Why is the force required to lift an object equal to the force with which the gravity pulls it down?

Question

What i mean is that suppose i want to lift 50kg of a stone. The force of gravity on it is 500N(approx) in downward direction. Now If i lift the object with a force of 500N in upward direction that means that two forces are acting on the stone of same magnitude but in opposite direction. In that case the stone should not move up , but it does Why is it so? are we applying more force to lift the stone?

Answer 1

If you apply 500N to a 500N weight, it will have 0 acceleration.
So to lift the weight to height h, you would have 4 distinct phases:

1. You need to apply a small additional force for a short time to accelerate the weight to a certain speed.
Let say you want 1 m/s^2 for 0.1 second
F = mg + ma
F = 500 + 50*1 = 550 N
After 0.1s the weight will have speed
v = at = 1*0,1 = 0.1 m/s

2. You then apply the exact weight to follow through your motion with zero acceleration and constant speed.
F= mg
v = 0.1m/s

3. At height h minus delta, you start decelerating by reducing the force for a short time
Let say you want -1m/s^2 deceleration rate then
F = mg + ma = 500N + 50*(-1m/s^2) = 450 N
And you apply this reduced force of 0.1 s
v = 0.1m/s - 0.1s*1m/s^2 = 0 m/s

4. FInally you reapply a force equal to the full weight to keep the weight in place at alatitude h.

Answer 2

You are correct in that a force greater than 500N must be applied to the stone of make it accelerate.

However, once it has accelerated to a velocity v, balancing out the force of gravity, with cause the stone to continue to move with velocity v as per Newton's First Law (inertia).

Caveat: No other forces are acting on the stone.

Answer 3

That's because: when you are lifting the stone with the force equal to gravity, as you know the force is opposite in direction. The force acting on stone is force of gravity, the stone is lifted upwards only when the force applied is greater than force of gravity. I think you should not get such doubts.

Answer 4

It is true that : F action= -F reaction but an object will move only when the force becomes unbalanced that means the net resultent of all the forces applied is not zero . Moreover the weight of a body is the force applied by the body on the earth so, its weight = F or F=m*a or F=m*g or weight = mass*acceleration due to gravity .

Answer 5

If you only applied 500n to the stone then it would have a neutral weight. It would take more than 500n to lift it.

If you lift a 10lb object and hold it stationary, then you are only applying 10lbs of lifting power, but as lift it higher you are applying more force, hence more than 10lbs of force.

Answer 6

Newton's Third Law

A force is a push or a pull upon an object which results from its interaction with another object. Forces result from interactions! As discussed in Lesson 2, some forces result from contact interactions (normal, frictional, tensional, and applied forces are examples of contact forces) and other forces are the result of action-at-a-distance interactions (gravitational, electrical, and magnetic forces). According to Newton, whenever objects A and B interact with each other, they exert forces upon each other. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body. There are two forces resulting from this interaction - a force on the chair and a force on your body. These two forces are called action and reaction forces and are the subject of Newton's third law of motion. Formally stated, Newton's third law is:

"For every action, there is an equal and opposite reaction."

The statement means that in every interaction, there is a pair of forces acting on the two interacting objects. The size of the forces on the first object equals the size of the force on the second object. The direction of the force on the first object is opposite to the direction of the force on the second object. Forces always come in pairs - equal and opposite action-reaction force pairs.

A variety of action-reaction force pairs are evident in nature. Consider the propulsion of a fish through the water. A fish uses its fins to push water backwards. But a push on the water will only serve to accelerate the water. In turn, the water reacts by pushing the fish forwards, propelling the fish through the water. The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards). For every action, there is an equal (in size) and opposite (in direction) reaction force. Action-reaction force pairs make it possible for fish to swim.

Consider the flying motion of birds. A bird flies by use of its wings. The wings of a bird push air downwards. In turn, the air reacts by pushing the bird upwards. The size of the force on the air equals the size of the force on the bird; the direction of the force on the air (downwards) is opposite the direction of the force on the bird (upwards). For every action, there is an equal (in size) and opposite (in direction) reaction. Action-reaction force pairs make it possible for birds to fly.

Consider the motion of a car on the way to school. A car is equipped with wheels which spin backwards. As the wheels spin backwards, they grip the road and push the road backwards. In turn, the road reacts by pushing the wheels forward. The size of the force on the road equals the size of the force on the wheels (or car); the direction of the force on the road (backwards) is opposite the direction of the force on the wheels (forwards). For every action, there is an equal (in size) and opposite (in direction) reaction. Action-reaction force pairs make it possible for cars to move along a roadway surface.

Answer 7

force = mass X accelleration

Answer 8

yes you are applying more.......this is the principle used to attain zero gravity in flights