Newton´s law of motion

In summary, the student was having problems with Newton's Laws of Motion. He needed to find the direction of the third force, which was measured counterclockwise from the x-axis. Using vectors and simple geometry, he was able to find the resultant between the vectors and find that the third force was equal and opposite to the sum of the two forces.
  • #1
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I got problems with with some of my homework problems in physics. Its about Newtons Laws of motion and i have really no idea what to do :cry:

I have an object beind acted upon by three forces and moves with a constant velocity. One force is 60N along the x-axis, the second is 75N along a direction making a counterclockwise angle of 150° with the x-axis. What I need to find out is the direction of the third force, measured conterclockwise from the x-axis.

I am not very good at vectors and trigonometry, so if someone can give me some advice where to start and with what formula, I would very appreciate it.
 
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  • #2
Please note that we DO have a homework help section.

Zz.
 
  • #3
Draw a diagram to start, as they can help (even if they're quite simple). What condition must the forces on the object satisfy for it to be moving at a constant velocity?
 
  • #4
As the object moves with constant velocity, its acceleration is zero , therefore this implies that net force on this object is zero.Place the particle at origin, and display the forces in directions as described in your post . Make use of simple geometry and methods to find resultant between vectors when the angle between them is given . Find the condition when net force on object is zero.

BJ
 
  • #5
well I am pretty sure what your looking for is a blaance from the counter clockwise angle... in such a case, the third force would be 75N acting in a direction that would create a clockwise angle of 150 degerrs with the x axis... that would put it back to a constant stright velocity along the x axis
 
  • #6
@ZapperZ
oops, sry :blushing: first post

@Nylex
I did, but i guess it was wrong or not helpful

What I need to find out is the angle of the third force counterclockwise to the x-axis
 
  • #7
ChinToka said:
What I need to find out is the angle of the third force counterclockwise to the x-axis
Add the two forces (vectors) that you are given. The third force must be equal and opposite to that sum.

Can you find the x & y components of the forces?
 
  • #8
now while your asking... i don´t know. Adding vectors is done by squareroot( x^2+y^2) and I guess the components are 60N and 75N.Squareroot(3600+5625) equals squareroot(9225) equals 96.04. Then 360° - 96 because it is counterclockwise which gives me 263°.
But I think its wrong.

BTW I am doing an independent study of "General Physics I" college, that´s why i have no clue of anything o:)
 
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  • #9
Yes, it's wrong. The first step is to find the x & y components of each force. Given the angle to the x-axis, the components of a vector (A) are:
[tex]A_x = A \cos \theta[/tex]
[tex]A_y = A \sin \theta[/tex]

Given this, find the components of each force. (I suggest you do some reading on vectors and vector addition. Here's a good place to start: http://hyperphysics.phy-astr.gsu.edu/hbase/vect.html#vec2)
 
  • #10
the only angle that is given is 150°, so x and y are "60 cos 150" and "60 sin 150"? I have some trouble identifying all necessary components.
 
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  • #11
150° is the angle of the 75 N force, not the 60 N force.
One force is 60N along the x-axis
Thus its x component is 60 N and its y component is 0. (The angle it makes with the x-axis is 0 degrees.)

Now find the components of the 75 N force.
 
  • #12
Allright, the y component is 0 because there is no angle given. Then the y component of 75N is 210 because of the counterclockwise 150°. The only difference i get when i calculate it is with 150° is (-64.95;37.5) and with 210° (-64.95;-37.5). The y component just turned negative.
 
  • #13
ChinToka said:
Allright, the y component is 0 because there is no angle given.
The y component of the 60 N force is 0 because the angle is 0. (The x-axis is at 0 degrees to itself.)
Then the y component of 75N is 210 because of the counterclockwise 150°.
Huh? How can a component of a vector be greater than the vector itself?
The only difference i get when i calculate it is with 150° is (-64.95;37.5)
These are the correct components of the 75 N force.
and with 210° (-64.95;-37.5).
Where does the 210° come from? The angle is given as 150°.
 
  • #14
my mistake, there were too many numbers flying around in my head

I was able to catch my instructor in his office hours and we solved the problem. BUT he did it too quick for me to understand and the notes he gave me are made by his thoughts, so I can´t follow them. Maybe we can still work on this problem so that I can understand it in full, can we?
 

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 with a constant velocity, unless acted upon by an external force.

What is Newton's second law of motion?

Newton's second law of motion states that the acceleration of an object is directly proportional to the net force acting on the object and inversely proportional to its mass. This can be represented by the equation F=ma, where F is force, m is mass, and a is acceleration.

What is Newton's third law of motion?

Newton's third law of motion states that for every action, there is an equal and opposite reaction. This means that when one object exerts a force on another object, the second object will exert an equal and opposite force back on the first object.

How do Newton's laws of motion apply to everyday life?

Newton's laws of motion can be seen in many everyday activities, such as driving a car, playing sports, or even just walking. For example, when you push a shopping cart, your force is causing the cart to accelerate (Newton's second law) and the cart pushes back on you with an equal and opposite force (Newton's third law).

Can Newton's laws of motion be applied to all types of motion?

Yes, Newton's laws of motion can be applied to all types of motion, including linear, circular, and rotational motion. They are the foundation of classical mechanics and are used to explain the behavior of objects in motion.

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