What is the force required to stop a moving object?

Answer 1

Any force can stop a moving object, so long as the force is opposite in direction. however, the object will only be stopped for a single point of time before the force pushes the object in the opposite direction

for example, if you throw a ball in the air, gravity will slow down the ball, stop the ball at its azimuth, and then propel the ball back to earth

At this point in your physics career you might be aware of Newtons first law, inertia: "An object will continue to remain at rest or travel in its current direction unless acted on by an unbalanced force." Therefore, in an ideal situation (i.e. most physics questions given to you in the classroom), your object will have no forces acting to slow it down or resist its motion. As a result, any deceleration on the object, whether caused by the brakes or crashing into another object, will stop the motion of the object. Please note that this above example refers only to ideal situations in physics questions.

More importantly, in real life, there are all sorts of forces that act to stop the motion of an object. Without any internal input of force (i.e. our own braking, etc), there are already many forces that will resist the motion of an object. Taking the motion of a car along a road for example, we can see several at work. Firstly, there is air resistance, which is basically the presence of molecules of air hitting the surface area of your car as you push forward. There is also friction upon the road, which allows you to actually move your car - without friction there would be no differential in force, and hence you would be unable to drive. This friction is then turned into heat energy as well, as you may notice that your tyres get rather hot after driving for a long time. Any internal force that we apply will seek to decrease the force by reducing our input of force, and by adding a resistance in the opposite direction.

On the molecular scale, we can also take in to fact a couple of other forces that may reduce the motion of an object. The Brownian motion of molecules dictates that their random collisions will exert a net pressure upon the system, and the more that we compress the gas, we find that we will increase the temperature (remember that temperature is a measure of the average kinetic energy of the system), and the pressure, and hence the force exerted on other molecules within the system.

Hope this isn't too complicated! I've arranged it so that we move from simple --> difficult in this response :) PM if you need more specifics :)

Friction is a force that always opposes motion. No matter what direction the object is moving in, friction will always slow down an object.

The Normal reaction is another force that prevents an object from moving. Consider a box on a table. The box exerts a force downwards on the table but it doesn't 'sink' into the table. This is because the table is providing a force equal to the weight of the box to stop the box 'sinking'.

The two forces are linked together, Frmax = μR

A heavier object will result in a greater Normal reaction, R, so the maximum magnitude of friction, Fr, increases proportionally with mass. μ is the coefficient of friction which is between 0 and 1.

Physics studies tells that friction is the force which causes the objects to stop.

Example. When the car has to stop on red signal, it slows due to the friction between brake and wheels.

Answer 2

You only asked for two examples, so I will try to elaborate slightly more off of my examples.

Forces that can stop objects can be any force that pretty much exists (for the most part) Very common examples include air resistance Fair, friction Ff, gravity Fg, any normal forces Fn, etc.......

Moving objects have inertia (Newton's First Law) and therefore, will travel on for an infinite distance unless acted upon by another force that can counteract the original force. This force that counteracts the original force applied to an object has to create a Net Force of 0 in the object (FNET). When the object has a net force of 0 it means there more than are no net forces acting on the object in any direction nother. This means the object is not accelerating. This does NOT mean it is at rest. The state of equillibrium is caused by an equilibrant force. The equilibrant force is what puts an object at rest, because all forces are cancelling each other out. The equilibrant force cancels out the other forces.

This puts an object at rest.

Answer 3

That would be friction.
On a related note, air resistance will also show similar effects.

Answer 4

If you specify what object and what force and in what conditions, it might be possible to answer without the generic answer, "an equal and opposite force".