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# M1 modelling assumptions watch

1. hi for part c of this question, I thought that the string is modelled as a light string since the string does not have any mass however the only answer is that the string is inextensible. But surely it's also light right ?

Also for part e the last part , I don't get why fr would also decreed the reaction force. Fr does not act veritically . It is parallel to the plane. I thought adding fr would only decrease the weight but I don't get why it decrease R too..... Thanks.
2. (Original post by coconut64)
hi for part c of this question, I thought that the string is modelled as a light string since the string does not have any mass however the only answer is that the string is inextensible. But surely it's also light right
Yes, the string would need to be light too.

Also for part e the last part , I don't get why fr would also decreed the reaction force. Fr does not act veritically . It is parallel to the plane. I thought adding fr would only decrease the weight but I don't get why it decrease R too..... Thanks.
Perpendicular (to plane) component of Reaction = mg cos 25

Since m decreases, so does the perpendicular component of the reaction.

Component parallel to the plane remains the same.

So, overall, the magnitude of the reaction decreases.

What paper is this from? Is it a real exam one?
3. (Original post by ghostwalker)
Yes, the string would need to be light too.

Perpendicular (to plane) component of Reaction = mg cos 25

Since m decreases, so does the perpendicular component of the reaction.

Component parallel to the plane remains the same.

So, overall, the magnitude of the reaction decreases.

What paper is this from? Is it a real exam one?
Oh okay. Is this right -light string =tension is constant on string. Smooth pulley=no friction on pulley. Inextensible string = same acceleration on string. This is from the Solomon paper thanks.
4. (Original post by coconut64)
hi for part c of this question, I thought that the string is modelled as a light string since the string does not have any mass however the only answer is that the string is inextensible. But surely it's also light right ?

Also for part e the last part , I don't get why fr would also decreed the reaction force. Fr does not act veritically . It is parallel to the plane. I thought adding fr would only decrease the weight but I don't get why it decrease R too..... Thanks.
with regards to assumptions, what would one say when it asks what does it being a particle mean? can you just say that its force/weight acts from one exact pinpoint kind of thing

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