Centripetal Force• Centripetal force is the force that keeps

an object rotating in a circle.• If you spin a ball attached to a string

around your head, the force comes from the tension of the string.

• The centripetal force points towards the center of the circle.

Centripetal Force

• But if it points towards the center of the circle then why do I feel like I’m being pushed to the outside?

• This is called the “centrifugal” force, and is not real.

• Objects want to continue to move in a straight line, so when they are forced to turn they resist this change.

• For example, if we cut the rope that is attached to the ball above, the ball will not fly away from the center of the circle but will continue in a straight line.

Planetary Orbits• Just like the tennis ball above, the planets in our solar system rotate

around the sun.• The path that these planets follow is called an orbit.• Instead of a string connecting them to the sun, the force of gravity

between the sun and the planets is what keeps the planets in their orbit.

• Planetary orbits aren’t always circles. Sometimes they follow an oval shape, called an ellipse.

Angular Momentum• Whenever an ice skater holds their

arms out they can spin around pretty fast.

• But if they move their arms towards their body they can spin around even faster.

Planet Distance from Sun (Earth = 1)

Days to Orbit Sun (Earth = 1)

Mercury 0.39 0.24Venus 0.72 0.62Earth 1 1Mars 1.52 1.88

Jupiter 5.2 11.86Saturn 9.54 29.46Uranus 19.18 84.01Neptune 30.06 164.8

• The planets in our solar system behave in a similar manner.

• The planets closer to the sun move around faster.

• The planets further away from the sun move slower.

Effects in the Solar System• Ever wonder why comets are only

around for a short time?• As they get close to the Sun, they

move faster, just like the Skater drawing in her arms!So when they are close enough to see, they are flying by quickly!

Planet Distance from Sun (Earth = 1)

Days to Orbit Sun (Earth = 1)

Mercury 0.39 0.24Venus 0.72 0.62Earth 1 1Mars 1.52 1.88

Jupiter 5.2 11.86Saturn 9.54 29.46Uranus 19.18 84.01Neptune 30.06 164.8

• The planets in our solar system behave in a similar manner.

• The planets closer to the sun move around faster.

• The planets further away from the sun move slower.