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The physics a2 megathread

Reply 200

Clark20

9

Need help understanding cii.

Original post by Clark20

Need help understanding cii.

You can set the location of the centre of the Earth to be at the origin and the centre of the Moon to be at x = +60R_E.

The position where the vector sum of the gravitational field strength is zero when the gravitational field strength due to the Earth cancelled that of the gravitational field strength due to the Moon.

Let x₀ be the position where the gravitational field strength is zero and take direction to the right to be positive.

-\dfrac{G{{M}_{\text{E}}}}{x_{0}^{2}}+\dfrac{G{{M}_{\text{M}}}}{{{\left( 60{{R}_{\text{E}}}-{{x}_{0}} \right)}^{2}}}=0

Solve for x₀.

Reply 202

Clark20

9

Original post by Eimmanuel

You can set the location of the centre of the Earth to be at the origin and the centre of the Moon to be at x = +60R_E.

The position where the vector sum of the gravitational field strength is zero when the gravitational field strength due to the Earth cancelled that of the gravitational field strength due to the Moon.

Let x₀ be the position where the gravitational field strength is zero and take direction to the right to be positive.

-\dfrac{G{{M}_{\text{E}}}}{x_{0}^{2}}+\dfrac{G{{M}_{\text{M}}}}{{{\left( 60{{R}_{\text{E}}}-{{x}_{0}} \right)}^{2}}}=0

Solve for x₀.

Thanks. Makes sense.

Reply 203

Clark20

9

Is elastic restoring force equal to resultant force in every case? Is elastic restoring force and resultant force the same?

Since here too, if you assume you let go of the trolley after displacing it and calculate the net force at the moment it starts to move, the net force is equal to 2kx.

Original post by Clark20

Is elastic restoring force equal to resultant force in every case? Is elastic restoring force and resultant force the same?

Since here too, if you assume you let go of the trolley after displacing it and calculate the net force at the moment it starts to move, the net force is equal to 2kx.

Is elastic restoring force equal to resultant force in every case?

No. In A level Physics, you should avoid generalizing the situation. If the spring is in the vertical position and a mass is displaced vertically, the resultant force in the vertical direction is the vector sum of the weight and the spring force (elastic restoring force). Although the vector sum of the weight and the spring force (elastic restoring force) can be simplified to the spring force by “adjusting” the equilibrium position, this does not imply that at the “new” equilibrium” position, the spring force is zero.

Is elastic restoring force and resultant force the same?

Technically, they meant different things. Elastic restoring force usually means spring force and is a real force.
Resultant force is the vector sum of all the real forces such as normal force, weight, frictional force, spring force, etc.

If a single real force such as spring force is equal to the resultant force, we can say that spring force is equal to resultant force, they can do the same thing but does not imply that they are the same.

Original post by Clark20

… Since here too, if you assume you let go of the trolley after displacing it and calculate the net force at the moment it starts to move, the net force is equal to 2kx.

Not sure what are you trying to ask here.

Reply 205

Clark20

9

Original post by Eimmanuel

No. In A level Physics, you should avoid generalizing the situation. If the spring is in the vertical position and a mass is displaced vertically, the resultant force in the vertical direction is the vector sum of the weight and the spring force (elastic restoring force). Although the vector sum of the weight and the spring force (elastic restoring force) can be simplified to the spring force by “adjusting” the equilibrium position, this does not imply that at the “new” equilibrium” position, the spring force is zero.

Technically, they meant different things. Elastic restoring force usually means spring force and is a real force.
Resultant force is the vector sum of all the real forces such as normal force, weight, frictional force, spring force, etc.

If a single real force such as spring force is equal to the resultant force, we can say that spring force is equal to resultant force, they can do the same thing but does not imply that they are the same.

Not sure what are you trying to ask here.

Alright.

Can you please explain how the elastic restoring force is calculated here in this question?

Original post by Clark20

Alright.

Can you please explain how the elastic restoring force is calculated here in this question?

It is (vector) sum of the 2 spring forces exerting on the trolley.

Reply 207

Clark20

9

Original post by Eimmanuel

It is (vector) sum of the 2 spring forces exerting on the trolley.

