# Resultant forces

In my text book they said that:

1. If no resultant force acts on an object, it will not accelerate; it will remain at rest or it will continue to move at constant speed
2. If an object is at rest or is moving at a constant speed in a straight line ,we can say that there is no resultant force on it.

Now my question as it concentrates more on the first point.

How if the resultant forces or force are zero still the object moves at a constant speed and some times it does not move (or it is at rest as they say)?

Welcome to the Stack Exchange community Yusuf. The first thing that you need to understand before you understand forces and their effect on bodies is relative motion. Whenever you mention distance or speed of any object, you always mention it with reference to a fixed point. For example, if someone asks you how far away is the school, you will reply that the school is x km away from something. This something can be your house or your friend's house or the leaning tower of Pisa or it can be the center of our galaxy and your answer will differ in each of the above cases.

Speed is also mentioned in reference to something. For example, if you tell your car's speed on a highway to someone, you would say something like, "My car is running at the speed of 120 km/h". This 120 km/h would be in reference to the road but it could also be relative to one of the other cars on the highway or relative to a flight from New Delhi to Dubai or a comet zooming past the sun and as before, the speed would be different in each of these cases.

Now to answer your question, I will take two cases, one in which the object is at rest and one in which the object is in motion relative to you.

Case 1: Imagine a small car that is standing in the neutral gear and has the handbrake disengaged. If a person and his friend are equal in strength and if the person pushes the car from behind and his friend pushes the car from ahead and if they both push the car with same force, the car will not move an inch as both of them would be canceling each other's force.

Case 2: Now imagine the car and the two people doing their pushing on a train that is moving at a constant speed in a constant direction. Let the railway track be frictionless and imagine yourself standing beside the railway track and watching the two people push the car. You would see the car moving at a constant speed but you would not notice any change in the speed of the car. To the two people on the train, it would seem as if the car is still but to you the car would appear to be moving at a constant speed.

So it is simply a matter of perspective because nothing is ever truly in motion if we don't mention the point of reference.

Recall Newton's Second Law, F=ma

This means that resultant forces acting on an object will produce acceleration, which is a change in velocity. Remember that velocity is a vector, which includes both speed and direction. So if an object is accelerating, it has either its speed and/or direction of movement undergoing a change.

If an object is traveling at constant speed and still moving in the same direction, it is not experiencing acceleration, and thus the resultant forces acting on it are zero. This is exactly the same for an object at rest, except that its speed happens to be zero.

• So @Flora Su when they say forces makes objects moves they that it makes it accelerate it does not get it in to moving with a constant speed?? and to get the object at a constant speed you do not need any forces to act on it ?? – Youssef Ahmed Apr 21 '16 at 17:28
• Right, exactly. In real life, we don't see this happening - you send something flying through the air, or rolling along the ground, and it'll stop eventually. The reason for this is because frictional forces act on the object, which cause it to lose kinetic energy and come to a stop. So, then, in real life, if you wanted to keep something moving at a constant speed, you keep pushing on it with enough force to cancel out the frictional forces acting on the object. And then you will have essentially no resultant forces acting on the object :) – Flora Su Apr 21 '16 at 23:42
• oh that helped alot....!!!! – Youssef Ahmed Apr 29 '16 at 8:18

A small and simple analogy to get the basic insight:

If each month, you spent as much as you earn, you will have constant savings. If you started without savings, you will continue to have zero savings. If you started with \$1000, it will remain \$1000. In short: with net revenue zero, your savings are constant. Change revenue in force and savings in velocity, and you get what you need.

Basically, if no **net ** force is acting on a body, it's acceleration is zero...

Now, acceleration is the change in velocity in unit time...

So, if velocity is constant or zero, it's acceleration is zero...

Hope this helps!!!

For an object to change from its state of rest to motion needs some force. When a force is applied the velocity of the object changes from 0 to some velocity V (and it keeps increasing as long as the force is present). This change in velocity is termed as acceleration.

Now if you remove the force, the object will move with the constant velocity V provided no other force is acting on it.

You might be confusing with pushing an object forward to maintain its motion.

For example, we pushed a CART forward and we got to maintain pushing it to keep it moving forever. Then, when we let go it will stop.

This is just because there is friction, this friction will cause the cart to stop, remove that and the cart will move forever! In real life Earth scenario however, this seldom happens, but push a cart in outer space and viola! It says goodbye us!