# Net Force Questions

#### kye6338

Hi,
I was hoping someone could help me with some general net force questions that will help me answer some of my problems. For example, if a net force acts on an object, does the object's speed and/or velocity change? Furthermore, is the net force that acts on an object which remains at rest equal to zero?

Going off of the velocity question from above, will a car, for example, initially moving at a constant velocity slow down unless a small net force were to be applied? And my final question would be that if two objects have the same acceleration, are they under the influence of equal forces? This is probably only true if the masses of the objects are the same, but I just wanted to be sure. Any help with these questions would be great.
Thanks!

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#### Doc Al

Mentor
kye6338 said:
Hi,
I was hoping someone could help me with some general net force questions that will help me answer some of my problems. For example, if a net force acts on an object, does the object's speed and/or velocity change? Furthermore, is the net force that acts on an object which remains at rest equal to zero?
Newton's 2nd law tells us that $F_{net} = m a$. Now apply it to your two questions. (1) If there's a net force on an object, it must be accelerating. (What does acceleration mean?) (2) If an object remains at rest, what is it's acceleration? Thus, what must be the net force on it?

Going off of the velocity question from above, will a car, for example, initially moving at a constant velocity slow down unless a small net force were to be applied?
If the net force on the car were zero, the car would never slow down (or turn). In real life, friction and air resistance act on the car to slow it down. So an additional force must be applied to balance those forces. When you are driving down a straight road at constant speed, Newton's law tells you that all the forces acting on the car (air resistance, the ground, gravity, etc.) must add to zero.

And my final question would be that if two objects have the same acceleration, are they under the influence of equal forces? This is probably only true if the masses of the objects are the same, but I just wanted to be sure.
Once again, turn to Newton. If an object is accelerating, the net force on it must equal "mass x acceleration". So the net force must be greater on the more massive object.

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