B Why pulling large objects is easier than pushing? Or vice versa sometimes?

[hyunxu]

Assume a situation: A delivery man wants to place a large object in the first floor. When he is passing through a stair case , he pulls that object.Why can't he push it upwards?When he reached the first floor , he wanted to move that object forward ,so he is pushing?Why can't he pull that object?Please explain me well.I need to know whose impact is greater? Push or pull.

 

[Drakkith]

When he is passing through a stair case , he pulls that object.Why can't he push it upwards?

He can, it's just easier to pull since our bodies are configured with our shoulders at the top, allowing us to exert a vertical force without having to bend down and get up under the object like you would have to do if you tried to push it upwards. Pulling it also means that the front side of the object is being lifted instead of the back side, helping to keep it from catching on the edges of the stairs.

Note that our bodies are also configured in such a way as to make it easier to provide a large force through pushing instead of pulling. It's easier to lock your upper body and push with your legs since the major muscle groups in your torso being used are your abdominal muscles, which are much stronger than your back muscles, the ones you'd have to use if you try to pull an object. Pulling also requires grip, and your hands are relatively weak and tire quickly.

 

[sophiecentaur]

There are many different scenarios included in the OP, by implication and it's not always clear cut which is the easier method. Getting the best out of your basic strength is a matter of smart use of your body. As @Drakkith says, it's always good to 'lock' parts of your skeleton when possible and then use the strongest muscle set.
If you are pushing a heavy object, you will often be pushing downwards, against the ground which will increase the Normal Force on the ground. This will increase the friction force along the ground, which often means More Work Done when pushing and will lift a dragged object out of ruts and bumps.. That can explain many examples and it's a typical example in basic Mechanics test questions.
Then there is the way our bodies /skeletons work. We can lean backwards tp pull and use our skeleton as a lever (fulcrum being the feet). Some of the force on the pulled object comes 'for free' when using this leverage and also, much of the effort can come from the legs. (Muscles at the front of the thighs, in particular)
When pushing, there is no equivalent lever to use. OTOH, we can sometimes use our weight force for free, such as when pushing a car out of a ditch, using our butt and in that case the legs are doing the work mostly.
My strength is nothing like it used to be and I find it more and more useful to think before trying to move something. No dead weight lifting for me, in any direction, I use the lever method whenever possible.
When you next see a deliver man with a heavy item on the stairs, why not engage him in conversation and ask him about his method. This would be preferably carried out when he is actually in the process of lifting. Other ways to make friends . . . . . . . .

 

[256bits]

Assume a situation

Did you actually witness a person pulling up when on the stairs, and pushing when on the floor?
Or is this just a thought experiment?
Thought experiments do need constraints, and so what are they?

Dependency of push or pull rests with some criteria such as the nature of the object, its weight, strength of the person.
If it is say a chair with legs I think I could maybe grab it by the shoulder rest and pull it up so that the legs bounce over each step.
Or I could be behind the chair and lift it up each step to the next and kind of walk it up.
Any attempt at just pushing it up, well, the legs would just be caught on the step and go nowhere no matter how much umph one wants to put into it.

A big heavy square box - well that one I might want to roll it up the stairs.
I can't get a very good grip for pulling, and by just pushing, there is a good possibility that the box would catch at each step.
On the other hand, a rectangular long box, such as for an LCD TV - that I would push up the stairs, as the length would allow an easy slide over multiple steps t a time.

Etc...

 

[hyunxu]

I guess pulling is much easier because total mass and energy required is less while comparing to pushing.

 

[sophiecentaur]

I guess pulling is much easier because total mass and energy required is less while comparing to pushing.

Really? Where is the inherent mass difference? In either case, you can choose whether or not to use 'body weight'.

 

[hyunxu]

In either case, you can choose whether or not to use 'body weight'.

Let me explain it through simple equations

N _pull = mg - F cosθ
N _push = mg+F cosθ

so these equations show that pulling is easier than pushing.

But in practical life , I feel pushing is easier.So that's my question.

 

[sophiecentaur]

Let me explain it through simple equations

You are not including 'body weight' in the calculation and that is frequently very relevant.
Those equations show a difference between the required force for just one selected example of pulling and pushing. But, even there, you are showing the resultant Normal force - which is not the one that counts. You don't explain the actual situation that your are trying to describe because no one exerts a Normal force in order to drag or push something along.
The classic schoolboy example is pulling or pushing a lawn roller with the handle pointing downwards (a well defined angle for the applied force). In that case the friction force is less because part of the effective weight force is reduced when pulling upwards and so is the friction / rolling resistance. Hence, pulling is less effort - BUT it achieves less in the way of grass flattening.
Bottom line is that the particular situation needs to be considered before any 'simple' equation can be written.

 

[hyunxu]

Bottom line is that the particular situation needs to be considered before any 'simple' equation can be written.

Oh Thank you for that! Thank you for guiding me in a correct way! I hope to learn a lot from you!

 

[sophiecentaur]

You're welcome.