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Chapter 5 Edexcel Mixed Excercise 5 Question 9 and Challenge Question.

First, question 9 part (a):

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I did $\ F = 700 + 2150 \times9.8\cos10 ^\circ = 4400N \ (2sf)$.

But the answer in the back is $3700N$. They forgot to include the $700N\ $ or I am not understanding something?

$$$$ Secondly, the challenge question:

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I got:

After the cable snaps, the boat decelerates up the slipway for a distance of $\ 2.8m (2sf)\ $ and time $\ 1.1\ $ seconds.

Then assuming that, when the cable snaps the distance from the boat to the water is $\ 8m\ $, it takes $\ 5.8\ $ seconds for the boat to accelerate back down the slipway until it reaches the water. So total time is $\ 1.1 + 5.8 = 6.9\ $ seconds $\ (2sf)$.

But the answer in the back is $\ 5.8\ $ seconds $\ (2sf).\ $ So again, is this me doing something incorrectly or is it sloppy book answers?

Adam Rubinson
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  • In the first question your value for the air-resistance would prevent the car from moving (since you're opposing the 700N of engine force as well). After the air-resistance is taken into account, the car is still moving though. For the second question, since the boat is being pulled up the slipway you should assume that when the cable snaps the boat instantaneously stops and immediately starts descending the slip-way. – postmortes Feb 19 '20 at 10:55
  • "In the first question your value for the air-resistance would prevent the car from moving (since you're opposing the $700N$ of engine force as well)". No, constant speed (acceleration = $0$) implies the components of the forces in the direction of the slope sum to zero.$$$$ "Since the boat is being pulled up the slipway you should assume that when the cable snaps the boat instantaneously stops and immediately starts descending the slip-way." Really, what's the justification for this? – Adam Rubinson Feb 19 '20 at 12:52

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