Does AddForce increase force over time?

I had a plane with a sphere on top. I used AddForce to move it in the update (*deltaTime) , but it seemed like it went faster with time.
How do i make it stay the same speed?

 

AddForce, as the name implied, adds a force to the object. This accelerates it into some direction. So by definition, adding a force is basically acceleration. Doing it each frame results in going faster and faster, just like if you kept the pedal pressed in a car. If you want to move with a constant speed there are quite a few options. Staying with the car analogy, you'll have to set a max speed, stop applying force once some velocity has been reached and reapplying it when you dip below that, or simply apply the correct amount of force to stay at some velocity. Some options that come to mind:

• Manually set the position. This gives you full control over the speed, or even acceleration.
• Put a cap on rigbidybody.velocity such that you allow acceleration but have some max speed.
• Instead of a cap, stop applying a force once you reach some velocity, and reapply it once you fall below.
• Directly set velocity. Keep in mind that this is not intended and generalls makes using physics a questionable choice in most situations.
• Properly calculate the force you need to apply to reach some desired velocity. Dont just add the same force each frame. This would probably be the best option, but you'll have to do the math.
  
• ... and whatever else you can think of

 

Just as a complement to Yoreki's comment, everything he said is right, when you use AddForce, the rigidbody's speed increases with time, and this may even generate problems with colliders/triggers depending on how fast you are.

The way I fixed this was by simply adding a "Constant Force" in the inspector for a certain object, that way the object's speed remained, well, constant throughout. Another alternative, as similar as Yoreki mentioned above is using Mathf.Clamp to limit the force you are applying on such rigidbody, which then limits your force between a minimim and a maximum value.

I hope this helps.

 

You should not use

AddForce

in

Update

and also do not multiply the force with

deltaTime

.

Everything interacting with physics should be done in

FixedUpdate

. Many of the physics methods apply to physics timesteps and

Update

can be called multiple times or not at all for a given timestep.

FixedUpdate

, on the other hand, is called for every timestep.

This is important because

AddForce

, depending on the force mode, already takes time into account. I.e.

Force

and

Acceleration

mean applying force/acceleration over the duration of the timestep the method was called in. Adding in another

deltaTime

, which might also reflect the

Update

delta and not the physics delta, will mess things up.

(

Impulse

and

VelocityChange

, however, are intended for forces applied at once and not over a time period. It's therefore safe to call them outside of FixedUpdate but you still don't want to multiply with deltaTime.)

 

I've had fine results by directly setting velocity in Update:

if(isActiveHyperDrive) g.GetComponent<Rigidbody>().velocity=Vector3.forward*10;

keeps it at a constant speed (+10z/second). There's no need to worry about deltaTime or fixedDeltaTime, since velocity is in meters/sec -- the physics system handles movement per frame.

In theory, FixedUpdate is a better spot than Update. If Updates occur more often then I'm wasting CPU time, and if FixedUpdate runs more often then your rigidbody might slow down a little before an Update resets the speed. But I've never noticed a problem. Even going from desktop to mobile the results are consistent.

Using AddForce isn't exactly the same as changing velocity directly. AddForce has a delayed effect -- they seem to be "saved up" until the physics step applies them to the velocity. That might have some effect on certain collisions, but, again, I've never seen anything weird happen.

 

I feel like adding a drag of 1 on the rigidbody fixed it somehow.. That doesent make sense though. What is happening?

 

Drag results in a maximum velocity. The drag force increases with velocity, until it matches your added force and prevents the object from accelerating further. The higher the drag, the lower the maximum velocity will be.

 

I know this post is 4 years old, but still
To anyone trying to understand how to resolve this issue, I've found that subtracting the rigidbody.velocity.magnitude from a speed value and multiplying the force vector by that achieves the constant moving speed effect
Like this

Code (CSharp):

1. void FixedUpdate()
2. {
3.     rb.AddForce(forceDirection * (speed - rb.velocity.magnitude));
4. }

You basically tell AddForce function here to adjust force for the desired speed, not just add it. And if you move an object or player like that the movement speed feels pretty consistent but still realistic
Hope this helps!