# Exploring Forces and Motion

• forty
In summary, the conversation discusses whether knowing the net force acting on a moving object allows one to determine the direction of the object's motion. The initial belief is that for a non-zero net force, the direction of the motion will be in the direction of the net force. However, this only applies to a specific case and cannot be generalized. Counter examples are presented, such as an object moving at a constant velocity with a sideways force, highlighting the need for considering initial conditions and the iterative nature of the problem. It is also mentioned that a net force of 0 does not necessarily mean there is no motion, but rather no acceleration. The conversation concludes with the suggestion of creating a force diagram to better understand the relationship between net force and direction

#### forty

If you know all of the forces acting on a moving object, can you tell the direction the object is moving? If yes, explain how. If no, provide a counter example.

At first i thought the answer was yes, that if you knew the net force on the object the object would be moving in the direction of the net force but that only works for a nonzero net force, and also doesn't tell you the direction of motion only the direction of the accelleration which would could be opposing the motion but the object still moving in the same direction (slowing down). If the net force is 0 then the object keeps moving in the direction it was set in originally. So i figure the correct answer is no.

Is my logic right? and as a result my conclusion?

no one will use the term "nonzero net force" because net force itself is non zero so simply by saying there is a net force or no net force is enough.yes there is nothing wrong with ur logic,it is according to Newton 1nd law,a body will continue its state of "uniform" motion or rest if no external force(net force)acts upon it.to know the direction of a body the initial state of motion is needed to determine the direction of the body if it acts by a net force,because the Newton 1st law applies to 2 cases--->body initially at rest and body initially in motion.for a body at rest if the net force is act towards the right on the body then the body will continue its motion towards the right direction with acceleration since F=ma if there is a net force then there will be acceleration.if the motion is initially moving with uniform speed towards the right then if the force acts to the left its direction still remains towards the right but with a deceleration and eventually the body might move to the left with acceleration.if the net force is cause by the friction then it simply stop at its final position(why?anyone can explain this?)if the body in motion towards the right is act by a force towards the same direction as the motion then the motion will move towards the right with acceleration.

that would be a pretty cool thing to model in MATLAB or something actually.

first of all i agree with yc90, but i can probably add to it a bit. you cannot simply determine a direction in which the object traveling because it shall depend on the time the force is acting. my solution would be resolve all the forces on the object into one resultant force so you are left with moving object with force applied to it. To determine the direction it takes it becomes an iterative problem. Obviously after an infinite time, the object will eventually move in the direction of the force because the force will either overcome the objects initial momentum or add to it depending on the direction of the applied force.

I would iterate the problem using Newton's laws of motion and plot a graph. if you were that keen, and know how to do it, set up variables you can enter such as forces, directions, mass of object etc and have your program create a video using your timesteps.

forty said:
At first i thought the answer was yes, that if you knew the net force on the object the object would be moving in the direction of the net force but that only works for a nonzero net force …

Hi forty!

No, it doesn't even work for a non-zero net force … the direction of velocity of a fast-moving object experiencing a given sideways net force will depend on its speed, won't it?

To add to the discussion, you should always look for counter examples first. Don't underestimate them, as they are quite powerful. You don't even need to think about the theory if you have a counter example.

For this question, there are many good counter examples. Just image you have a object moving at constant velocity south with a force acting west. Can you tell from the force that it is moving west? If yes, what if the same object (still moving west) had the force acting on it north? I'll let you finish it... :)

kkrizka said:
Just image you have a object moving at constant velocity south with a force acting west. Can you tell from the force that it is moving west?

if an object is moving at a constant velocity it doesn't have a force in that direction. Fnet=ma. Force=mass*acceleration, not velocity. that's momentum. a net force of 0 just means there is no acceleration. a net force shows an objects acceleration. i would say the easiest way to look at this is draw a force diagram.

rockerdoctor said:
if an object is moving at a constant velocity it doesn't have a force in that direction.

I guess I worded it wrong. I should have said "initial moving at a constant velocity, then a force..."

A good, and quite familiar example, is that of an object being thrown straight up in the air.

The force (gravity) acts downward for the entire time the object is in the air.

Is the velocity always in the same direction as the downward force, the entire time the object is in the air?

## What is the definition of forces and motion?

Forces and motion refer to the physical interactions between objects and the resulting movement or change in position of those objects. It is a fundamental concept in physics that explains how objects behave in response to different forces.

## What are the different types of forces?

There are four main types of forces: gravitational force, electromagnetic force, strong nuclear force, and weak nuclear force. Gravitational force is the force of attraction between two objects with mass, electromagnetic force is responsible for electric and magnetic interactions, and strong and weak nuclear forces govern interactions between subatomic particles.

## What is Newton's first law of motion?

Newton's first law of motion, also known as the law of inertia, states that an object at rest will remain at rest and an object in motion will continue in motion at a constant velocity unless acted upon by an external force.

## How do forces affect motion?

Forces can cause an object to accelerate, decelerate, or change direction. The magnitude and direction of the force determine the resulting motion of the object. The net force acting on an object is the sum of all the forces acting on it and is responsible for any changes in its motion.

## What is the relationship between force and mass?

According to Newton's second law of motion, the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. This means that the greater the mass of an object, the more force is required to accelerate it, and vice versa.