# Relationship between constant velocity and force

### how do you move an object at constant velocity? It doesn't matter where in the universe you are, a newton of force always feels the The question is: what is the relationship between acceleration and velocity . There is no force on an object moving with constant velocity v in a vacuum. Understanding The Relationship Between Force And Acceleration: Example Question #2. A crate slides along the floor with a constant velocity. What is the net . Newton's second law says that when a constant force acts on a massive body, it causes it to accelerate, i.e., to change its velocity, at a constant.

This stationary background is called a reference frame. The change in position in a reference frame is measured in terms of the distance traveled by an object from a fixed point. The distance that an object moves from this reference point per unit of time is called speed.

### Big Ideas for Forces and Motion

When the direction of motion is included with speed, this is called the velocity of the object. Most objects do not move at a constant velocity through the distance that was traveled. Sometimes there are times of acceleration and deceleration, in which the object is speeding up or slowing down. Acceleration can be calculated by dividing the change in velocity by the time.

A force is a push or a pull that can change an objects velocity and therefore its acceleration. The net force is the combination of all of the forces acting on an object, which determines whether the velocity of the object will change. When the combination of forces acting on an object combines to produce a net force of zero, the forces are balanced. When the combination of forces acting on an object combines to produce a net nonzero force, the forces are unbalanced. We know that this is 49 newtons. We know the component of gravity that is perpendicular to the plane-- we figured out this two videos ago-- is 49 square roots of 3 newtons.

We know that this block is not accelerating in this normal direction, so there must be some force counteracting gravity in that direction.

And that's the normal force of the wedge on the block. So that is going in that direction at 49 square roots of 3 newtons. And now instead of assuming that this block is stationary, let's assume that it's moving with a constant velocity. So now we're dealing with-- let me do that in a different color. So now we're dealing with a scenario where the block has a constant velocity. And for the sake of this video, we'll assume that that constant velocity is downward.

And so the constant velocity, v, is equal to-- I don't know. Let's say it is 5 meters per second down the wedge, or down the ramp. Or I guess we could say in the direction that is parallel to the surface of the ramp. So it's in this direction right over here. So that's the constant velocity. So what are all the forces at play?

And be very careful here. There might be a temptation that says, OK, there's a net force here. So maybe that's the net force that's causing the move. But remember, this is super important. This is Newton's first law. If you have a net force, if you have an unbalanced force, it will cause it to accelerate. And we are not accelerating here. We have a constant velocity. We are not accelerating here. So if you're not accelerating in that direction, then that means that the force in that direction must be balanced.

## Force of friction keeping velocity constant

So there must be some force acting in the exactly opposite direction that keeps this thing from accelerating downwards. And so it must be exactly 49 newtons in the opposite direction. And as you can imagine, this is the force of friction. This right over here is the force of friction. And the difference between this video and the last video is last time friction was static. Even at 49 newtons, the box was stationary. You had to keep nudging until you get to 50 newtons, and then it started moving.

Here we're just jumping into this picture where we just see a box that's moving down the slope at 5 meters per second. So we don't know how much force it took to overcome static friction.

But we do know that there is some force of friction that is keeping this box from accelerating, that's keeping it at a constant velocity, that is completely negating the parallel component of the force of gravity, parallel to the surface of this plane. So given this, let's calculate another coefficient of friction. But this is going to be the coefficient of kinetic friction, because now we are moving down the block.

And I'll do a video on why sometimes a coefficient of static friction can be different than the coefficient of kinetic friction. So the coefficient of kinetic friction-- we'll write it.

So this is the Greek letter mu, and we put this k here for kinetic, or we can kind of say moving friction. It's going to be equal to the force of friction, or I should say the magnitude of the force of friction over the normal force. I should say the magnitude of the normal force. And you can derive this experimentally. One, if you just observe this whole thing going on and you knew the mass of the block, so you knew this component of gravity that's going in this direction.

If you knew this angle was 30 degrees from the last situation, you could figure out this coefficient of kinetic friction.