Mixing
Force required to lift an object
Clash Royale CLAN TAG#URR8PPP
up vote
0
down vote
favorite
The force required to lift an object with weight mg must be slightly greater than mg to produce a net force upwards. There's one thing I'm struggling to understand with this concept though and would appreciate if someone can clarify it for me. If an object is resting on a table then it will have a reaction force (the normal force) also acting upwards on the object. Since the net force on this object is zero as the normal/weight forces cancel out, then shouldn't any force applied upwards to the object cause it to start moving, regardless of whether the force's magnitude is less than mg or not?
forces
asked 2 hours ago
user552217
41
add a comment |Â
up vote
0
down vote
favorite
The force required to lift an object with weight mg must be slightly greater than mg to produce a net force upwards. There's one thing I'm struggling to understand with this concept though and would appreciate if someone can clarify it for me. If an object is resting on a table then it will have a reaction force (the normal force) also acting upwards on the object. Since the net force on this object is zero as the normal/weight forces cancel out, then shouldn't any force applied upwards to the object cause it to start moving, regardless of whether the force's magnitude is less than mg or not?
forces
asked 2 hours ago
user552217
41
add a comment |Â
up vote
0
down vote
favorite
up vote
0
down vote
favorite
The force required to lift an object with weight mg must be slightly greater than mg to produce a net force upwards. There's one thing I'm struggling to understand with this concept though and would appreciate if someone can clarify it for me. If an object is resting on a table then it will have a reaction force (the normal force) also acting upwards on the object. Since the net force on this object is zero as the normal/weight forces cancel out, then shouldn't any force applied upwards to the object cause it to start moving, regardless of whether the force's magnitude is less than mg or not?
forces
asked 2 hours ago
user552217
41
The force required to lift an object with weight mg must be slightly greater than mg to produce a net force upwards. There's one thing I'm struggling to understand with this concept though and would appreciate if someone can clarify it for me. If an object is resting on a table then it will have a reaction force (the normal force) also acting upwards on the object. Since the net force on this object is zero as the normal/weight forces cancel out, then shouldn't any force applied upwards to the object cause it to start moving, regardless of whether the force's magnitude is less than mg or not?
forces
forces
asked 2 hours ago
user552217
41
asked 2 hours ago
user552217
41
asked 2 hours ago
user552217
41
asked 2 hours ago
user552217
41
asked 2 hours ago
user552217
41
41
add a comment |Â
add a comment |Â
2 Answers
2
active
oldest
votes
up vote
2
down vote
The normal force only acts on that object while that object is in contact with the surface it is resting on. It is also proportional to the force being applied onto the surface - which is not necessarily $mg$ but would be if there were no other forces involved. Say you went to lift the object off that surface, and you applied a force $F<mg$ upwards. What you would get is a gravitational force going down of $mg$ and a normal force of $N=mg-F$ going up and your force of $F$ going up. In the end, the situation is still balanced and the object doesn't move. The normal force does not always provide a $mg$ force going up. It's only enough to keep the object from moving down through the surface.
answered 1 hour ago
enumaris
3,4551520
add a comment |Â
up vote
2
down vote
In a sense, you are correct.
Consider the mass resting not on a table, but on a very easily stretched trampoline.
The mass is at rest; the force of gravity is exactly balanced by the force exerted by the elastic stretching of the trampoline.
If you apply the slightest upward force, the mass will begin to accelerate upward. However, this allows the elastic trampoline to contract, so it exerts less upward force. The net upward force thus decreases, bringing the upward acceleration down to (and past) zero. The object eventually stops at a new level, with gravity balanced by the reduced upward force and the new upward force from you.
Now consider the table to be a really stiff trampoline...
answered 46 mins ago
DJohnM
8,57821824
add a comment |Â
2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
2
down vote
The normal force only acts on that object while that object is in contact with the surface it is resting on. It is also proportional to the force being applied onto the surface - which is not necessarily $mg$ but would be if there were no other forces involved. Say you went to lift the object off that surface, and you applied a force $F<mg$ upwards. What you would get is a gravitational force going down of $mg$ and a normal force of $N=mg-F$ going up and your force of $F$ going up. In the end, the situation is still balanced and the object doesn't move. The normal force does not always provide a $mg$ force going up. It's only enough to keep the object from moving down through the surface.
answered 1 hour ago
enumaris
3,4551520
add a comment |Â
up vote
2
down vote
The normal force only acts on that object while that object is in contact with the surface it is resting on. It is also proportional to the force being applied onto the surface - which is not necessarily $mg$ but would be if there were no other forces involved. Say you went to lift the object off that surface, and you applied a force $F<mg$ upwards. What you would get is a gravitational force going down of $mg$ and a normal force of $N=mg-F$ going up and your force of $F$ going up. In the end, the situation is still balanced and the object doesn't move. The normal force does not always provide a $mg$ force going up. It's only enough to keep the object from moving down through the surface.
