Quick Physics Questions I found online and need explanations solving

[Candice123]

Homework Statement

1. A wooden box weighing 40 dag lies on a flat surface. Draw to scale all the forces acting on the box.

2. For the force in the following examples, write how it is distributed: point, planar or spatial.
a. Using the tip of a pencil, push an eraser across a table. What is the force of the pencil on the eraser?
b. The wind blows against the sail and pushes a sailboat. What is the force of the wind on the sail?
c. A string on which the weight is hung is attached to the ceiling. What is the force of the string on the ceiling?
d. Magnet attracts pieces of iron. What is the magnet's force on iron?
e. A crate lies on the ground. What is the force of the crate on the ground?
f. An apple falls from a tree. What is the force acting on the apple while falling?

Questions source for more context: [Spam link redacted by the Mentors]

Relevant Equations

dont have any.

Umm… I’m not super sure, but I think the only force acting is the weight.
40 dag is like… 40 N i guess? So maybe just draw a downward arrow labeled 40 N for weight?
I don’t think there’s any other force since it’s just lying there… so I guess that’s it? Just the weight pointing down.

 

[PeroK]

Homework Statement: 1. A wooden box weighing 40 dag lies on a flat surface. Draw to scale all the forces acting on the box.

Relevant Equations: dont have any.

What about Newton's laws of motion, as relevant equations?

Umm… I’m not super sure, but I think the only force acting is the weight.
40 dag is like… 40 N i guess? So maybe just draw a downward arrow labeled 40 N for weight?
I don’t think there’s any other force since it’s just lying there… so I guess that’s it? Just the weight pointing down.

What would Newton's second law say about that?

 

[Candice123]

Newton's second law says the net force should be zero if the box isn’t moving.

 

[PeroK]

Newton's second law says the net force should be zero if the box isn’t moving.

And is the net force zero?

 

[Candice123]

So... If there was no upward force balancing it, the box would definitely be accelerating downwards because of gravity.

But since it’s just lying there, the surface must be pushing up with an equal force—that’s the normal force, right? So they cancel out, and that’s why it’s not moving.

Thanks for breaking it down like that!

 

[kuruman]

Newton's second law says the net force should be zero if the box isn’t moving.

Newton's second law is summarized by $F_{\text{net}}=ma$ and says, among other things, that zero acceleration means zero net force. "Isn't moving" means that the velocity is zero. An object can have zero velocity and non-zero acceleration, e.g. a rock thrown straight up in the air and is at the top of its trajectory. At that point, the velocity is zero at the same time that the net force is mg down.

It would have been more correct to say "Newton's second law says the net force should be zero if the box is at rest and remains at rest." I am mentioning this because a lot of students confuse acceleration and velocity and believe that the acceleration of the rock at the top of its trajectory is zero because its velocity is zero. Not true.

What is the meaning of "dag"?

 

[haruspex]

What is the meaning of "dag"?

It is either a dekagram or, in Australia, dirty, wet wool besmirching the back end of a sheep; but neither is a weight.

 

[DaveC426913]

Donning my Googles for a moment, it seems 'a "dag" is the abbreviation for dekagram, a unit of mass in the metric system, equivalent to 10 grams'.

A 40dag box would mass 400g, and weigh 400g on Earth.

 

[jbriggs444]

A 40dag box would mass 400g, and weigh 400g on Earth.

If by "weigh" you mean "has a gravitational downforce of" then that would be either approximately 4000 newtons or approximately 400 grams force (deprecated unit).

If by "weigh" you mean "has a mass of" then a 40 dekagram box obviously weighs exactly 400 grams, on Earth or elsewhere.

 

[weirdoguy]

What is the meaning of "dag"?

Wow, that's interesting. In Poland we use dags in shops, buying cold cuts or cheese, so everyone knows this unit from an early age, probably first unit one learns (along with km/h). Daily reminder that different parts of the world are, well, different

 

[Lnewqban]