# Somebody solve this mystery for me.

#### avito009

As I was explaining Newtons First Law. I said " An object will remain at rest or in uniform motion in a straight line unless acted upon by an external force."

Dan said "What if a person is inside the car and handbrakes aren't applied and he pushes the car from the inside, the force is internal since he is inside the car."

My argument is that it is still an external force. How?

Let me make it easier. Tyre generates friction in three major ways adhesion, deformation and wear.

Adhesion: Adhesion is the property of tyre that causes it to stick to other material. Adhesion generally thought to be the result of momentarily molecular bonding between two surface. Adhesion friction is the major contributor in tyre traction. Tyre in contact with a smooth surface generates friction force mainly by adhesion.

Deformation and wear friction has less contribution in tyre traction on smooth surface.

So it could mean that Adhesion is not all that great in this case or happens later on after the car moves a certain distance.
Deformation and wear friction are ruled out.

So the car isnt moving because the car is pushed forward by Dan but because the friction is reduced to zero at the point. But even in case of a ball
when you slide it horizontally on a frictionless surface you are exerting a force initially then it moves horizontally forever due to no friction

Which makes us find out whether the internal force of Dan is unbalanced.

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#### studiot

There is nothing to find out.

All internal forces are balanced, unless the object is changing shape, which rigid bodies don't, by definition.

(Gosh I'm a poet that don't know it)

He pushes with his hands and arms onto the dash.

But unless he also pushes back onto the seat with his back or feet or something with a force that is equal in magnitude, but opposite in direction, he will simply move himself within the vehicle.

I will leave you to put this in formal terms, perhaps using N1,N2 and N3.

A further twist, what is the situation if the man is standing on the ground and leans into the car to push on the dash?

• 1 person

#### avito009

Simplification.

Let us simplify this way

Dan's man is pushed towards the seat as he exerts force on the dashboard. So here:

The force of the seat pushing on him = force of this man on the dashboard + force required to move the car forward.

Which explains the internal forces to be equal. Is this correct?

#### studiot

Let us simplify this way

Dan's man is pushed towards the seat as he exerts force on the dashboard. So here:

The force of the seat pushing on him = force of this man on the dashboard + force required to move the car forward.

Which explains the internal forces to be equal. Is this correct?
Well that's progress. However you have not expressed it formally, as I requested.

Here is a start.

By Newton's third law (N3) the dash pushes back on Dan's man with equal but opposite force to the push the man exerts on the dash.

You need to employ two of Newton's laws to complete this.

It may sound stuffy and pedantic but this chain of reasoning leads to important conclusions in every part of statics and dynamics, including rotational dynamics.

And how about my second question?

Thinking about these helps you learn.

• 1 person

#### avito009

Newtons Laws.

Studiot, you have explained the third law of newton working here. Now let me put up the first and second law of newton here.

Newtons First Law: The car moves ahead as Dan's man pushes on the dashboard. Here there is an external force which is the force of Gravity. You see if the road was perfectly smooth the car would have not moved on its own. But there must be a slight slope on the earth where the car stands. Similar to a ball on an inclined plane. On the inclined plane there is a force of gravity. So here gravity is the external force. Due to the net external force of gravity and reduced friction the car moves ahead.

Newtons Second Law: F= ma. There is force only when there is acceleration. The car is initially accelerated from zero speed to say one unit of speed. Which is acceleration of 1 unit. The car moves due to this acceleration. Again read what I said in other posts. F=ma works as a formula for gravity. Lets say mass of the car is 1 kg. So force due to gravity = 1 x 1 = 1.

So we are right that it is the force of gravity due to which the car moves.

Newtons Third Law: You exert a force on the dashboard and the car moves a bit ahead. Equal this with the force pushing on you by the stationary seat. There is an equal and opposite force acting. Which means the force is balanced.

According to N3 there is a balanced force. But according to N1 and N2 force of gravity is unbalanced from the force of friction. But gravity is an external force.

Conclusion: Internal forces on the car are balanced and external forces namely gravity is unbalanced.

