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  • Intro 0:00
  • Question 1 0:19
  • Question 2 1:05
  • Question 3 2:09
  • Question 4 2:53
  • Question 5 3:17
  • Question 6 4:00
  • Question 7 4:41
  • Question 8 5:20

Transcription: Gravity

Hi everyone and welcome back to

In this mini-lesson, we are going to go through the first page of the APlusPhysics worksheet on gravity and you can download that worksheet from the link right below the video here.0003

I recommend taking a minute, printing that out, giving it a try and then we will come and check our answers.0013

Number 1 -- A space probe is launched into space from Earth's surface.0018

Which graph represents the relationship between the magnitude of the gravitational force exerted on Earth by the space probe and the distance between the space probe and the center of Earth.0023

We know that the gravitational force equals (g)m1m2/r2, so we have this inverse-square law relationship between the distance and the gravitational force.0034

You can also see that as you have greater distances, you are going to have lower forces, so that eliminates Number 1 and Number 4.0048

Because it is an inverse-square law, not a linear relationship, Number 2 must be our best answer.0056

Number 2 -- The diagram shows two bowling balls (A) and (B), each having a mass of 7 kg placed 2 m apart.0066

What is the magnitude of the force exerted by ball (A) on ball (B)?0072

To do this I am going to go to Newton's Law of Universal Gravitation, where force of gravity is m1m2/r2 -- (G) is 6.67 × 10-11 N-m2/kg2, and mass 1 is 7 kg and mass 2 is 7 kg divided by the square of the distance between them, 22.0077

This implies then that we have 6.67 × 10-11 × 7 × 7/4 (22)...0100

...and I end up with an answer of around 8.17 × 10-10 N corresponding to answer Number 3.0113

Number 3 -- A 60 kg physics student would weigh 1560 N on the surface of Planet (X).0130

What is the magnitude of the acceleration due to gravity on the surface of Planet (X)?0137

If the weight (mg) is 1560 N, and if we want (G), the acceleration due to gravity, that is going to be 1560 N/60 kg (mass)...0142

...therefore, the acceleration due to gravity must be 1560/60 or about 26 m/s2, so the correct answer is Number 4.0156

Number 4 -- Earth's mass is approximately 81 times the mass of the moon.0174

If the Earth exerts a gravitational force of magnitude (F) on the moon, the magnitude of the gravitational force of the moon on Earth is...?0179

If Earth exerts a force (F) on the moon, the moon must exert (F) back on Earth, and that is Newton's Third Law.0187

Number 5 -- An object weighs 100 N on Earth's surface. When it is moved to a point 1 Earth radius above Earth's surface, it will weigh...?0197

If there is the Earth -- initially it is on Earth's surface at a distance (r) away, then if it is moved so that it is now a distance of 2(r) away, we have doubled the distance.0206

Remember that the force of gravity is an inverse-square law with a distance -- so effectively, we have doubled the distance, therefore we have cut the force in four, or 1/4, so 1/4 of 100 N is going to be answer Number 1, 25 N.0221

Number 6 -- A container of rocks with a mass of 65 kg is brought back from the moon's surface where the acceleration due to gravity is 1.62 m/s2.0242

What is the weight of the container of rocks on Earth's surface?0251

Weight is just mass times the acceleration due to gravity and our mass is 65 kg, the acceleration due to gravity on Earth is 9.8 m/s2, so that is going to be right around 638 N, answer Number 1.0255

We did not need to know the acceleration due to gravity on the moon since we already knew what its mass was.0274

Number 7 -- The graph below represents the relationship between gravitational force and mass for objects near the surface of the Earth.0281

What does the slope of the graph represent?0289

Slope is going to be rise/run, so we are going to have change in gravitational force divided by mass.0293

If you remember, the gravitational force is also (mg), so (Fg) -- if I divide both sides by (m), Fg/m = G (the acceleration due to gravity), so our slope is going to give us the acceleration due to gravity, answer Number 1.0301

Let us take a look at one more -- A person weighing 785 N on the surface of Earth would weigh 298 N on the surface of Mars.0319

What is the magnitude of the gravitational field strength on the surface of Mars?0327

The way I would do that is we know that on Earth, their weight (mg) is 785 N, so let us find their mass.0332

If I divide both sides by (g), 9.8 m/s2, then mass is going to be 785/9.8 m/s2 or about 80 kg, so on Mars they weighed 298 N, so on Mars, (m) times the acceleration on Mars is 298 N.0340

If we want (g) now, we will divide both sides by (m), which will be 298 N/80 kg (mass), so 298/80 = 3.72 N/kg, so the correct answer there is Number 2.0364

That completes page 1. Thanks so much for your time everybody. Make it a great day!0389