TOPIC 6: Fields and Forces

What is gravity?Is there gravity in space?Why do astronauts float?What keeps the moon from flying off in space?

6.1 Gravitational Force and Field6.1.1 State Newton’s universal law of gravitation.

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6.1 Gravitational Force and Field6.1.1 State Newton’s universal law of gravitation.

Galileo (1564-1642) – g = 9.81m/s2, even with different masses.David Scott – feather and hammer dropped on the moon, Apollo 15Isaac Newton(1643-1727) –

Idea about a cannon ball that never hit the ground.Orbit period of the moon – 27.3daysRadius of moons orbit – RM = 3.844 x 108m, RE = 6.378 x 106m

mid-1600s Earth’s and Moon’s masses had been determined

MM = 7.35 x 1022kg ME = 5.98 x 1024kg)

6.1 Gravitational Force and Field6.1.1 State Newton’s universal law of gravitation.

From all that data Newton calculated the centripetal acceleration on the moon due to the earth’s gravitational attraction.

ac= v2/r= (2πr/T)2 x 1/ r = 0.00272m/s2

Compared this with the “g” 3600 times lower.Concluded gravitational force of attraction is inversely proportional to the square of the distance between the centers of the two object. FG ∝ 1/r2

6.1 Gravitational Force and Field6.1.1 State Newton’s universal law of gravitation.

Also concluded that the gravitational force was proportional to the product of the two masses.FG ∝ m1m2

Combined = FG ∝ m1m2/r2

6.1 Gravitational Force and Field6.1.1 State Newton’s universal law of gravitation.

IB Equation and Formula

Newton’s Law of Universal Gravitation – every object attracts every other object with a force that is proportional to the product of the two masses and inversely proportional to the square of the distance between them.

F = G(m1m2/ r2)

Universal law of gravitationG = 6.67 x 10-11 Nm2kg -2

6.1 Gravitational Force and Field6.1.1 State Newton’s universal law of gravitation.

Practice 1Calculate the gravitational force of attraction between you and the person sitting next to you! Assume your mass is 65kg, their mass is 55kg and the distance is 2mAnswer 5.9x10-8N

6.1 Gravitational Force and Field6.1.2 Define gravitational field strength.

Gravitational field is like a “force field” that exist around every object.It is dependent on the mass of an object. So larger mass means a larger field. Smaller mass means smaller field.

IB Definition and Formula

Gravitational field strenghth – the force per unit mass acting on mass in a gravitational field

g = F/m

6.1 Gravitational Force and Field6.1.2 Define gravitational field strength.

Practice 2A 2.45kg object feels a gravitational force of 4.0N at the surface of the Moon. Calculate the Moon’s gravitational field strength at its surface.Answer: 1.63 N/kg

6.1 Gravitational Force and Field6.1.3 Determine the gravitational field due to one or

more point masses.6.1.4 Derive an expression for gravitational field strength at the surface of a planet, assuming that all its mass is concentrated at its Center.6.1.5 Solve problems involving gravitational forces and fields.

6.1 Gravitational Force and Field6.1.3, 6.1.4, 6.1.5

Gravitational Field strength is a vector. This means we must use the rules of vector addition!Given that g = F/m and F = G(Mm)/r2

g = gravitational field strengthF = gravitational forceM = mass of the planetm = mass of object in gravitational field

Substitute for F and you get:g = G(Mm)/r2m = GM/r2

6.1 Gravitational Force and Field6.1.3, 6.1.4, 6.1.5

Problem 3Use the following data to calculate the gravitational field strength at the surface of the Earth.RE = 6.378 x 106 m

ME = 5.98 x 1024 kg

Answer: 9.80 N/kg

6.1 Gravitational Force and Field6.1.3, 6.1.4, 6.1.5

Problem 4If we take a look at the Earth-Moon system it becomes apparent that there must be a point somewhere between them where the gravitational field strength is zero. Meaning a mass, a spacecraft, is not pulled in either direction at that point. Calculate the position of this point – distance, x.Earth mass ME = 5.98 x 1024 kg

Moon mass MM = 7.35 x 1022 kg

Earth Moon distance 3.84 x 108 mHint: this will be gravitational field strengths are equalAnswer: 88.9% of the distance from the Earth to the Moon or 3.41 x 108m

6.1 Gravitational Force and Field6.1.3, 6.1.4, 6.1.5

Problem 5The Sun, Earth and Moon, at a particular moment in time are perpendicular to each other. (See board for diagram). Calculate the gravitation field strength and direction at the moon’s position in space, due to the Earth and Sun. Dist. for Sun to Moon – RS = 1.49x1011m

Sun Mass – MS = 1.99x1030

Answer: g= 0.0066N/kg @ 24º as seen on the diagram.