If an object is at rest, there must be no force acting on it. Is this statement correct? Explain?

Answer 1

This is incorrect. An object being at rest can have forces acting on it however the vector sum of those forces, called net force, must be zero. This can easily be shown using Newton's Second Law.

F(net) = m a, object at rest implies a = 0 so F(net) = m (0) = 0

An example would be a glass sitting on a table. The force of gravity (weight) is acting on the glass in a downward direction and a normal contact force from the table is acting on the glass in an upward direction. When you add these two forces together, you get zero net force, which means zero acceleration. Therefore, the glass at rest on a table will not move in this situation.

As an interesting side note, the exact same argument works for any object moving at a constant velocity. The constant velocity case also means acceleration is zero and therefore net force on the object is zero.

Answer 2

Forces On Object At Rest

Answer 3

Totally incorrect. In fact, an object might be either moving or flat out still and have forces acting on it.

For example, sit still in that chair in front of your computer. Are you moving? Not if you're sitting still. Are there forces acting on you? Let's see.

Even though you're sitting still, do you weigh anything? Sure you do...your bathroom scale says so. Well, that's the force of gravity acting on you...we call it weight W = mg; where m is your mass and g is acceleration due to gravity (9.81 m/sec^2 on Earth's surface). So there's one force on you whill you are sitting still...at rest, as you put it.

Are there others? Yep...unless you are sinking into the chair, then the floor, then the ground under the house, and so on. That is, there is a force acting upward on you to keep you from sinking into the center of Earth under the weight of gravity, which is pulling you downward.

What is that force...it's your chair pushing back up on your backside. It's the equal, but opposite force that Newton has been known to talk about a lot. So, that's force number two acting on you while at rest in front of your computer screen.

Strangely enough, you could be floating out in space, moving with some velocity v and have no forces acting on you at all. This is yet another of Newt's laws. This one says a body will continue on a straight line of travel (with a constant velocity) without a force applied. Momentum p = mv is you floating outside the gravity, with no forces on you. But you are still moving at constant velocity v. But if you were to want to change that velocity (either magnitude or direction or both), that would take a force to make that change.

So there you have it...a body, your body, could be moving along in space at constant velocity v without any forces on it. On the other hand, while sitting at rest in your chair, you have at least two, balanced forces acting on you.

Answer 4

If an object is at rest the forces acting upon the object are balanced. Balanced forces are equal to each other and opposite in direction. If the forces are unbalanced the object will be in motion.
Example: the unit used to measure force is the newton (N)

------50 N -------><----------50 N --------these forces are balanced so the object is at rest.

-------50 N-----><----------30 N --------these forces are unbalanced, there is movement in this ---------->
direction

Answer 5

No. Gravity is acting on my ball even though it is on the floor at rest. It just mens it is not in motion.

Answer 6

yeah that guy is right. Also if you pushed up against a brick wall, there would be a force against it (you pushing) , but it wouldn't move. Thats just another example.