BailaS
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I'm stuck on the second part of this Q. I got all parts of a correct.

A trailer of mass 600 kg is attached to a car of mass 900kg by means of a light inextensible tow-bar. The car tows the trailer along a horizontal road. The resistances due to motion of the car and the trailer are 300N and 150N respectively.


a) Given that the acceleration of the car and trailer is 0.4ms^-2, calculate,
i) the tractive force exerted by the engine of the car,
ii) the tension in the tow - bar.

b) Given that the magnitude of the force in the tow-bar must not exceed 1650N, calculate the greatest possible deceleration of the car.

For ai I got 1050N and for aii I got 390N, both of which are right according to the book. I got confused on part b. It says the car is decelerating, so the force from the engine must not exist, and the resultant force is opposing the motion of the car. I know you have to use F=ma to get the answer, but I'm not sure which forces to use, and that's where I get confused.

Thanks
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BailaS
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Can anyone help me?
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TenOfThem
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(Original post by BailaS)
I'm stuck on the second part of this Q. I got all parts of a correct.

A trailer of mass 600 kg is attached to a car of mass 900kg by means of a light inextensible tow-bar. The car tows the trailer along a horizontal road. The resistances due to motion of the car and the trailer are 300N and 150N respectively.


a) Given that the acceleration of the car and trailer is 0.4ms^-2, calculate,
i) the tractive force exerted by the engine of the car,
ii) the tension in the tow - bar.

b) Given that the magnitude of the force in the tow-bar must not exceed 1650N, calculate the greatest possible deceleration of the car.

For ai I got 1050N and for aii I got 390N, both of which are right according to the book. I got confused on part b. It says the car is decelerating, so the force from the engine must not exist, and the resultant force is opposing the motion of the car. I know you have to use F=ma to get the answer, but I'm not sure which forces to use, and that's where I get confused.

Thanks

Look at the car

Acting against motion you will have 300N and ma
Acting with motion will be the thrust 1650N
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BailaS
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(Original post by TenOfThem)
Look at the car

Acting against motion you will have 300N and ma
Acting with motion will be the thrust 1650N
Ok so that will give me 1650 = 900a + 300

1650 - 300 = 1350

1350 = 900a

1350/900 = a

That gives me an acceleration of 1.5ms-2, but the book says 3ms-2 is the answer.

Did I go wrong somewhere?
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TenOfThem
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(Original post by BailaS)
Ok so that will give me 1650 = 900a + 300

1650 - 300 = 1350

1350 = 900a

1350/900 = a

That gives me an acceleration of 1.5ms-2, but the book says 3ms-2 is the answer.

Did I go wrong somewhere?

Ah

Do the trailer instead
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BailaS
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(Original post by TenOfThem)
Ah

Do the trailer instead
Ok so if I do that I get 1650 = 600a + 150

1650 - 150 = 600a

That gives a=2.5ms-2. Would that be correct?
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TenOfThem
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(Original post by BailaS)
Ok so if I do that I get 1650 = 600a + 150

1650 - 150 = 600a

That gives a=2.5ms-2. Would that be correct?
Not if the answer is 3

Sorry - I am not being any help
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BailaS
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(Original post by TenOfThem)
Not if the answer is 3

Sorry - I am not being any help
It's ok haha I was totally confused by this Q anyway. I tried every different combination of forces and nothing gave me the answer of 3.
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k4l397
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(Original post by BailaS)
Ok so if I do that I get 1650 = 600a + 150

1650 - 150 = 600a

That gives a=2.5ms-2. Would that be correct?
You kind of have the right idea here except for the direction you are taking tension as. Because it's decelerating tension acting on the trailer is acting in the same direction as friction.

So 1650 + 150 = 600a
1800 = 600a

It's difficult to think about but if you think of it accelerating the tension is the force pulling the trailer in the direction of motion, but if it's decelerating then the trailer is working with friction against the direction of motion.

Hope this helps If you don't get why tension is that way still let me know and I'll try to explain it better.
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BailaS
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(Original post by k4l397)
You kind of have the right idea here except for the direction you are taking tension as. Because it's decelerating tension acting on the trailer is acting in the same direction as friction.

So 1650 + 150 = 600a
1800 = 600a

It's difficult to think about but if you think of it accelerating the tension is the force pulling the trailer in the direction of motion, but if it's decelerating then the trailer is working with friction against the direction of motion.

Hope this helps If you don't get why tension is that way still let me know and I'll try to explain it better.
Oh yeah that makes sense thank you!
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error666
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how did u answer a)i). i have a question more or less the same
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