When an object is moving at constant "steady" velocity, the resultant force is equal zero.

But I can not understand how the object moves and at the same time, backwards and forwards forces are the same, because it moves forwards?

To solve this problem, I imagine that the car has already moved at the road so there is a resultant force on it forwards, but if there is an additional force acting on car forward force equal to 100 N, so at the same time must be another force acting on the car at the opposite direction with the same value to keep it moving at the steady speed.

Does the above explanation is right or wrong?

(I know that F=ma, so if acceleration is equal zero, then resultant force will be zero , but I want to understand this case logically).

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    $\begingroup$ There are frictional forces on the car that are equal in magnitude and opposite in direction to the forward force provided by the car's engine. Also note that Newton's first law applies here. An object in motion tries to maintain its state of motion unless acted on by an outside force. $\endgroup$ – David White May 27 '19 at 17:54
  • $\begingroup$ When frictional force on the car is equal to forward force so the car stops!!! $\endgroup$ – Rami ki May 27 '19 at 18:10
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    $\begingroup$ @Ramiki Why would it stop; there's no unbalanced force to reduce the velocity. $\endgroup$ – JMac May 27 '19 at 18:35
  • $\begingroup$ Rami, you need to get over your misconception that it takes a net force to keep an object moving ... it doesn't. $\endgroup$ – David White May 27 '19 at 19:40
  • $\begingroup$ Thank you so much $\endgroup$ – Rami ki May 27 '19 at 20:21

Force causes a body to accelerate - if $F=0$, then $a=0$, and so the body continues to move at the velocity it has.

In your example, a force is required to move the car from rest ($v=0$) to a non-zero velocity. Once it is at a velocity, it requires no force to maintain that velocity. Thus, if forward and backward forces are equal, $F_{\text{net}} = 0$, and so the body continues to move at the velocity it had before forces were applied.

  • $\begingroup$ So can we say that the total force acting on the car in the same direction of non-zero velocity because it is moving in this direction? $\endgroup$ – Rami ki May 27 '19 at 18:03
  • $\begingroup$ @Rami No, we cannot say that. If the velocity is constant, the acceleration is zero, and so is the total force. $\endgroup$ – PM 2Ring May 27 '19 at 18:40

I find this to be a common challenge when learning Newton's laws. Newton uses a very precise meaning of force. This is close-but-not-quite the same as your intuition, so it takes a little while to get used to this. Netwon's law states that if the forces on an object are equal, then the object does not accelerate. This means it will keep a constant velocity.

Where your intuition is likely faltering is that many of the most natural forces we deal with are not constant. They change with position. For example, you'll learn a spring pulls with more force if you stretch it further. In these sorts of cases, you'll typically see unequal forces appear and you'll see the changes in velocity that you and I both intuitively expect to happen.

If I may offer an example you can try yourself, find a nice flat section of road with no cars (like a parking lot) and a friend/parent/etc. Have them sit in the driver's seat (for safety) and put the car in neutral. The car wont move. Now, go to the rear bumper and start pushing it. No surprise, the car will start to accelerate. The forces on it are not in balance.

Get it up to a reasonable speed, and then stop pushing. Now we should all agree that there are no forces on the car acting in the direction of motion (just gravity and the normal force from the ground... which goes up and down rather than forward and backwards).

What do you expect to happen? Do you expect the car to suddenly halt because you stopped pushing it? No, you'll find the car happily keeps moving in a straight line, at whatever velocity you pushed it. Remember that friend I mentioned was in the driver's seat? Yeah... this is where they'll probably need to apply the brakes.

Now you will notice the car slows down. Why? Because the friction of the bearings on the axles isn't 100% perfect. But they should be close enough to perfect to get you some gut intuition as to what would happen if the axles were indeed perfect. Without their friction, the car would just keep moving forever (or until it hits a wall. Remember your friend in the driver's seat?) The makers of cars have been very interested in minimizing that friction because lower friction means better gas mileage. So they've done a pretty darn decent job of it.

  • $\begingroup$ I appreciate your effort to help me, It is really helpful. So can we state that the forward force of the car will resist the frictional force between the car and the tyre so it keeps moving at the steady speed? $\endgroup$ – Rami ki May 28 '19 at 0:28
  • $\begingroup$ Yes. At a steady speed (such as when driving at a steady speed down the highway), the force of the car's wheels on the road is exactly balanced to the forces of drag in the opposite direction. $\endgroup$ – Cort Ammon May 28 '19 at 0:30
  • $\begingroup$ And, interestingly enough, this happens to be a stable example. If you set the accelerator to a particular position, so that the force from the car's wheels is constant and above the drag forces, the car will accelerate. This increase in speed will increase the drag forces. The car will accelerate slower and slower, until it eventually stabilizes at exactly the point where the forward forces balance the backwards forces. $\endgroup$ – Cort Ammon May 28 '19 at 0:31

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