# Simple, but confusing question about Newton's laws

• AznBoi
In summary: There will also be a moment of inertia about the vertical axis for each object in the stack. In summary, the normal force for a stack of objects is the sum of the individual forces acting on the objects. The magnitude of the normal force depends on the mass and velocity of the objects in the stack.
AznBoi
Ok, if you need to determine a tension in a string that is being pulled by a human that is on a sled (the sled is accelerating outwards). Is the normal force the two weights combined??

I have Friction force + (- Tension)= ma

But for the friction force I don't know whether or not I need to find the human's normal force or the sled or both.

The human isn't moving, the sled is pulling away ---> but he is holding onto a string attatched to a wall

l------O The O is the human and he is standing on the sled.

Thanks!

AznBoi said:
Ok, if you need to determine a tension in a string that is being pulled by a human that is on a sled (the sled is accelerating outwards). Is the normal force the two weights combined??

I have Friction force + (- Tension)= ma

But for the friction force I don't know whether or not I need to find the human's normal force or the sled or both.

The human isn't moving, the sled is pulling away ---> but he is holding onto a string attatched to a wall

l------O The O is the human and he is standing on the sled.

Thanks!
It depends what you're considering to be your system. If it's just the human, then the normal force is applied by the sled on the human. If the system is the sled and the human, then the normal force is the force applied by the Earth on the sled. Remember that Newton's laws don't apply when the relative body is itself accelerating.

So which one do I use? Do I add their weights together to get the normal force? Only the human is holding onto the string, but he is not moving. The sled is accelerating from below him, he is standing on the sled and resisting the motion because he is holding the string. =/

AznBoi said:
So which one do I use? Do I add their weights together to get the normal force? Only the human is holding onto the string, but he is not moving. The sled is accelerating from below him, he is standing on the sled and resisting the motion because he is holding the string. =/
For stacked objects in general there are multiple normal forces. Draw a free body diagram for each object in the stack. If there is no vertical acceleration, each object must have zero net force in the vertical direction. As you move lower in the stack, the normal force magnitudes will increase.

## 1. What are Newton's laws of motion?

Newton's laws of motion are three fundamental laws that describe the behavior of objects in motion. The first law states that an object at rest will stay at rest, and an object in motion will stay in motion at a constant velocity unless acted upon by an external force. The second law states that force is equal to mass multiplied by acceleration. The third law states that for every action, there is an equal and opposite reaction.

## 2. How do Newton's laws apply to everyday life?

Newton's laws can be applied to everyday life in many ways. For example, the first law explains why objects stay in motion unless acted upon by a force, such as a ball rolling on a flat surface. The second law can be seen when pushing a shopping cart - the harder you push, the faster it will go. The third law can be seen when a rocket engine propels a rocket forward by expelling exhaust gases in the opposite direction.

## 3. What is the difference between mass and weight in relation to Newton's laws?

Mass is a measure of the amount of matter in an object, while weight is a measure of the force of gravity acting on an object. In relation to Newton's laws, mass is used in the second law to calculate the force needed to accelerate an object, while weight is a factor in the gravitational force acting on an object.

## 4. How do Newton's laws relate to each other?

Newtons's laws are interconnected and build upon each other. The first law provides the foundation for the second law, which uses the concept of force from the first law. The third law can be seen as a result of the first two laws, as it explains the equal and opposite forces acting on objects.

## 5. Why are Newton's laws important?

Newton's laws are essential in understanding the behavior of objects in motion and are the basis of classical mechanics. They are used in various fields such as physics, engineering, and astronomy to explain and predict the motion of objects. Without these laws, many technological advancements, such as airplanes and satellites, would not be possible.

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