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Course: Class 11 Physics (India)>Unit 9

Lesson 3: Newton's second law

Systems and Objects

Systems are collections of objects. Objects can be treated as if they have no internal structure. You can treat a system as an object if the internal structure is not relevant to the question. Created by David SantoPietro.

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• Am I missing something because the practice for this is giving us equations that were not covered? Did I miss something?
• I am very confused about how this course works. So far, I've watched the videos, then when we get to the questions, they require knowledge/equations that were not in the videos. Is there something else I'm supposed to be reading or watching as we go through the course? Every other KhanAcademy class I've done, the videos have all the information you need to solve the problems. For example, the problems after this video deal with sum of forces and tension, which hasn't been discussed anywhere up until this point.
• I feel the same way. In order to answer those questions, go to the AP Physics 1 formula sheet. You will use those formulas to answer the questions.
• What is Newton's law?
• Newton has 3 laws, so you can't refer to a specific 'Newton's Law'.

The first of Newton's Laws states that an object at rest will stay at rest unless some external force acts on it.

The second states that Force = (mass)(acceleration).

The third states that when two objects interact, they apply forces to each other of equal magnitude and opposite direction.

Hope this helps!
• So in the last example that he gave, why is the mass that he plugged in 2 kg? Wouldn't it be 1 kg since the box 1 is exerting the force on box 2? Confused.
(1 vote)
• No, it is 2 kg because we are finding the force that is exerted on the 2kg object by the 1 kg object. Therefore we are plugging in the second objects mass which is 2kg.
• The One thing that I don't understand with this whole problem is when David uses the acceleration equation to find the acceleration of the system as 3m/s^2 and then uses the same acceleration to find the internal force of the 2kg box. Wouldn't the acceleration be different for the 2kg box when the "internal" force is being taken than that of a "system "being the collection of objects as mentioned in the video?
(1 vote)
• Both boxes are in contact, and the applied force is transferred through the 1 kg box to the 2 kg box. Because they are connected, they must move together with the same acceleration. If they had different accelerations, they would separate or compress, which contradicts the problem's conditions.
The force applied to the 1 kg box results in an acceleration that is transmitted through the 1 kg box to the 2 kg box. The interaction force between the two boxes ensures that the 2 kg box experiences the same acceleration.

Hope this helps.
• Im confused because I thought calculating it is 9/ 2 = 4.5
How did he get 6 m/s^2
Thanks!
(1 vote)
• I don't see where he got 6 m/s^2. If you're referring to where he got F = 6N, then he simply did 3 m/s^2 * 2 kg = 6 N, which was the net force he was attempting to find.