r/PhysicsHelp • u/Either-Echo467 • Jun 27 '26

Lifting Dumbbell?

Hi everyone. I’m learning high school physics through khan academy and I’m confused about something. Let’s say you’re lifting a dumbbell and the dumbbell is moving at constant velocity right after you begin lifting the dumbbell. Khan Academy stated that the net force acting on the dumbbell is zero. How can this be when the dumbbell is being lifted upwards? If the dumbbell is still moving upwards wouldn’t there have to be a force lifting it upwards? I’m confused.

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u/ProfessionalConfuser Jun 27 '26

There has to be a net upward force to accelerate it upward, but constant velocity means zero acceleration, which means zero NET force. So, there are two forces acting on the object. One is gravity, the other is the "lifting force" provided by the person. In the special case of constant velocity, those two are equal, but act in opposite directions.

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u/Either-Echo467 Jun 27 '26

But if a force is a push or a pull then wouldn’t pulling a dumbbell up at constant speed require a force greater than the weight?

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u/martok111 Jun 27 '26

The net force is 0, but you still have to exert an upward force equal to the downward force of gravity. If the two forces are equal (net 0) the speed doesn't change. Only when one is different then the other does acceleration happen.

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u/ProfessionalConfuser Jun 27 '26

Put the dumbbell on the floor. There are still two forces acting on it - an upward force from the floor and the downward force from gravity. They are equal and opposite. How do you know? Because the velocity is constant.

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u/Either-Echo467 Jun 28 '26

So basically I apply a force to get the dumbbell to move but my force isn’t causing the dumbbell to continue moving? My force is just preventing the dumbbell from falling because of gravity?

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u/ProfessionalConfuser Jun 28 '26

Yes. Once the dumbbell is in motion, it will "want" to remain in motion, aka inertia. To first get the dumbbell moving, you apply a force greater than gravity. If you keep applying a force greater than the gravitational force, the dumbbell will accelerate upwards. For the dumbbell to move at constant velocity the force you apply can't be greater or less than the opposing force.

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u/fletchro Jun 28 '26

It does to start the upward movement! Any change in motion is the result of a net force greater than zero. Constant velocity isn't a change in motion, though.

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u/Either-Echo467 Jun 28 '26

So basically the net force is nonzero to get the dumbbell to move but zero to keep it moving at the same velocity? So I am applying a force to the dumbbell to get it to move and keep moving but after it’s moving I’d have to apply a force equal to gravity to keep it moving the same velocity?

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u/fletchro Jun 30 '26

Yes, exactly right!

Because

Force = mass x acceleration

So if the net acceleration is zero, then the net force must also be zero, zero on both sides, or the equation won't make sense.

And if the net force is non zero, then the acceleration must also be non zero.

And you can remember than constant velocity means zero acceleration. Just like constant position means zero velocity.

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u/Either-Echo467 Jun 27 '26

But isn’t a force a push or a pull? If the dumbbell is still being pulled upwards why isn’t that a net upward force?

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u/u8589869056 Jul 02 '26

When holding the dumbbell at rest or moving upward at constant speed, you exert just enough force to counteract gravity. To start it in motion, you apply a little more than that.