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### Course: Mechanics (Essentials) - Class 11th>Unit 6

Lesson 3: Applying Newton's laws to a group of objects

# 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.

## Want to join the conversation?

• Am I missing something because the practice for this is giving us equations that were not covered? Did I miss something?
• Most of these equations are slight adaptations of equations I have learnt in 9th grade. So I don’t really understand why people keep commenting this. That being said, you could look up the equations or watch another khan academy video about them.
• 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.
• 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.