Point particle model for a person pushing a block on ice

[salqmander]

Homework Statement

The following image shows a person pushing a large block on a horizontal ice surface in a straight line to the right at a constant speed.

The figure presents a person pushing a large block toward the right. The block is slightly larger than the person. The person and the block are both on a horizontal surface labeled Ice.

The mass of the block is 20 kg. Frictional forces between the block and the ice are negligible. The block has a wide cross-sectional area, so air resistance is acting on the block, but air resistance on the person is negligible. The person's shoes do not slip on the ice.

As the person pushes the block, the person moves with the same constant speed as the block. In one situation, the person pushes the block with a force of 78 N moving it at a constant speed of 4.00 m/s
(I attached the image with the problem below)

a) Using a dot as a particle model (representing the person), draw and label the forces (not components) exerted on the person. Each force must be represented by a distinct arrow starting on, and pointing away from, the dot.
b) The person now stops and releases the block. Determine the magnitude and direction of the block's acceleration at the instant the block is released. Show your work.

Relevant Equations

F = ma

a) For the model, I'm not sure how to include all the forces I think are present. Gravity, normal force from the ice, contact force from ice to person, and person to ice. Is there also a normal force keeping the person's hands from going "into" the ice?

b) As for part b, I would think to use newtons second law, but I'm not sure if that's correct because the force of the person is no longer acting on the block
78 = 20a
a= 78/20
a=3.9m/s^2?

image given with question:

my attempt at a model part a:

 

[PeroK]

Note that it's better to say point particle, rather than dot particle.

 

[PeroK]

Homework Statement: The following image shows a person pushing a large block on a horizontal ice surface in a straight line to the right at a constant speed.

The figure presents a person pushing a large block toward the right. The block is slightly larger than the person. The person and the block are both on a horizontal surface labeled Ice.

The mass of the block is 20 kg. Frictional forces between the block and the ice are negligible. The block has a wide cross-sectional area, so air resistance is acting on the block, but air resistance on the person is negligible. The person's shoes do not slip on the ice.

As the person pushes the block, the person moves with the same constant speed as the block. In one situation, the person pushes the block with a force of 78 N moving it at a constant speed of 4.00 m/s
(I attached the image with the problem below)

a) Using a dot as a particle model (representing the person), draw and label the forces (not components) exerted on the person. Each force must be represented by a distinct arrow starting on, and pointing away from, the dot.
b) The person now stops and releases the block. Determine the magnitude and direction of the block's acceleration at the instant the block is released. Show your work.
Relevant Equations: F = ma

a) For the model, I'm not sure how to include all the forces I think are present. Gravity, normal force from the ice, contact force from ice to person, and person to ice. Is there also a normal force keeping the person's hands from going "into" the ice?

b) As for part b, I would think to use newtons second law, but I'm not sure if that's correct because the force of the person is no longer acting on the block
78 = 20a
a= 78/20
a=3.9m/s^2?

image given with question:
View attachment 332813my attempt at a model part a:
View attachment 332815

There's only one contact force acting on the person from the block. That's $F'_c$ in your diagram. The block isn't pulling the person forward, so $F_c$ is the force exerted by the person on the block.

 

[salqmander]

Note that it's better to say point particle, rather than dot particle.

 

[salqmander]

There's only one contact force acting on the person from the block. That's $F'_c$ in your diagram. The block isn't pulling the person forward, so $F_c$ is the force exerted by the person on the block.

thank you

is part b correct?

 

[PeroK]

is part b correct?

No. You need to list all the forces on the block.

 

[salqmander]

No. You need to list all the forces on the block

There was a force of friction which is negligible because the ice is not slippery so are there no more forces acting on the ice? will it continue at a speed of 4m/s so 0 acceleration?

 

[haruspex]

There's only one contact force acting on the person from the block. That's $F'_c$ in your diagram. The block isn't pulling the person forward, so $F_c$ is the force exerted by the person on the block.

What about friction from the ground?

 

[salqmander]

What about friction from the ground?

I think because the surface is ice friction is negligible, right?

 

[haruspex]

I think because the surface is ice friction is negligible, right?

"The person's shoes do not slip on the ice."

 

[salqmander]

"The person's shoes do not slip on the ice."

yea i'm not sure

 

[salqmander]

"The person's shoes do not slip on the ice."

do you know how to solve part b? i'm not sure what to do my textbook is not very clear

 

[haruspex]

yea i'm not sure

If the only horizontal force on the person is the 78N reaction force from the ice, why isn't the person accelerating away from the ice?

 

[haruspex]

F = ma
a = F/m
a = g x m/ m
a=g
a= 9.8m/s^2?

This makes no sense.
What are the horizontal forces on the ice while the person is pushing? What equation relates them?
What forces change immediately the person stops pushing?

 

[salqmander]

yes i realized that g shouldn't be in a horizontal question, let me see

 

[haruspex]

do you know how to solve part b? i'm not sure what to do my textbook is not very clear

You have not solved part a yet. What are the horizontal forces on the person while pushing the ice? What equation relates them?

 

[salqmander]

to the person, i believe there is friction force from the block,

 

[haruspex]

to the person, i believe there is friction force from the block,

The person is pushing the block horizontally, not (we assume) up or down. So why would there be friction between the person and the block?

 

[salqmander]

The person is pushing the block horizontally, not (we assume) up or down. So why would there be friction between the person and the block?

oh that makes sense. is it just contact force?

 

[haruspex]

oh that makes sense. is it just contact force?

Between person and block, yes. The person is pushing on the block with a force of 78N, so what does Newton say about the force of the block on the person?

 

[salqmander]

78N back to the person

 

[haruspex]

78N back to the person

Right. But the person is not accelerating away from the block, so what other horizontal force acts on the person?

 

[salqmander]

Right. But the person is not accelerating away from the block, so what other horizontal force acts on the person?

is it static friction

 

[PeroK]

What about friction from the ground?

We never got that far! I was expecting the OP to realise that an unopposed horizontal force $F_c$ couldn't be correct.

oh that makes sense. is it just contact force?

You need to have a serious think about what Newton's law tell you:

1) The first law implies that unless an object is accelerating, there can be no net force on it.

2) The second law tells you how to calculate the acceleration from the mass and the net external force.

3) The third law tells you that forces come in pairs. Those pairs, however, are exerted on different objects. Third laws pairs never apply to the same object. That's why your $F_c$ was wrong.

I'll leave the thread to @haruspex, but I think you haven't really understood how all three of Newton's laws apply to this scenario.

Personally, I would start by listing all the forces on both the man and the block. Even if the question doesn't ask for it, there's no reason not to do it.

 

[haruspex]

is it static friction between the persons feet and the surface

Yes. And what is the magnitude of that?

 

[LynnS]

anyone has the asnwer yet lol?

 

[erobz]

anyone has the asnwer yet lol?

The OP either gave up, or solved it without updating us.