Speed and force are quite different things.
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Drag force, or the force of air friction for a falling body, increases with speed. A falling object will reach a speed at which the force of air friction will be equal to and opposite the force of gravity. At that point, the object will no longer accelerate. It's speed will remain constant, and we call that speed (and direction) its terminal velocity.
The 'speed' of a body cannot be negative. But if you are stating the 'velocity' then it can be negative. Negative velocity means it is going opposite to the direction that you decided to call the positive direction.
The speed. Also, if a positive slope represents the speed in one direction, the negative slope is the speed in the opposite direction.
Velocity is a vector, which means it has a direction, but speed isn't. Speed is the absolute value of velocity. Velocity can be negative, meaning that the speed is opposite to the direction that you're calling the positive direction.
(-)11,666.67 N. To calculate this, you need to use the impulse-momentum principle, whereby the change in momentum is equal to the force multiplied by the time over which the force is applied. The change in momentum here is the final speed x the mass - the initial speed x the mass. Then divide the answer by the time (six seconds) and the answer will be the force applied (in this case the answer is negative as the force is applied in the direction opposite to the direction of the truck's motion.)