Universal gravitation

Science · Space Systems & Earth's History · ages 12-13

Name ______________________ Date ____________

Where would you weigh less: on Earth or on the Moon?
- Earth
- the Moon
- You would weigh the same on both
You and your backpack both have mass, so you pull on each other. Why do you never feel your backpack tugging you sideways?
- Only Earth is allowed to have gravity
- Gravity works only on objects that are moving
- The backpack's mass is far too small to give a pull you could feel
A cannonball fired sideways from a very tall mountain curves down to the ground. Which change could make it keep missing the ground and go into orbit?
- Much faster sideways
- Straight up instead
- A heavier cannonball
- Slower sideways
Which statement best describes gravity?
- Gravity only pulls objects toward Earth
- Gravity is a pulling force between any two objects with mass
- Gravity pushes objects apart
The International Space Station orbits high above Earth, where gravitational field strength is about 8.7 N/kg. At Earth's surface it is 9.8 N/kg. What percentage of Earth's surface gravity does the station feel? Round your answer to the nearest whole percent.
Answer: ______________
Three things move in curved paths. Which one is held in its path by the Sun's gravity rather than Earth's?
- The Moon
- A weather satellite circling Earth
- Jupiter
Look at the chart. Compared to Earth, which has the strongest surface gravity?
- Earth
- the Moon
- Jupiter
On the lesson's chart the Moon's bar is 0.17 while Earth's is 1. What does that tell you about a rock taken to the Moon?
- The rock would weigh about 17 times more there than on Earth
- The rock would weigh 0.17 newtons less there than on Earth
- The rock would weigh about one sixth of its Earth weight there
Suppose the Moon somehow lost all of its sideways motion while Earth's gravity kept pulling. What would happen next?
- It would carry on looping around Earth as usual
- It would fall straight toward Earth
- It would drift slowly away from Earth
Earth and the Sun pull on each other. Why is it Earth that loops around the Sun, and not the Sun looping around Earth?
- Earth has no gravity of its own to pull with
- The Sun pushes Earth around in a circle
- The Sun's mass is so huge it barely moves

Answer key

For grown-ups. Fold this page away before handing over the rest.

Universal gravitation W1-mt_oLjz18CxDg-s1

the Moon · The Moon's gravity is weaker than Earth's, so you weigh less standing on the Moon, even though your mass is the same.
The backpack's mass is far too small to give a pull you could feel · The pull is real, it is just tiny. A pull only grows big enough to feel when one of the two objects has an enormous mass, like a planet.
Much faster sideways · An orbit needs a pull toward Earth and fast sideways motion. Fired fast enough sideways, the ball falls but keeps missing the ground.
Gravity is a pulling force between any two objects with mass · Gravity acts between any two objects with mass, not just near Earth. Earth, the Sun, and even two rocks in space all pull on each other.
89 · Divide the gravity up there by the gravity at the surface and multiply by 100: 8.7 / 9.8 x 100, which is about 89 percent. Gravity gets weaker with distance, but only a little at that height.
Jupiter · Jupiter loops around the Sun, so the Sun's pull is what bends its path. The Moon and a weather satellite both loop around Earth, so Earth's pull shapes theirs.
Jupiter · The tallest bar is Jupiter, so its gravity pulls hardest. An object would weigh the most there.
The rock would weigh about one sixth of its Earth weight there · The chart numbers are multipliers next to Earth's 1. Multiplying by 0.17 is close to dividing by 6, so the rock's Moon weight is about a sixth of its Earth weight.
It would fall straight toward Earth · An orbit needs the inward pull and the sideways speed together. With the sideways speed gone, only the pull is left, and a pull on its own leads straight in.
The Sun's mass is so huge it barely moves · Every pair of masses pulls both ways, so Earth's pull on the Sun is real. The Sun's mass is enormous next to Earth's, so the Sun stays nearly still while Earth does the looping.