Force of car pushing on a truck

[syllll_213]

Homework Statement

Which of the following is true?

Relevant Equations

F=ma?

HI, when I looked at this question, I thought that due to the acceleration, we should conclude that there is an overall force acting on that direction, and therefore the two bodies will accelerate. And since the car is the only with functional battery, it should exert greater force? Am I wrong at looking at them separately?

 

[phinds]

Is the car exerting a force on anything but the truck?

 

[PeroK]

Homework Statement: Which of the following is true?
Relevant Equations: F=ma?

View attachment 358492
HI, when I looked at this question, I thought that due to the acceleration, we should conclude that there is an overall force acting on that direction, and therefore the two bodies will accelerate. And since the car is the only with functional battery, it should exert greater force? Am I wrong at looking at them separately?

Which of Newton's laws have you forgotten about?

 

[syllll_213]

Which of Newton's laws have you forgotten about?

Is it the third law? But I am unsure why the third law should be the main factor here?

 

[syllll_213]

Is the car exerting a force on anything but the truck?

Its exerting forces on the ground too?

 

[haruspex]

since the car is the only with functional battery, it should exert greater force?

The question concerns the force the car exerts on the truck (and vice versa). What would the car need to exert a force on to accelerate the car?

Is it the third law? But I am unsure why the third law should be the main factor here?

It is not merely a factor, it is an iron law. What does it say?

 

[Orodruin]

Is it the third law? But I am unsure why the third law should be the main factor here?

It is the only factor here. The third law states that the force from object A on object B is always equal in magnitude and opposite in direction to the force from object B on object A. The question is directly testing your understanding of the third law.

 

[syllll_213]

It is the only factor here. The third law states that the force from object A on object B is always equal in magnitude and opposite in direction to the force from object B on object A. The question is directly testing your understanding of the third law.

I see. I was confused because I thought if there is an acceleration, then there should be a net force to the right. But I just realised (through your comment) that they are just asking about the impact of car on truck and truck on car in this care, therefore I should focus on the third law and so the magnitude will be the same? If they were to consider other forces, then I should shift my focus to the net force?

 

[kuruman]

Yes. Suppose the car is pushing on the truck but the two vehicles stay in place because the truck's handbrake is engaged. You are given the same options A - E. Would your answer to the question be different?

 

[syllll_213]

Yes. Suppose the car is pushing on the truck but the two vehicles stay in place because the truck's handbrake is engaged. You are given the same options A - E. Would your answer to the question be different?

yes? I think I would have chosen C instead. That makes some more sense now , thank you so much!

 

[kuruman]

yes? I think I would have chosen C instead. That makes some more sense now , thank you so much!

You are welcome. Now let's complete your understanding of the given problem with the two vehicles accelerating as one. Since the acceleration is not zero, there must be a net horizontal force acting on the vehicles. Where does this force come from?

 

[syllll_213]

You are welcome. Now let's complete your understanding of the given problem with the two vehicles accelerating as one. Since the acceleration is not zero, there must be a net horizontal force acting on the vehicles. Where does this force come from?

Does it come from the car? Maybe the car's friction with the ground when the engine starts to move the car? Does that create an equal magnitude but opposite force on the truck as well which contributes to the net force of the car?

 

[kuruman]

Does it come from the car? Maybe the car's friction with the ground when the engine starts to move the car? Does that create an equal magnitude but opposite force on the truck as well which contributes to the net force of the car?

Yes. Would the car be able to push on the truck if the vehicles were on very smooth and slippery ice?

 

[syllll_213]

NO? Because the friction is too little to contribute to the net force of the car, and so the net force will not be enough to push the truck?
// May I clarify my understanding? So when the car starts its engine, it will exert force horizontal to the truck (3rd law discussed above) . Meanwhile, its tyre will also have friction with the ground, and the ground will push the car with the same force that give it a net force that can move itself and the truck forward? D:

 

[haruspex]

// May I clarify my understanding? So when the car starts its engine, it will exert force horizontal to the truck (3rd law discussed above) . Meanwhile, its tyre will also have friction with the ground, and the ground will push the car with the same force that give it a net force that can move itself and the truck forward? D:

Close.

The engine tries to turn the axle relative to the chassis. I.e. the chassis and the drive wheels exert equal and opposite torques on each other. The torque on the chassis pushes the back wheels down while easing the load on the front wheels; the torque on the wheels pushes the bottoms of the tyres backwards against the road.

