Hi Faziah! 

Since this question is about force, it looks like we’ll be using the f=ma and SUVAT equations.

First thing we need to do is figure out what velocity the car is going at before it breaks. Using v = s/t, we get v = 100/12 = 8.33ms^-1.

With this, we know when the car breaks;

t = 9 (it takes 9 seconds to stop moving)
u = 8.33 (calculated above)
v = 0 (the car breaks until it stops)
a = ?

So using the SUVAT equations, we can choose: v=u + at

Rearranging this equation to get a we arrive at the equation:

(v - u)/t = a

Plugging in the numbers from above:

(0- 8.33)/ 9.81 = -0.85 (2 s.f)

Since we require force, we use the equation stated above: 

F = ma
F = mass x 0.85

We use the positive value for acceleration here as the question asks for the magnitude of the force, which is always a positive number! 

The mass hasn’t been given in the question you wrote but of you know it, convert the mass of the car from tonnes to KG (because we have to use SI units!). The conversion equation is:

1 tonne = 1000kg

Plug the mass into the equation and you’ll arrive at your answer, Hope this helps! 


Maths

A car, of mass Tonnes, accelerates from rest of over 100 m in 12 seconds. it then immediately applies the brakes. given that the car stops nine seconds later, find the magnitude of the braking force.

The amount of trees in the world that aren’t elm are 25%, (100 - 75 = 25).

For the smallest possible amount of elm trees to be diseased, we must assume all of the non-elm trees are diseased. 

40% (total diseased trees) - 25% (non-elm trees) = 15%

This is the smallest possible amount of elm trees that are diseased! 

English

40% of the trees in a wood are disease . 75% of the trees in the world are elm . What is the least possible percentage of trees in the wood that are diseased elm ?

So for this question we need to consider two different thing, the cost of the car hire and the cost of petroleum. 

Taking the cost of the car hire first, the car costs 99.4 CHF per day, if we hired the car for four days, the total cost will be:

4 x 99.4 = 397.6CHF

now for the cost of petrol, if the car uses 0.08l of petrol for every km it drives, it will use a total of:

0.08 x 525 = 42l of petrol

If petrol costs 1.35CHF per litre then the total petrol cost is:

42 x 1.35 = 56.7CHF

adding these all together you get:

56.7 + 397.6 = 454.3CHF

Hope this helps! 

Xavier hires car for four days in Switzerland. Currency in Switzerland is CHF. The car costs 99.40 CHF per day to hire. The care uses 0.08 l of petrol for every km it travels. Petrol costs 1.35 CHF per litre . He drives 525km over 4 days. Total?

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