simple mechanics question

OP

why dont they consider the forward force of the car?

OP

sholdnt they find the forward force to?

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
the answer is this

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.

Reply 4

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.

Reply 5

OP

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Here

Reply 6

Original post by interlanken-fall

Here

So, as I said, no mention of a driving force.

Reply 7

OP

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

Reply 8

mqb2766

21

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.

(edited 2 years ago)

Reply 9

OP

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?

Reply 10

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.

Reply 11

OP

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

Reply 12

davros

16

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 :smile:

Reply 13

OP

Original post by davros

A constant (net) force produces a constant acceleration as per Newton 2.

Not quite sure what you're asking here :smile:

i get it, if a object is stationary or constant motion all forces equal

Reply 14

mqb2766

21

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".

(edited 2 years ago)

Reply 15

OP

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?

Reply 16

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.

Reply 17

OP

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?

Reply 18

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".

Reply 19

OP