# simple mechanics question

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#1
a car of mass 800kg travels a horizontal road with constant regarding force of F N reduced the speed from 18 ms to 12ms in 2.4s
find F
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#2
why dont they consider the forward force of the car?
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#3
sholdnt they find the forward force to?
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4 weeks ago
#4
(Original post by interlanken-fall)
a car of mass 800kg travels a horizontal road with constant regarding force of F N reduced the speed from 18 ms to 12ms in 2.4s
find F
Since the car is decelerating, the force is acting against the foward movement of the vehicle, while acceleration can be found by determinig the difference in velocity over time (v-u)/t
that is (12-18)/2.4
this gives us -2.5 m/s2
Force according to Newton's second law of motion is mass multiplied by accelation, F=ma
800kg * -2.5 m/s2
gives a force of 2000 N in the opposite direction.
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4 weeks ago
#5
(Original post by interlanken-fall)
why dont they consider the forward force of the car?
You haven't posted the exact question, but I'm assuming it never mentioned a driving force, in which case you shouldn't assume there is one. (The car might be coasting, after all).

I grant you that in an exam this would be a bit disconcerting (this is where you need to use the "exam technique" of "this is obviously not answerable if there's an unknown driving force, so I'd better assume it's zero), but in a given solution which doesn't mention it then it's pretty clear what you're supposed to do.
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#6
Here
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4 weeks ago
#7
(Original post by interlanken-fall)
Here
So, as I said, no mention of a driving force.
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#8
(Original post by DFranklin)
So, as I said, no mention of a driving force.
but there should be one right? its still moving forward meaning there should be a force making it go
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4 weeks ago
#9
(Original post by interlanken-fall)
but there should be one right? its still moving forward meaning there should be a force making it go
No, you don't brake and accelerate in a car at the same time.

The re tarding force is probably braking, but it could involve friction as well. They're referring to the net force, so it doesn't matter what component forces make it up.
Last edited by mqb2766; 4 weeks ago
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#10
(Original post by mqb2766)
No, you don't brake and accelerate in a car at the same time.

The re tarding force is probably braking, but it could involve friction as well. They're referring to the net force, so it doesn't matter what component forces make it up.
thank you, another question is when finding the force of the car we do F=MA why do we use use -f=ma is it because its going in the opposite direction?
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4 weeks ago
#11
(Original post by interlanken-fall)
but there should be one right? its still moving forward meaning there should be a force making it go
No. You seem to be fundamentally misunderstanding Newton's Laws of Motion. Newton's first law explicitly says that a body can continue moving forwards without a force making it do so.
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#12
(Original post by DFranklin)
No. You seem to be fundamentally misunderstanding Newton's Laws of Motion. Newton's first law explicitly says that a body can continue moving forwards without a force making it do so.
meaning if theres a constant force do we ignore or no
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4 weeks ago
#13
(Original post by interlanken-fall)
meaning if theres a constant force do we ignore or no
A constant (net) force produces a constant acceleration as per Newton 2.

Not quite sure what you're asking here
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#14
(Original post by davros)
A constant (net) force produces a constant acceleration as per Newton 2.

Not quite sure what you're asking here
i get it, if a object is stationary or constant motion all forces equal
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4 weeks ago
#15
(Original post by interlanken-fall)
i get it, if a object is stationary or constant motion all forces equal
Yes. Thats Newton 1. Bold should be the "sum of all forces acting on the object is zero".
Last edited by mqb2766; 4 weeks ago
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#16
(Original post by mqb2766)
Yes. Thats Newton 1. Bold should be the "sum of all forces acting on the object is zero".
can you help explain the 3rd law equal and opposite reaction, that's always confused because eg if a ball is falling shouldn't the force pulling it down be the same pulling it up as equal and opposite so how is it falling?
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4 weeks ago
#17
(Original post by interlanken-fall)
can you help explain the 3rd law equal and opposite reaction, that's always confused because eg if a ball is falling shouldn't the force pulling it down be the same pulling it up as equal and opposite so how is it falling?
The opposite force acts on the Earth, pulling it up towards the apple. This force is equal in magnitude to that on the apple, but because the Earth is rather a lot heavier than an apple(!), the effect on the Earth is not noticeable.
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#18
(Original post by DFranklin)
The opposite force acts on the Earth, pulling it up towards the apple. This force is equal in magnitude to that on the apple, but because the Earth is rather a lot heavier than an apple(!), the effect on the Earth is not noticeable.
ohhh so the apple hits the earth at 9N and the earth hit the apple with 9N ??
so a rope will get tension when pulled on it but will get equal tension from it pushing back?
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4 weeks ago
#19
(Original post by interlanken-fall)
ohhh so the apple hits the earth at 9N and the earth hit the apple with 9N ??
so a rope will get tension when pulled on it but will get equal tension from it pushing back?
I wouldn't use the word "hit" - this is true whether or not the apple is in contact with the ground. The apple exerts a force of F newtons on the Earth, and the Earth exerts a force of N newtons in the opposite direction on the apple.

Regarding the rope: remember that the forces are equal and opposite. So if a man is pulling the rope to the right with a force F, he will feel as if the rope is pulling him to the left with a force F. That is, both experience a "pull", not one a pull and the other a push.

Similarly if you push on a wall, you will feel like the wall is "pushing back", not that it is "pulling back".
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#20
(Original post by DFranklin)
I wouldn't use the word "hit" - this is true whether or not the apple is in contact with the ground. The apple exerts a force of F newtons on the Earth, and the Earth exerts a force of N newtons in the opposite direction on the apple.

Regarding the rope: remember that the forces are equal and opposite. So if a man is pulling the rope to the right with a force F, he will feel as if the rope is pulling him to the left with a force F. That is, both experience a "pull", not one a pull and the other a push.

Similarly if you push on a wall, you will feel like the wall is "pushing back", not that it is "pulling back".
THANK YOU! so in a pulley theres tension due to the weight of the masses pulling on the ropes but at the same time the rope is pulling back
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