answered 1 hour ago
enumaris
3,4551520
add a comment |Â
up vote
2
down vote
up vote
2
down vote
The normal force only acts on that object while that object is in contact with the surface it is resting on. It is also proportional to the force being applied onto the surface - which is not necessarily $mg$ but would be if there were no other forces involved. Say you went to lift the object off that surface, and you applied a force $F<mg$ upwards. What you would get is a gravitational force going down of $mg$ and a normal force of $N=mg-F$ going up and your force of $F$ going up. In the end, the situation is still balanced and the object doesn't move. The normal force does not always provide a $mg$ force going up. It's only enough to keep the object from moving down through the surface.
answered 1 hour ago
enumaris
3,4551520
The normal force only acts on that object while that object is in contact with the surface it is resting on. It is also proportional to the force being applied onto the surface - which is not necessarily $mg$ but would be if there were no other forces involved. Say you went to lift the object off that surface, and you applied a force $F<mg$ upwards. What you would get is a gravitational force going down of $mg$ and a normal force of $N=mg-F$ going up and your force of $F$ going up. In the end, the situation is still balanced and the object doesn't move. The normal force does not always provide a $mg$ force going up. It's only enough to keep the object from moving down through the surface.
answered 1 hour ago
enumaris
3,4551520
answered 1 hour ago
enumaris
3,4551520
answered 1 hour ago
enumaris
3,4551520
answered 1 hour ago
enumaris
3,4551520
3,4551520
add a comment |Â
add a comment |Â
up vote
2
down vote
In a sense, you are correct.
Consider the mass resting not on a table, but on a very easily stretched trampoline.
The mass is at rest; the force of gravity is exactly balanced by the force exerted by the elastic stretching of the trampoline.
If you apply the slightest upward force, the mass will begin to accelerate upward. However, this allows the elastic trampoline to contract, so it exerts less upward force. The net upward force thus decreases, bringing the upward acceleration down to (and past) zero. The object eventually stops at a new level, with gravity balanced by the reduced upward force and the new upward force from you.
Now consider the table to be a really stiff trampoline...
answered 46 mins ago
DJohnM
8,57821824
add a comment |Â
up vote
2
down vote
In a sense, you are correct.
Consider the mass resting not on a table, but on a very easily stretched trampoline.
The mass is at rest; the force of gravity is exactly balanced by the force exerted by the elastic stretching of the trampoline.
If you apply the slightest upward force, the mass will begin to accelerate upward. However, this allows the elastic trampoline to contract, so it exerts less upward force. The net upward force thus decreases, bringing the upward acceleration down to (and past) zero. The object eventually stops at a new level, with gravity balanced by the reduced upward force and the new upward force from you.
Now consider the table to be a really stiff trampoline...
answered 46 mins ago
DJohnM
8,57821824
add a comment |Â
up vote
2
down vote
up vote
2
down vote
In a sense, you are correct.
Consider the mass resting not on a table, but on a very easily stretched trampoline.
The mass is at rest; the force of gravity is exactly balanced by the force exerted by the elastic stretching of the trampoline.
If you apply the slightest upward force, the mass will begin to accelerate upward. However, this allows the elastic trampoline to contract, so it exerts less upward force. The net upward force thus decreases, bringing the upward acceleration down to (and past) zero. The object eventually stops at a new level, with gravity balanced by the reduced upward force and the new upward force from you.
Now consider the table to be a really stiff trampoline...
answered 46 mins ago
DJohnM
8,57821824
In a sense, you are correct.
Consider the mass resting not on a table, but on a very easily stretched trampoline.
The mass is at rest; the force of gravity is exactly balanced by the force exerted by the elastic stretching of the trampoline.
If you apply the slightest upward force, the mass will begin to accelerate upward. However, this allows the elastic trampoline to contract, so it exerts less upward force. The net upward force thus decreases, bringing the upward acceleration down to (and past) zero. The object eventually stops at a new level, with gravity balanced by the reduced upward force and the new upward force from you.
Now consider the table to be a really stiff trampoline...
answered 46 mins ago
DJohnM
8,57821824
answered 46 mins ago
DJohnM
8,57821824
answered 46 mins ago
DJohnM
8,57821824
answered 46 mins ago
DJohnM
8,57821824
8,57821824
add a comment |Â
add a comment |Â
Sign up or log in
StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
StackExchange.ready(
function ()
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fphysics.stackexchange.com%2fquestions%2f438554%2fforce-required-to-lift-an-object%23new-answer', 'question_page');
);
Post as a guest
Sign up or log in
StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Sign up or log in
StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Sign up or log in
StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
