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Newton's 3rd 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 0:17
  • Question 2 0:44
  • Question 3 1:14
  • Question 4 1:51
  • Question 5 2:11
  • Question 6 2:29
  • Question 7 2:53

Transcription: Newton's 3rd Law

Hi everyone and welcome back to 0000

I am Dan Fullerton and today we are going to continue our mini-lessons as we do the first page of the APlusPhysics worksheet on Newton's Third Law. 0003

Take a minute and download that from the link below and see if you can do it and then let us check our answers together. 0010

Number 1 -- A student pulls a 60 N sled with a force having a magnitude of 20 N. 0017

What is the magnitude of the force that the sled exerts on the student? 0022

If the student pulls with a force of 20 N, then the sled must exert that force back on the student by Newton's Third Law. 0026

The 60 N sled is just referring to the weight of the sled, so this one is all about reading the question. 0036

Number 2 -- If we have a 65 kg astronaut and the astronaut exerts a force with a magnitude of 50 N on a satellite, that she is repairing, the magnitude of the force that the satellite exerts on her is...?0045

Well, it has to be the exact same according to Newton's Third Law -- the force of object 1 on 2 is equal in magnitude and opposite in direction to the force of 2 on 1. 0058

Number 3 -- A 400 N girl -- so that is the weight of the girl -- standing on a dock exerts a force of 100 N -- so the girl is exerting a force of 100 N on a 10,000 N sailboat and that is the weight of the sailboat. 0072

As she pushes it away from the dock, how much force does the sailboat exert on the girl? 0087

Well that is going to be equal and opposite to the force that the girl exerts on the sailboat. 0092

That is 100 N, so this also must be 100 N. 0098

Do not let the weight of the girl or the weight of the sailboat throw you off. It is all about reading these questions carefully. 0103

Number 4 -- A carpenter hits a nail with a hammer. 0111

Compared to magnitude of the force the hammer exerts on the nail, the magnitude of the force the nail exerts on the hammer during contact is...?0114

It must be the exact same -- again Newton's Third Law -- forces come in pairs with the same magnitude, but opposite in direction. 0121

Number 5 -- A woman is pushing a baby stroller. Compared to the magnitude of the force exerted on the stroller by the woman, the magnitude of the force exerted on the woman by the stroller is...?0132

Again, the exact same thing and hopefully we are catching a pattern here -- the same Newton's Third Law. 0142

Number 6 -- When a child squeezes the nozzle of a garden hose, water shoots out of the hose toward the east, so if water is shooting out toward the east, what is the direction of the force being exerted on the child by the nozzle? 0150

Again, according to Newton's Third Law, if the water is shooting out towards the east, the force must be to the opposite side or the west. 0162

Last question -- A 100 kg boy and a 50 kg girl each holding a spring scale pull against each other as shown right here. 0175

The graph below shows the relationship between the magnitude of the force that the boy applies on a spring scale in time. 0182

If you notice it goes from 0 to about 12 N, linearly, over 3 seconds and the question we are being asked now is which graph best represents the relationship between the magnitude of the force that the girl applies on her spring scale in time?0188

Again, they have to be the exact same, but opposite in direction by Newton's Third Law, so the graph that goes from 0 - 12 N linearly is this one, Number 1. 0205

That concludes page 1 of the Newton's Third Law worksheet. 0218

Thanks so much for your time everyone and make it a great day!0222