Forces and vectors: pulling a baby buggy on soft ground

[Jeff97]

Homework Statement

A buggy with a child has a mass of 25kg. On the soft ground, it's easier to pull the buggy than push it. The buggy is pulled with a force of 100N at an angle of 40degrees to the horizontal.

Calculate the vertical component of the force and then us it to find the total force exerted by the wheels on the ground.

Relevant Equations

Not aware of any, please advise me of any.

I don't know exactly where to start with this problem. But I'm going to try this, cos θ= adj/hyp cos 40° = Fx/100 Fx=100 cos 40° =76.60= 77N??

 

[hutchphd]

Start by drawing a picture. Then draw a free body diagram showing all forces and angles and a coordinate system. Then do vector algebra.
Where does 42 come from?

 

[hutchphd]

How does this relate to the problem??

 

[Jeff97]

Yes, I'm sorry got two different problems confused, edited original post.

 

[haruspex]

Problem Statement: A buggy with a child has a mass of 25kg. On the soft ground, it's easier to pull the buggy than push it. The buggy is pulled with a force of 100N at an angle of 40degrees to the horizontal.

Calculate the vertical component of the force and then us it to find the total force exerted by the wheels on the ground.
Relevant Equations: Not aware of any, please advise me of any.

Fx=100 cos 40° =76.60= 77N??

That's the horizontal component of the applied force, but you are not asked for that.

 

[Jeff97]

64.2516?

 

[hutchphd]

Units please and what is this exactly? (I think you are on the right track)

 

[Jeff97]

It's the vertical component. V = cos(θ) × f which is: V=sin40°×100N = 64.27=4N?

 

[haruspex]

It's the vertical component. V = cos(θ) × f which is: V=sin40°×100N = 64.27=4N?

64.28N. What is the "=4"?

 

[Jeff97]

typo 64N?

 

[haruspex]

typo 64N?

Ok. So on to the next part, what is the total force exerted on the ground?

 

[Jeff97]

@haruspex For the total force do I need to know acceleration? so do I need to do a=f/m?

 

[haruspex]

@haruspex For the total force do I need to know acceleration? so do I need to do a=f/m?

Excellent question. In principle, yes, but you have no way to determine it, so you will have to assume there is no acceleration.

 

[Jeff97]

This may sound a little stupid, but I'm not aware of that formula.

 

[haruspex]

This may sound a little stupid, but I'm not aware of that formula.

What formula?
If the buggy is not accelerating then you can consider the force balance on it.

 

[Jeff97]

I just don't know how to progress, I see your comment but don't quite understand it. Are you saying that the mass of the buggy is also the same force being pushed up at the ground?

 

[haruspex]

I just don't know how to progress, I see your comment but don't quite understand it. Are you saying that the mass of the buggy is also the same force being pushed up at the ground?

You mean the weight of the buggy, not the mass.
Have you drawn an FBD of the buggy? What forces act on it?
Since we are assuming it is not accelerating, what equations can you write?

 

[Jeff97]

F=mg? I have not drawn an FBD but I think there's the pull force to the right, then drag to the left, weight acting down and reactive force upwards

 

[haruspex]

F=mg? I have not drawn an FBD but I think there's the pull force to the right, then drag to the left, weight acting down and reactive force upwards

Yes, but you can think of the drag and normal force from the ground combined as a single total force from the ground.

 

[hutchphd]

Start by drawing a picture. Then draw a free body diagram showing all forces and angles and a coordinate system. Then do vector algebra.

F=mg? I have not drawn an FBD but I think there's the pull force to the right, then drag to the left, weight acting down and reactive force upwards

Why should we bother?

 

[Jeff97]

@hutchphd, F=mg, mass is 64x9.8=627.2N this is the total force exerted?

 

[sojsail]

Where did the 64 come from? Certainly not the 64 N in frame #8.
In which direction are you now trying to work out the total force -- total force exerted on the ground?

 

[haruspex]

I have not drawn an FBD

Then do so, and you deserve no further assistance until then.
According to Newton, the total force the buggy exerts on the ground is equal and opposite to what?

 

[Jeff97]

Ok,@haruspex I have "drawn" one. So what your saying is the force being exerted by the buggy, the ground will exert the same amount of force in the opposite direction.(up)In my FBD, I have Identified A pulling force of 100N, downward force,weight, of 25kg and a normal force acting up.

 

[haruspex]

Ok,@haruspex I have "drawn" one. So what your saying is the force being exerted by the buggy, the ground will exert the same amount of force in the opposite direction.(up)In my FBD, I have Identified A pulling force of 100N, downward force,weight, of 25kg and a normal force acting up.

With one force straight up, one straight down and one up at an angle it can't be constant velocity, can it? What have you forgotten? Hint: reread my post #19.

 

[Jeff97]

drag, Is there a formula for this?

 

[haruspex]

drag, Is there a formula for this?

You do not need a formula for it; since there is no acceleration you can deduce it from the other forces.
As I wrote in post #19, you do not need to think of drag and normal force separately. Just treat that combination as the total reaction force from the ground. That leaves you with only that unknown force and two known forces.

 

[Jeff97]

Yes, but you can think of the drag and normal force from the ground combined as a single total force from the ground.

Ok, Is the unknown force gravity? or mg? (as I called the weight downwards) Deduction, interesting, I assume I need to list my force/s numerical values. Weight down (=mg= 25x9.8=245N) Pull force (100N) ?

 

[haruspex]

Is the unknown force gravity? or mg?

How is that unknown? You know the mass and you know g, so the weight is known.

I need to list my force/s numerical values

And their directions. Forces are vectors, directions matter.

Weight down (=mg= 25x9.8=245N) Pull force (100N) ?

Yes. The third force is the one of unknown value. What name did I give it in post #27?

 

[Jeff97]

"total reaction force downwards"