Homework Help: Mechanics & Properties Of Matter

1. Sep 12, 2006

glasgowm

I just really dont 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?:yuck:

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

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

2. Sep 12, 2006

Hootenanny

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

Last edited: Sep 12, 2006
3. Sep 14, 2006

glasgowm

That didnt really help me :-/

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

4. Sep 14, 2006

Hootenanny

Staff Emeritus
Yes it is accelerating, but what causes acceleration?

5. Sep 14, 2006

glasgowm

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

6. Sep 14, 2006

Hootenanny

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

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.

7. Sep 14, 2006

glasgowm

So...

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

8. Sep 14, 2006

Hootenanny

Staff Emeritus
You are forgetting the frictional force.

9. Sep 14, 2006

glasgowm

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

is it

2000N - the friction = 1500N ?

10. Sep 15, 2006

Hootenanny

Staff Emeritus
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.
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 FA 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?

Last edited: Sep 15, 2006