Newtons second law basically states that a force can be calculated by the product of a mass and its acceleration. Since we know acceleration is the rate of change in velocity, it stands to reason that when were not moving were not exerting a force, right? Actually, one force that always is exerted on you is the gravitational force (gravity), which essentially causes you to always accelerate in the y-direction. Remember this fact when you think of a nonmoving object as one thats not under acceleration. Thankfully because of Newtons 3rd law, which well learn about next, we arent falling through the Earth.
FBDs are tools for visualizing forces on a single object and writing equations to represent a physical situation.
The acceleration of an object is directly proportional to the net force experienced and inversely proportional to its inertial mass.
The net force on an object is the vector sum of the individual forces.
Newton's 2nd Law of Motion
Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.
Hi, I am Dan Fullerton. Welcome back to Educator.com.0000
Let us talk about Newton's Second Law of Motion.0004
Now our objectives are going to be to draw and label a free-body diagram showing all the forces acting on an object and also draw a pseudo-free body diagram showing all components of forces acting on an object.0006
We will explain the relationship between acceleration, net force, and mass of an object, and use Newton's Second Law to solve a variety of problems.0020
Finally, we want to make sure we understand the difference between mass and weight, and the conditions required for equilibrium.0028
Free-body diagrams or FBD's -- these are tools that we use to analyze physical situations.0037
What they do is they show all the forces acting on a single object.0044
Some folks like to draw it as a box. It does not matter, either one.0052
Now when you draw a FBD, choose the object of interest and draw it as either a dot or a box. 0058
Then you are going to label all the external forces acting on the object and only forces go on that diagram.0064
Finally, sketch a coordinate system showing the direction of the object's motion as one of the positive axis.0070
For example, let us take a look at a circus elephant falling off a tight rope -- sad story -- it is just pretend, do not worry.0077
Neglecting air resistance -- draw a free-body diagram for the falling elephant.0085
I am going to use my amazing physics artistic skills to draw an elephant.0089
There it is in FBD terms and I am going to label all the forces acting on it.0094
The weight of the elephant -- the force of gravity -- which I typically write on FBD as mg -- the force of gravity on an object on a FBD can save yourself a little bit of work if you write weight as mg. 0100
Or, we could draw it as a box -- there is our elephant and the one force acting on it is the weight of the elephant pulling it down.0115
How about if we had the falling elephant with air resistance?0127
We have a 25N horizontal force northward and a 35N horizontal force southward acting concurrently -- that means at the same place and at the same time -- on a 15 kg object on a frictionless surface.1277
What is the magnitude of the object's acceleration? Again, let us start with the FBD.1290
There are horizontal forces both North and South, but I am going to draw an overhead view.1296
We have a force of 25N North and we have 35N South.1302
The net force should be pretty easy to see. It is going to be 10N South.1311
The acceleration is going to be the net force divided by the objects mass, which is going to be 10N South, divided by 15 kg, or 0.67 m/s 2 South.1320
What is the weight of the astronaut on Planet X, where the gravitational field strength is 6 m/s 2.1402
On Earth, mg, the object's weight, is 1,000N, therefore, we could say that the mass of the object -- what does not change, is going to be 1,000N/g on Earth -- 10, or about 100 kg.1409
If we go over here to Planet X, mg on Planet X must equal the mass, 100 kg -- that does not change, times g on Planet X, 6 m/s2 -- 100 x 6 = 600N.1427
An alien on Planet X weighs 400N. What is the mass of the alien?1448
On Planet X, mg(x) for the alien must be 400N, therefore, the mass of the alien on X is 400N/g on x, 6 m/s 2, or about 66.7 kg.1454
Take the alien to Earth, it is going to have a different weight.1475
It will not be 400N, but the mass will be the same, 66.7 kg.1478
Coming back to equilibrium. Translational equilibrium occurs when there is no net force on an object, therefore, acceleration is 0.1487
The equilibrant is a name for a single force vector that you add to any unbalanced forces you have on an object in order to bring the object into translational equilibrium.1497
For example, if I have a force that is 25N that direction -- if I want its equilibrant, I need a force that is 25N in that direction so that you add them together -- you get 0.1506
You get no unbalanced forces. You have 0 acceleration.1520
In the diagram here we have a 20N force due North, and a 20N force due East acting concurrently, again at the same place and same time on an object. 1530
What additional force is required to bring the object into equilibrium? Or we are looking for the equilibrant.1540
Now the way I do this is, is if I look here we have 20N and 20N.1549
Let us add them together to get the net force.1554
I am just going to slide this vector over so that they are lined up tip to tail so I can add them -- 20N -- and my resultant, the sum of the two vectors is going to be a vector with the length square root of 20 squared plus 20 squared.1556
That is going to be square root of 20 20 + 20 2 = 28.3N.1572
I could replace that 20N North and 20N East with one vector, 28.3N to the northeast.1583
It's equilibrant, the vector I would have to add to that system to bring it back into equilibrium, must be the exact opposite of that.1589
The equilibrant must be that red vector, which would be 28.3N to the southwest. 1596
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