Okay. That's true and is coming out as equal to 2kx as it should. But, why is the restoring force determined that way? The vector sum is Net force. I dont see any difference between resultant force and restoring force.

Is there any particular way to determine the restoring force acting on an object undergoing SHM?

(edited 4 years ago)

Reply 208

Clark20

9

Need help understanding one thing here.

We calculate gain of an op amp amplifier using gain = output voltage ÷ input voltage

And input voltage = Voltage in non inverting terminal - Voltage in inverting terminal.

But input voltage used here is only through the non inverting terminal (across resistor R). Why isnt the voltage across X (voltage through the inverting terminal) subtracted from it?

Thank you very much.

(edited 4 years ago)

Original post by Clark20

Okay. That's true and is coming out as equal to 2kx as it should. But, why is the restoring force determined that way?

Again, I don’t really know what you are trying to ask. Your question (to me) is like as followed. You are taught of adding two 2 real numbers and then ask why the real number is determined that way.

There are 2 springs in the problem and the two springs are stretched when the trolley is displaced. This means that there are 2 spring forces exerting on the trolley. To find the total spring force or restoring force is adding the 2 spring (or restoring) forces vectorially. If there are 3 forces, you add 3 forces vectorially, if there are 4 forces, you add 4 forces vectorially. Because forces are vector quantity and they follow rules of vector operations. To me, this a circular reasoning.

Original post by Clark20

… The vector sum is Net force. I dont see any difference between resultant force and restoring force.

Explain more on what you mean that I don’t see any difference between resultant force and restoring force. Don’t expect people to know what you mean without giving more info.

As I already mention in post # 205 (read it again and slowly) about the differences between resultant force and restoring force. Resultant force exerting on an object is the vector sum of all the real forces exerting on the object.

The mathematical statement of equality does not mean they are the same.

In this problem, there are 2 spring (or restoring) forces exerting on the trolley along the horizontal direction, so the resultant force along the horizontal direction is just the vector sum of the 2 spring (or restoring) forces.

Original post by Clark20

… Is there any particular way to determine the restoring force acting on an object undergoing SHM?

It is again the vector sum of all the forces exerting on the object. “Every” so-called "special" way originates from the fundamental (or systematic) way of doing physics.

Reply 210

Clark20

9

Original post by Eimmanuel

Explain more on what you mean that I don’t see any difference between resultant force and restoring force. Don’t expect people to know what you mean without giving more info.

Actually, I meant to confirm, is the restoring force acting on an object in SHM (in every case of SHM) MATHEMATICALLY equal to the net force acting on it?

Asking of this because there are cases in SHM where no springs are involved. So, if asked to determine the restoring force acting on an object at a specific moment during its oscillations, we can simply determine the net force acting on it at that specific moment? Yes, I understand that only mathematically can these 2 forces be equal. I understand there is a different meaning to both 'restoring force' and 'net force'. They are not the same.

Reply 211

Clark20

9

I have this one confusion in the question attached above, of the topic "Electronics". It isnt related with the solution to the answer to it.

We know that Vin (input voltage) is determined by subtracting V- from V+.

In the question, why do they take the potential V+ directly as input voltage when V- isnt earthed? Shouldnt the voltage V- be equal to the voltage across the 1.5 kilo-ohm resistor? (Or the voltage across resistor R?). And hence have a value, to be subtracted from V+ to determine the input voltage?

(edited 4 years ago)

Original post by Clark20

Actually, I meant to confirm, is the restoring force acting on an object in SHM (in every case of SHM) MATHEMATICALLY equal to the net force acting on it? ….

This question is very different from the question that you raised previously.

Any objects undergoing SHM must act by a net force that must be a restoring force.

Original post by Clark20

….Asking of this because there are cases in SHM where no springs are involved. So, if asked to determine the restoring force acting on an object at a specific moment during its oscillations, we can simply determine the net force acting on it at that specific moment?

Yes.

Reply 213

Clark20

9

Original post by Eimmanuel

This question is very different from the question that you raised previously.

Any objects undergoing SHM must act by a net force that must be a restoring force.

Yes.