#### avito009

Gravity as External Force.

Here "Car" is the system so gravity is an external force. But if Car-Earth system is taken then gravity is an internal force. This is because the mechanical energy is conserved in the Car-Earth system.

#### studiot

You have posted in 'Advanced Mechanics', but let me take to back to early stuff for a moment.

When we first meet 'forces' in mechancis we are often told that a force is a push or a pull.

No distinction is made as to types of force, that comes later in the more advanced stuff.

So moving on we distinguish external forces (engineers call them loads) and internal forces. The difference being that the conditions of the body have no influence on the loads, they are entirely determined by external conditions.
Internal forces, however are determined by both the loads and the internal conditions of the body.
The internal conditions are the spatial arrangement/configuration of the body for rigid bodies and both the spatial arrangement and the material properties for non rigid (deformable) bodies.

Further consideration of the interaction between internal and external forces leads to the idea of surface forces and body forces.

Surface forces act (suprisingly ) at the surface of the body and may be resolved into two forces one parallel to and one perpendicular to the surface at any point.
It is these types of force are considered point loads.

A small difficulty arises with body forces since they act within the body, but are still 'external' loads.
That is why we should be careful with the term 'external'.

Gravity is one such force.

The point is that these body forces interact act differently within the body's own internal forces and with the external loads.

From the point of view of the external loads we consider the body force to act at a single point, called the centre of gravity. The force acting at the COG is the combined effect of all the internal actions of the body force.

From the point of view of the internal forces a body force acts in a distributed way throughout the body, as though is was lots of tiny parallel forces acting on each point of the body.
Alternatively we can consider the body to be subdivided into lots of tiny but touching free body parcels with the effects of passed on through the body via the surface forces that act at the touching boundaries, according to Newton's laws.

In statics the study of the interaction between the external loads and the internal forces is called structural mechanics, which does not involve the use of the material properties of the body (apart from self weight). This is used for calculations involving rigid bodies and also for calculation of the imposed load distribution on deformable bodies.
When the material properties are used to calculate deformation/displacement, the subject is then called striuctural analysis.

• 1 person

#### avito009

Well the answer is really simple if you see. When a person applies a force on the dashboard and the car moves forward he is applying an external force.

Take for example a baseball bat hitting a ball. Here there is an external force on the ball by the baseball bat. So in our example we can say that the hand of the person inside the car is the baseball bat and the ball is the dashboard.

So answer is simple it is an external force applied on the car which moves it forward. So the person who asked me the question stating that it was an internal force since the driver was in the car was wrong.

Internal forces are: The gravity forces, magnetic force, electrical force, and spring force. Since none of these are involved in our question it proves that the force applied is external.

#### topsquark

Forum Staff
Well the answer is really simple if you see. When a person applies a force on the dashboard and the car moves forward he is applying an external force.

Take for example a baseball bat hitting a ball. Here there is an external force on the ball by the baseball bat. So in our example we can say that the hand of the person inside the car is the baseball bat and the ball is the dashboard.

So answer is simple it is an external force applied on the car which moves it forward. So the person who asked me the question stating that it was an internal force since the driver was in the car was wrong.

Internal forces are: The gravity forces, magnetic force, electrical force, and spring force. Since none of these are involved in our question it proves that the force applied is external.
However I was stating that no external forces act on the car. I should have mentioned gravity but the car does not move vertically either so it cancels out and has no effect on the car.

If the car is moving then we have to be doing work on it. Work, in it's most general definition, is defined as a change in one form of energy to another. In this case we have chemical energy (from ATP or some other biological process) being converted to kinetic energy. So there is an imbalance.

If it helps I was never considering a constant force on the car. It has to be done by "lunges" or some other gross body movement of Dan's man (I like that), not a steady push on the dashboard.

-Dan

#### lovebunny

Yes I agree and believe this is an extremely important fact that is being described but in every theory there is also a fundamental part that does not always seem real which I hope you will discuss more completely but In all that is being said it is extremely inspiring.