Tug of war

A truck of mass 3000 kg 3000~\mbox{kg} is harnessed to a trailer (Trailer A) of mass 1000 kg 1000~\mbox{kg} . The trailer is hooked to ANOTHER trailer (Trailer B) of mass 500 kg 500~\mbox{kg} . The truck begins to accelerate to the right at 1 m/s 2 1~\mbox{m/s}^2 , pulling the trailers along. What is the ratio of the force the truck exerts on Trailer A to the force Trailer A exerts on Trailer B?

Details and assumptions

  • Ignore friction.


The answer is 3.

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14 solutions

The total mass of the trailers is:
500 k g + 1000 k g = 1500 k g 500 kg + 1000 kg = 1500 kg
In order for the truck to get the trailers accelerating at 1 meter per second, the truck must exert:
1500 k g × 1 m e t e r s e c o n d = 1500 N e w t o n s 1500 kg \times 1 \frac{meter}{second} = 1500 Newtons
In order for trailer A to get trailer B accelerating at 1 meter per second, Trailer A must exert: 500 k g × 1 m e t e r s e c o n d = 500 N e w t o n s 500 kg \times 1 \frac{meter}{second} = 500 Newtons
Finally, dividing the force of the truck by the force of trailer A,
1500 N e w t o n s 500 N e w t o n s = 3 \frac{1500 Newtons}{500 Newtons} = 3 , so the final answer is 3 . \boxed{3}.


ok I got it ! nicely solved

Devesh Rai - 7 years, 6 months ago

the law of the conservation of momentum...and second &third law.

Thouhid Shourov - 7 years, 3 months ago

Got it. Thanks for the solving and explanation

Muhammad Kamal - 7 years, 3 months ago
Akshat Jain
May 20, 2014

We know that according to Newton's second law, F = m a F = ma

The force exerted by the Truck on Trailor A would be-

F = m × a F = m \times a

F = 1500 × 1 F = 1500 \times 1

F = 1500 N F = 1500 N

Now, the force exerted by Trailor A on Trailor B would be-

F = m × a F = m \times a

F = 500 × 1 F = 500 \times 1

F = 500 N . F= 500 N.

Therefore, the required ratio is- 1500 500 = 3 \frac {1500}{500} = 3

Jason Kristiano
May 20, 2014

Draw the free body diagram for truck, trailer A, and trailer B.

Let: \overline{Fta} = Contact force from truck to trailer A \overline{Fat} = Contact force from trailer A to truck \overline{Fab} = Contact force from trailer A to trailer B \overline{Fba} = Contact force from trailer B to trailer A \overline{Fm} = Force from the truck machine

Trailer A

\overline{F} = ma \overline{Fta} -\overline{Fba} = 1000 \cdot 1 \overline{Fta} -\overline{Fba} = 1000 ... (1)

Trailer B

\overline{F} = ma \overline{Fab} = 500 \cdot 1 \overline{Fab} = 500 ... (2)

\overline{Fab} = \overline{Fba}

Substitute Eq(2) inti Eq(1)

\overline{Fta} = 1000 + 500 = 1500

\frac {\overline{Fta}}{\overline{Fab}} = \frac {1500}{500} = 3

Vostro Del
May 20, 2014

Balancing of Forces.

common acceleration of the system,a=1 m/s^2
(note:we do not need the value of a in this problem)

force applied by truck on trailer A=(combined mass of trailer A and B) /times (their acceleration).

force applied by trailer A on trailer B=(mass of trailer B) /times (its acceleration).

required ratio=force applied by truck on trailer A / force applied by trailer A on trailer B.

since acceleration of A and B are the same,a cancels out, required ratio = (mass of A + mass of B) / (mass of B) = (1000 + 500) / 500

So,The Given Ratio Of Forces=3.

Mark Hennings
May 20, 2014

If T 1 is the force that trailer A exerts on trailer B, then T 1 = 500\times1 = 500 N (N2L on trailer B). If T 2 is the force that the car exerts on trailer A, then T 2-T 1=1000\times1, and so T 2 = 1500 N (N2L on trailer A). Thus the ratio T 2/T 1 is 1500/500=3.