By Newton III, the tyre and road exert equal and opposite forces against each other (both vertically and horizontally, but we're just interested in the horizontal forces).
Since the wheels cannot move forwards independently of the chassis, the reaction force from the road on the wheels leads to the wheels exerting a forward force on the chassis.

Try to draw a diagram of all that.

 

[jbriggs444]

So when the car starts its engine, it will exert force horizontal to the truck (3rd law discussed above)

The engine does not exert any force on the car.

Meanwhile, its tyre will also have friction with the ground, and the ground will push the car with the same force that give it a net force that can move itself and the truck forward?

Yes. This is the only force that propels the car forward.

 

[Lnewqban]

HI, when I looked at this question, I thought that due to the acceleration, we should conclude that there is an overall force acting on that direction, and therefore the two bodies will accelerate.

That is correct.
Car and truck accelerate at exactly the same rate.
As "the mass of the truck is much greater than the mass of the car", much greater force will be required on the truck than on the car to reach the same acceleration in both bodies.

And since the car is the only with functional battery, it should exert greater force? Am I wrong at looking at them separately?

That is also correct, if you are comparing the force exerted by the nose of the car when pushing the truck and the force exerted by the nose against the air (car accelerating alone with no truck in its way).

For accelerating only the car (little mass), the maximum possible torque from the engine will induce a greater acceleration than it would for accelerating car plus truck (much greater combined mass).

For accelerating only the car:
-Certain inertial resistance is opposing the torque of the engine from the center of mass of the car.
-No inertial resistance is exerted against the nose of the car.

For accelerating the car and the truck simultaneously:
-Same certain inertial resistance is opposing the torque of the engine from the center of mass of the car.
-Additional and greater inertial resistance is exerted against the nose of the car (transferred to that contact point from the center of mass of the truck.

Note that the second case could be idealized as a single body, or only one car of four axes having a greater mass (equivalent to the combined masses of original car and truck).

In such idealization, the point of contact between car nose and truck rear in our peoblem would behave, regarding internal forces and acceleration, just like any other point along our four axes bigger car.

 

[haruspex]

That is correct.
Car and truck accelerate at exactly the same rate.
As "the mass of the truck is much greater than the mass of the car", much greater force will be required on the truck than on the car to reach the same acceleration in both bodies.

A greater net force, yes, but the external force on the car will be greater than the force the car exerts on the truck.

That is also correct, if you are comparing the force exerted by the nose of the car when pushing the truck and the force exerted by the nose against the air (car accelerating alone with no truck in its way).

I don’t think that is the comparison @syllll_213 was making, but it is unclear what comparison was being made.

 

[Lnewqban]

HI, when I looked at this question, I thought that due to the acceleration, we should conclude that there is an overall force acting on that direction, and therefore the two bodies will accelerate.

That is correct.
Car and truck accelerate at exactly the same rate.
As "the mass of the truck is much greater than the mass of the car", much greater force will be required on the truck than on the car to reach the same acceleration in both bodies.

A greater net force, yes, but the external force on the car will be greater than the force the car exerts on the truck.

Sorry, I don't understand how your statement relates to the two above quotations.
Could you please explain the forces in bold?
Thank you.

 

[haruspex]

Sorry, I don't understand how your statement relates to the two above quotations.
Could you please explain the forces in bold?
Thank you.

It was not as clear as it should have been.
I meant that the force on the car that is external to the car+truck system will be greater than the force the car exerts on the truck.

 

[phinds]

As the truck is moving so I think option B is correct.

Jeez, have you even read this thread? Do you understand Newton's Laws?

 

[phinds]

If both are exerting same forces on each other then why is the truck moving?

see post #20

 

[phinds]

Okay

So, do you now understand what's going on?

 

[haruspex]

If both are exerting same forces on each other then why is the truck moving?

Consider a system that consists of two falling stones not in contact. They exert equal (zero) force on each other, yet both move.

 

[jbriggs444]

If both are exerting same forces on each other then why is the truck moving? I will be thankful to you if you make me clear.

The force that the car exerts on the truck is applied to the truck. This influences the acceleration of the truck.

The force that the truck exerts on the car is applied to the car. This influences the acceleration of the car. It does not influence the acceleration of the truck.

 

[PeroK]

As the truck is moving so I think option B is correct.

See post #3 in this thread:

Which of Newton's laws have you forgotten about?