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Newton's 2nd Law

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.

  • Intro 0:00
  • Question 1 0:16
  • Question 2 0:55
  • Question 3 1:50
  • Question 4 2:40
  • Question 5 3:33
  • Question 6 3:56
  • Question 7 4:29

Transcription: Newton's 2nd Law

Hi folks and welcome back to

Today we are going to do another mini-lesson and this one is on Newton's Second Law, so we are going to go through the first page of the APlusPhysics worksheet that you can download at the link here below.0002

Take a shot at that and then we will check out the answers.0012

Number 1 -- We have a constant, unbalanced force, so that means we have a net force that is going to be equal to some constant value that is not 0 -- it does exist.0016

It is applied to an object for a period of time.0028

Which graph best represents the acceleration of the object as a function of time?0030

Well, if Fnet is equal to a constant, net force is also equal to mass times acceleration.0035

If mass does not change, then acceleration must be constant as well.0042

We are looking for the graph where we have a constant acceleration and that obviously is right there, Number 4.0046

Number 2 -- The diagram shows a horizontal 12 N force being applied to two blocks, (A) and (B), initially at rest on a horizontal, frictionless surface.0055

Block (A) has a mass of 1 kg and block (B) has a mass of 2 kg. Find the magnitude of the acceleration of block (B).0064

The way I would do this, is, as I look at this (B) is not going to accelerate unless (A) does.0072

(A) and (B) are going to move together in this case as if they are one object, so let us make this simple.0078

Let us treat them as one object with a mass of 3 kg.0083

We know then that the net force is equal to mass times acceleration according to Newton's Second Law, so acceleration is net force divided by mass or 12 n/3 kg, which will be 4 N/kg or 4 m/s2.0086

Great. On to Number 3 -- Which body is in equilibrium, with no net force on it?0109

A satellite moving around Earth in a circular orbit -- it cannot be that one because anything moving in a circular orbit has a net force on it that has a centripetal force, a force pulling it toward the center of the circle.0115

Number 3 -- A cart rolling down a frictionless incline -- that is going to be accelerating because the longer it goes down the incline, the faster it is going to go.0126

An apple falling freely toward the surface of the Earth -- well, of course that has a net force on it -- it has gravity pulling it down.0133

A block sliding at constant velocity -- well there we go.0140

A constant velocity -- whenever you see that think acceleration = 0, therefore net force which is equal to mass times acceleration, must also be equal to 0, so 4 would be the one in equilibrium because it says constant velocity.0143

Number 4 -- The weight of a typical high school physics student is closest to...?0160

Well, I do not know that off the top of my head, but let us think about it for a minute.0165

The mass of a typical student is somewhere in the 60 - 70 kg range, so the weight which is the force of gravity on them would be mass times the acceleration due to gravity and notice how this formula parallels to net force equals mass times acceleration.0169

Acceleration due to gravity is acceleration; force of gravity is net force.0188

This is really just a specific version of net force = ma, Newton's Second Law.0193

In either case, (mg) will be the objects weight and if 60 kg is the weight, (mg), if we assume (g) is around 10 m/s2, that is going to be 600 N.0199

Number 5 -- Base your answers to the question on the diagram below, which shows a 1 N metal disc resting on an index card mounted on top of a glass.0214

What is the net force on it?0223

It looks like the disc is in equilibrium to me; it is just sitting there; it is not accelerating, therefore the net force on it must be 0 N.0225

As a follow-on, when the index card is quickly pulled away from the glass in a horizontal direction, the disc falls straight down.0235

What is that action a result of?0244

That is a result of the objects inertia or its resistance to being accelerated; it is happy where it is, so you pull the index card out quickly with very little friction and the disc falls straight down.0247

This same trick you have seen done with probably plates and silverware on a table.0260

Let us take a look at one more -- A student is standing in an elevator that is accelerating downward.0267

Here is our elevator, there is our student, and the elevator is accelerating downward.0273

The force the student exerts on the floor of the elevator must be...?0280

Well, we can do this one probably by common sense.0285

Think about how you feel in an elevator if the elevator accelerates down below you.0287

For a split second do you not feel like you are a lot lighter; your stomach may actually rise up in your innards for a few minutes and you feel a little queasy.0291

As that elevator is accelerating down, the normal force on the floor is actually less because it is accelerating away -- you do not have to have as much force from the floor of the elevator on you.0300

So, the force that the student exerts on the floor of the elevator must be less than the weight of the student when it rests as you feel lighter for a moment.0314

That concludes page 1 of the worksheet on Newton's Second Law.0324

If this went well for you -- Terrific -- move on to some of the AP level questions and if it did not, now would be a great time to go back and review the less on Newton's Second Law.0328

Thanks so much for your time everyone and make it a great day!0337