Getting a Rocket Into Space

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Physics Powerpoint Presentation on getting a rocket into Space and some of the theory behind it including Newton's Universal Gravitation, Projectile Motion and Travelling in orbit with Kepler's Laws.Classical Physics for the high school student.

Text of Getting a Rocket Into Space

1. The Earth has a gravitational field that exerts a force on objects both in and around it

Gravity and Weight on Other Planets Mass, Weight and Gravity

F = m.g Weight is a measurement of the force on an object caused by gravity trying to pull the object down. Weight is a Vector quantity measured in Newtons. Gravity is an attractive force. Mass is the amount of matter contained inside an objectMore Information and an activity on Weight Force

Weight Weight is a measurement of the force on a object caused by gravity trying to pull the object down. Mars has less gravity than Earth. Therefore if you were on Mars you'd weigh less because the force of gravity wouldn't be as strong as it is here on Earth. Does this mean you would suddenly be thinner on Mars? No. You would have the same amount of mass as you do on Earth. (Mass is the amount of stuff inside an object.)

Gravity Gravity is an attractive force. This doesn't mean it's pretty. What "attractive" means is that an object's gravity pulls other objects toward it. The Earth's gravity naturally pulls us, and everything else, toward the centre of the planet, which keeps us from drifting off into space. The Earth isn't the only thing that has gravity. In fact, every single object in the universe has gravity. The tables you're sitting at have gravity. They are pulling you towards them. You have gravity, and you are pulling the tables towards you. We can't see or feel these things happening because people and tables have a such a small mass that the effects of gravity cannot be seen.

Mass Mass is the amount of stuff contained inside an object. It takes a lot of mass to make a lot of gravity. The Earth has a lot of mass, so it has a lot of gravity. The moon's gravity is about 1/6 the amount of the Earth's because the moon has less mass than the Earth. You've probably seen video footage of astronauts walking on the moon. They seem to float between each step. Remember that the moon has about 1/6 the amount of gravity that the Earth has? Well, if you went to the moon, you'd weigh less than you do here on Earth. On the moon your mass would be the same -- there is no less of you on the moon. But your weight is different because the moon's gravity is different.

Calculating Gravity Sir Isaac Newton was the man that brought the heavens down to Earth. Remember Newtons 3rd Law Every action has an equal and opposite reaction Therefore the force of gravity works in both ways. That is, you are pulled down by gravity, but, at the same time you are pulling the earth up towards you. So from Newtons work it is possible to calculate the Force of Gravity between two objects.


m1m2 !G 2 r

The Law of Universal Gravitation


m1m2 !G 2 r

Fg=Force in Newtons G = Gravitational Constant m1= mass of object 1 in kg m2 = mass of object 2 in kg r = radius or distance between the centre of the two objects.

Calculating the Force between two objects.Problem Given the following data determine the magnitude of the gravitational attraction between: Answers a) The Earth and the Moon a) 1.98 x 1020N b) The Earth and the Sunb)3.52 x 1022N

Mass of Earth = 5.97 x 1024kg So even though the Sun Mass of the Moon = 7.35 x 1022kg is further away the Force of attraction is Mass of the Sun = 1.99 x 1030kg Average Earth-Moon distance = 3.84 x 108m greater than the moon. Average Earth-Sun distance = 1.50 x 1011m N.B. The Earth Sun Distance is also a unit of measure for space. 1 AU (Astronomical Unit) = 1.50 x 1011m 2 AU therefore would be 2 x 1.50 x 1011m = 3.0 x 1011m

Calculating Gravity on other Planets. Remember Weight is dependent upon the gravity of a planet. We can calculate the Gravity on a planet if we know the planets Mass in kg and its mean radius in m. We can use the following formula to calculate it:

Gravity planet ! G

m planet rplanet

Remember the Gravitational constant = 6.67x10-11 Nm2/kg2

Calculating it for Earthravity planet !planet

r 2 planet

Gravity = 6.67x10-11x 5.98x1024 (6.37x106)2 = 9.83 m/s2Now it is your turn to calculate it for each planet in our solar system.

Gravity on the PlanetsPlanet Earth The Moon Mars Jupiter Pluto Mercury Venus Saturn Uranus Neptune Mass(kg) 5.98 x1024 7.36 x1022 6.42 x1023 1.90 x1027 1.4 x1022 3.18 x1023 4.88 x1024 5.68 x1026 8.68 x1025 1.03 x1026 Mean Radius (m) 6.37 x106 1.74 x106 3.37 x106 6.99 x107 1.5 x106 2.43 x106 6.06 x106 5.85 x107 2.33 x109 2.21 x107 Acceleration due to Gravity (m/s/s) 9.8 1.6 3.7 24.8 0.7

Your Weight on other planets1. Estimate your mass. 2. Record your mass in the chart below. Your mass is your weight on Earth. 3. Multiply your mass times the gravity in each row to figure out your weight at each location. 4. Where do you weigh the most? Where do you weigh the least? To calculate your weight: mass x gravity = weight

Location Earth Earth's moon Venus Mars Mercury Jupiter Saturn Uranus Neptune Pluto


Gravity =9.8 =0.17 x 9.8 =0.90x9.8 =0.38x9.8 =0.38x9.8 =2.36x9.8 =0.92x9.8 =0.89x9.8 =1.13x9.8 =0.07x9.8


Lets Investigate Acceleration due to Gravity First Hand Investigation Determine a value for gravity using pendulum motion. Handout

Gravity and Potential Energy When we raise something off the ground we are giving it energy because if we drop it then it will fall to the ground. Therefore to raise the item we have to do work to raise it off the ground. Essentially we are working against gravity to lift the object. We must remember however that we are only observing gravity on the surface of the earth. What about when we are a long way from Earth? Does it affect us the same?Earth Gee gravity feels strong here I can hardly feel gravity out here

Gravitational Fields We can represent the gravity around an object with field lines. The field lines go in towards the centre of mass. The field on the surface of the Earth is straight down. E.g. in a room

Gravity and Work As we have said to move something in a gravitational field we must do work or use Energy. Therefore our idea of

PE=mghis good but only when we are close to Earth. How could we define it for the entire Universe?

Gravity is Inversely Proportional That means as an object moves away from a large object the force of gravity reduces. The faster we can go.

1 Gravity w 2 radius

Remember Newtons Gravitational ForceW ! F vs s ! radius ! r W ! F vr


m1m2 !G 2 r

E potential

m1m2 ! G r

m1m2 W ! F vr ! G 2 vr rNotice the negative sign, it will be explained in the next couple of slides.

Gravitational Potential Energy and Work Done.The gravitational potential energy of an object at some point within a gravitational field is equivalent to the work done in moving the object from an infinite distance to that point. It can be shown mathematically that the gravitational energy, Ep , of an object with mass, m1 , a distance, r , from the centre of a planet of mass, m2 , is given by:

GPE Information from Zona Land

Total Energy Total Energy = Potential Energy + Kinetic Energy

Therefore drawing graphs of both Ep and Ek would look like:

Graph of Gravitational Potential EnergyEarth






Graph of Kinetic EnergyEarth






GPEEp at suface