Mechanics & Properties Of Matter

[glasgowm]

I just really don't understand the wording of this question.

A car of mass 1200kg tows a caravan of mass 1000kg. The frictional force on the car and caravan is 200N and 500N respectively, The Car accelerates at 2 ms^-2

b) What force does the tow bar exert on the caravan?

Does it want me to find the tension of the bar?

M x A = T - Friction
4312N = T - 700N
T = 3612N ?

 

[Hootenanny]

I just really don't understand the wording of this question.

A car of mass 1200kg tows a caravan of mass 1000kg. The frictional force on the car and caravan is 200N and 500N respectively, The Car accelerates at 2 ms^-2

b) What force does the tow bar exert on the caravan?

Does it want me to find the tension of the bar?

M x A = T - Friction
4312N = T - 700N
T = 3612N ?

No, it wants you to find the force required to accelerate the caravan at 2m.s^-2.

 

[glasgowm]

That didnt really help me :-/

The caravan is already accelerating at 2m.s-2, isn't it?

 

[Hootenanny]

That didnt really help me :-/

The caravan is already accelerating at 2m.s-2, isn't it?

Yes it is accelerating, but what causes acceleration?

 

[glasgowm]

whats the formula that's like F = ma but with friction in it?

 

[Hootenanny]

whats the formula that's like F = ma but with friction in it?

$$\sum \vec{F} = m\vec{a}$$

In words, the vector sum of the forces is equal to the acceleration. You need to sum all the forces acting on the caravan to find the acceleration.

 

[glasgowm]

So...

A = f/m
2 = f/1000
F = 2000N ?

 

[Hootenanny]

So...

A = f/m
2 = f/1000
F = 2000N ?

You are forgetting the frictional force.

 

[glasgowm]

You are forgetting the frictional force.

I asked you how to do it with the Friction and you just gave me F = ma

is it

2000N - the friction = 1500N ?

 

[Hootenanny]

I asked you how to do it with the Friction and you just gave me F = ma

No, I gave you;

$${\color{red}\sum}\vec{F} = m\vec{a}$$

The important bit is the sum, this means the vector sum of all the forces acting on the caravan (the resultant force) is equal to the product of the mass and acceleration.

is it

2000N - the friction = 1500N ?

Not quite. I'll walk you through it.

(1) Sum the forces
Taking the frictional force to be negative the sum of the forces is as follows (let F_A be the force applied on the caravan by the tow bar)

$$\sum\vec{F} = F_{A} - 500$$

(2) Apply Newton's Second Law

$$\sum\vec{F} = m\vec{a}$$

Now, we have already summed the forces above, we also know the mass of the caravan (1000kg) and the acceleration (2 m.s^-1); substituting those values in gives;

$$F_{A} - 500 = 1000 \times 2$$

$$F_{A} - 500 = 2000$$

Can you know finish of the question? Do you follow my working? Intuitively you should know if your answer makes sense. If there was no friction would it take a larger or smaller force to accelerate the caravan at the same rate?