Junia Estequi
May 20, 2014

How Newton said, F=m.a. About F on Newton, "m" on kg and "a" on m/s². 1) The truck has 3000kg and accelerate to the right at 1 m/s², so he exerts 3000N force at trailer A. 2) The trailer A has 1000kg and the same acceleration, so the force Trailer A exerts on Trailer B is equal to 1000N. 3) The ratio is truck-trailer A and trailer A-trailer B, so 3000/1000 = 3. So the the ratio of the force the truck exerts on Trailer A to the force Trailer A exerts on Trailer B is equal to 3.

Consider the forces acting on B. The normal force and the weight cancel out. The net force is equal to the force exerted by A.

Consider the forces acting on A and B as a whole. The normal force and the weight cancel out. The net force is equal to the force exerted by the truck.

A and B is thrice as massive as B. A and B has the same acceleration as B. Thus, the net force on A and B is thrice the net force on B.

Therefore, the ratio between the force that the truck exerts on A and the force that A exerts on B is 3.

Michael Hirt
May 20, 2014

The truck is only connected with trailer A and cannot exert any direct force on trailer B.

Trailer B has a mass off 500kg and is accelerated by 1m/s², therefore the force on it is F=m*a=500N. This force only can be exerted by trailer A. Therefore trailer B exerts an equal and opposite force on trailer A.

The total force on trailer A has to be 1m/s²*1000kg=1000N to the right. We already know, there is a force of 500N to the left. In consequence the truck has to exert the force 500N+1000N=1500N on trailer A.

After dividing 1500N by 500N, we get the ratio of 3.

Tejas Kasetty
May 20, 2014

Since all the mass(Truck, Trailer-A, Trailer-B) are connected, the following observations should be made clear:

  1. The acceleration of all the bodies is the same. i.e., 1 m / s 2 1m/s^2 .

  2. Since friction is ignored, force on Truck due to its acceleration is equal to the force on Trailer-A.

3.And force on Trailer-A due to its accelerationis equal to force on Trailer-B the .

Here, we don't consider the force on Trailer-A due to the Truck because what force is applied on Trailer-A due to Truck is equal to the force applied on Trailer-B due to Truck. [ Reason: here we are only considering force on Trailer-B due to Trailer-A]

Force the truck exerts on Trailer-A is given by, F A T = m 1 a F_{AT}=m_{1}a where m 1 = 3000 k g m_1=3000kg and a = 1 m / s 2 a = 1m/s^2 [acceleration is same for all]

F A T = 3000 N \Rightarrow F_{AT} = 3000N [note: F A T F_{AT} is read as force on Trailer A due to Truck]

Similarly, Force Trailer-A exerts on Trailer-B,

F B A = m 2 a F_{BA} = m_{2}a where m 2 = 1000 k g m_{2}=1000kg . F B A = 1000 N \Rightarrow F_{BA} = 1000N

ratio of the force the truck exerts on Trailer A to the force Trailer A exerts on Trailer B is given by, F A T F B A = 3000 1000 = 3 \frac{F_{AT}}{F_{BA}}= \frac {3000}{1000} = 3

\therefore The ratio of the force the truck exerts on Trailer A to the force Trailer A exerts on Trailer B is 3 3

Sunil Jadhav
Jan 11, 2014

F=ma, here a = 1 m/s^2 truck pulls 1500 kg so F = 1500 * 1 = 1500 N and trailer pulls 500 kg so f =500 * 1 =500 N The ratio F/f =3

Prasun Biswas
Dec 21, 2013

The whole system moves with an acceleration of 1 m/ s 2 s^{2} . So, acc.(a)=1 m/ s 2 s^{2} .

Now, for the truck, since the truck is carrying its own mass plus the mass of the two trailers, so the force exerted by truck on trailer A = F t r u c k _{truck} =ma= ( 3000 + 1000 + 500 ) × 1 = 4500 × 1 = 4500 (3000+1000+500)\times 1 = 4500 \times 1 = 4500 N.

