diagram, kinda. is it where you draw a cartesian plane then draw arrows in different directions?

so what you mean is that i could solve this as one, or split it into two?

 

[Gilbert189]

+1

Imagine you are towing a car. If you want to calculate the tension in the tow rope using f=ma would you need to know the mass of every nut and bolt that makes up the car individually?

probably not? i guess just the mass of the car as a whole?

 

[haruspex]

freebody diagram, kinda. is it where you draw a cartesian plane then draw arrows in different directions?

You draw one rigid body and show all the forces that act on it. Do not show the reaction forces it exerts on other bodies - those would be shown on the FBDs for those bodies.
When you have that, you can write down the ΣF=ma equation(s) for the body.

i could solve this as one, or split it into two

That is the easiest way in this problem. The two blocks will clearly move as one, so treat them as a single rigid body.
I would guess that the question goes on to ask about the tension on the string connecting them. If so, you will need to consider one block in isolation.

 

[CWatters]

probably not? i guess just the mass of the car as a whole?

Yes that's the best approach for this question.

 

[Gilbert189]

You draw one rigid body and show all the forces that act on it. Do not show the reaction forces it exerts on other bodies - those would be shown on the FBDs for those bodies.
When you have that, you can write down the ΣF=ma equation(s) for the body.

That is the easiest way in this problem. The two blocks will clearly move as one, so treat them as a single rigid body.
I would guess that the question goes on to ask about the tension on the string connecting them. If so, you will need to consider one block in isolation.

The tension of a rope is the always the same on any side of a rope, right? Why is that?

 

[haruspex]

The tension of a rope is the always the same on any side of a rope, right? Why is that?

Apply ΣF=ma to the rope. If the rope is being pulled with different forces at each end and the rope is massless, what do you conclude?

 

[Gilbert189]

Apply ΣF=ma to the rope. If the rope is being pulled with different forces at each end and the rope is massless, what do you conclude?

Oh, right. So NetF is 0. Which means that on each side it is pulling the exact same? Does that have to do with Newton's third law?

 

[haruspex]

Oh, right. So NetF is 0. Which means that on each side it is pulling the exact same? Does that have to do with Newton's third law?

Second and third.
The third law tells you that the force the rope exerts on the object at one end equals the force the object exerts on it.
The second (combined with the zero mass) tells you that the forces exerted on the two ends are equal and opposite.
Tension is not exactly a force, more like a pair of equal and opposite forces.
Note that the tension is not uniform if the rope has mass and is accelerating.

 

[Gilbert189]

Second and third.
The third law tells you that the force the rope exerts on the object at one end equals the force the object exerts on it.
The second (combined with the zero mass) tells you that the forces exerted on the two ends are equal and opposite.
Tension is not exactly a force, more like a pair of equal and opposite forces.
Note that the tension is not uniform if the rope has mass and is accelerating.

Okay, so that means T1 is the same as T2.

 

[haruspex]

Okay, so that means T1 is the same as T2.

No, we were discussing the forces associated with one rope. T₁ and T₂ are the tensions in two different ropes.

 

[CWatters]

The problem doesn't actually ask you for T2 and you don't need to calculate it. Perhaps return to that after answering the question?