Circular motion

A-level Physics

Unit G484: The Newtonian World

Newton’s law of gravitation

A-level Physics

Unit G484: The Newtonian World

Newton’s law of gravitation

Circular motion

To do/answer

1. Complete the following:

‘a field is a region of space in which a particular type of object experiences a … ‘

2. Name the type of object needed to detect the following:

a. a magnetic field b. an electric field c. a gravitational field

3. Sketch a diagram to show the gravitational field in this lab.. Be ready to explain what you are attempting to show.

4. Sketch a diagram to show the Earth’s gravitational field. How do we describe this field? Where do the field lines i) come from, ii) go to? What have you assumed about the Earth?

5. Define ‘gravitational field strength’.

Gravitation – lesson 1 recall LOs

Circular motion

Learning objectivesAt the end of the lesson you will be able to:

Lesson focus• Newton’s law of universal gravitation

• state Newton’s law of gravitation;

• select and use the equation for centripetal force F = -GMm/r2 for the force between two point or spherical objects.

Circular motion

Learning outcomes

All of you should be able to• state Newton’s law of gravitation in words;• state this law as an equation and define the symbols used;• use this equation to solve simple problems.

Most of you will be able to• use Newton’s law of gravitation to solve more complex problems.

Circular motion

Imagine two masses

m1 m2

We know that masses give rise to gravitational fields. Each mass will, therefore, be in the gravitational field of the other.

1. Sketch the situation and show the forces on each mass.

2. List the factors that determine the size of the forces on the masses.

Isaac Newton and gravity: video

The gravitational force between objects LOs

Circular motion

This law states that the

‘gravitational force between two point masses is proportional to the

product of the two masses and inversely proportional to the square of

the distance between them’.

Notes:

• a point mass has no spatial extent

• the minus sign in the equation shows that the force is attractive

F = -G m1 m2

r2

G - universal gravitational constant (‘big G’)

Newton’s law of universal gravitation LOs

Circular motion

Newton conceived of this force extending (instantaneously) throughout the entire Universe (hence universal). He did not attempt to explain the origin of the force.

F = -G m1 m2

r2

where,

G - universal gravitational constant (‘big G’)

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Newton’s law of universal gravitation LOs

Circular motion

To do

Using the data given find a value for G.

F = -G m1 m2

r2

Data:• your mass• the mass of the Earth (5.97 x 1024 kg)• radius of the Earth (6.37 x 103 km)

G is one of the least well defined constants of nature (hw: why is this?). The accepted value is 6.674×10−11 N m2 kg−2 .

To do Calculate the percentage difference between this and your value.

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Newton’s law of universal gravitation LOs