Now, for trailer A, it carries its own mass plus mass of trailer B, so the force exerted by trailer A on trailer B = F A _{A} = ma = ( 1000 + 500 ) × 1 = 1500 × 1 = 1500 (1000+500)\times 1 = 1500\times 1 = 1500 N

Now, Ratio of the forces = F t r u c k F A = 4500 1500 = 3 \frac{F_{truck}}{F_{A}} = \frac{4500}{1500} = \boxed{3}

This solution is wrong. The mass of the truck is irrelevant.

Tong Choo - 7 years, 5 months ago

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How can you say that the mass of the truck is irrelevant? Due to the mass of the trailers and the acc. of the system, the trailers and the truck experience the forces.

Prasun Biswas - 7 years, 5 months ago

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If they change the mass of the truck in the question, your answer would be different and wrong. The ratio of forces of the truck on A and A on B has nothing to do with the mass of the truck, but rather, the mass of A and the mass of B. The answer is 1500/500, regardless of what the mass of the truck is, not 4500/1500 which is dependent on the mass of the truck and will be a different answer if the truck was heavier or lighter.

Tong Choo - 7 years, 5 months ago

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@Tong Choo @Tong choo, 1- why mass is specified here? 2- if u cange mass of trailer A then also whole solution gets changed here. if suppose mass of truck is 100kg then tell me the answer? and check it is practical or not?

arif mujawar - 7 years, 5 months ago

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@Arif Mujawar If mass of truck is 100kg, the answer is still the same. 1500/500 = 3. The force exerted on the trailer has NOTHING to do with the mass of the truck. The engine will need to do less work to pull the whole truck+trailers but the force acting on the trailer is always the same since the mass of the trailer is the same and its acceleration is still the same.

Tong Choo - 7 years, 5 months ago

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@Tong Choo Would you care to post a complete solution that you see correct here in the comments so that we can understand what you are trying to say ??

Prasun Biswas - 7 years, 5 months ago

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@Prasun Biswas Force on B = mass x acceleration = 500 x 1 = 500 N This is the force exerted on B, by A.

Resultant force on A = mass x acceleration = 1000 x 1 = 1000 N But force on A = force exerted by truck on A - opposite force exerted by B on A (equal and opposite force) Therefore, 1000 N = force exerted by truck on A - 500 N. So, force exerted by truck on A = 1500 N

Ratio = 1500/500 = 3

Tong Choo - 7 years, 5 months ago

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@Tong Choo sorry, its complicated to understand your comment? can you make its simple with providing directions to your forces?? 2- how can truck exert any force if its mass assumed to be 10kg means niglisible compared to trailer?? mass of truck is must be counted here.

arif mujawar - 7 years, 4 months ago

we can solve this easily...by applying newtons third law.....

Harish Kp - 7 years, 3 months ago
Khaled Mohamed
Feb 12, 2014

Force that the truck exerts on trailer A = m(truck) x a = 3000 N

Force that trailer A exerts on trailer B = mt(trailer A ) x a = 1000 N

then ratio is 3000 : 1000 = 3 :1 = 3

Abbas Bagwala
Jan 27, 2014

Since the acceleration of the body as a whole is the same, we simply equate them. Let f1 and f2 to be forces exerted BY the truck and the trailer respectively and m1, m2 their masses.

Therefore, from the first proposition

f1/m1 = f2/m2

Therefore, f1/f2 = m1/m2 = 3000/1000 = 3.

Gaber Boraey
Jan 10, 2014

while F = m * a, where m: is the mass, and a: is the acceleration So for the force the Truck exerts on Trailer A, F1 = 3000 * 1 = 3000 Kg.m/sec^2 and for the force Trailer A exerts on Truck B: F2 = 1000 * 1 = 1000 Kg.m/sec ^ 2

Therefore the ratio = F1/F2 = 3000/1000 = 3.0

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