Gravity, weight, and mass
Gravity
When you drop an object, it falls to the ground.
What pulls the object down?
The Earth you live on is a large object with a mass of about .
Objects with large mass, such as the Earth, cause strong forces of gravity.
All objects, even pens and pencils, cause forces of gravity. Objects with small mass, such as pens and pencils, cause very weak forces of gravity. That means we do not notice other objects being attracted to them.
The force of gravity caused by an object acts towards the centre of the object.
You can imagine the Earth as a giant ball in space. The force of gravity at positions around the Earth acts towards the centre. That means when you drop an object, the object falls in a line that points towards the centre of the Earth.
The strength of gravity decreases as you go further from a large object such as Earth. For example, if you travelled away from Earth in a spacecraft, the force of gravity from the Earth acting on you would get smaller.
Questions
1. Draw a circle to make a diagram of the Earth.
Put arrows around your diagram to show the direction of the force of gravity.
Show Answer
Arrows should point toward the center of the circle, representing how gravity pulls objects toward the Earth's center.
2. Use your diagram from question 1 to explain why people who go to the South Pole do not fall off the Earth. Discuss your answer with a partner.
Show Answer
Gravity pulls toward the center of the Earth from all directions. At the South Pole, gravity still acts downward relative to your feet, so you don’t fall off.
3. The diagram shows the Earth. It is not to scale.
A ball is dropped from four different places, A, B, C and D.
Draw arrows to show the direction in which each ball will fall. The first one has been done for you.
Show Answer
Balls at A, B, C, and D will fall toward the center of the Earth. The arrows should point inward from their respective positions.
4. The Moon has a mass of about _____.
Some people think there is no gravity on the Moon. Are they correct? Use the information in this question to explain why.
Show Answer
No, there is gravity on the Moon. Although weaker than Earth's, the Moon’s gravity causes tides on Earth and can pull objects toward it.
Weight
The force of gravity on an object is called its weight.
It is difficult to lift a heavy object because gravity is pulling it towards the centre of the Earth. By lifting, you are pulling against gravity.
Weight is a force and it is measured in newtons, N. The weight of an apple is about 1 N. That means gravity from the Earth is pulling on the apple with a force of 1 N. You need to apply a force of 1 N to hold the apple.
The weightlifter in the picture is holding about !
You can see the effect of the force bending the bar.
The contact force
When a book with a weight of 5 N is resting on a desk, gravity is still pulling it down with a force of 5 N. So why is the book not moving down through the desk?
The answer is because the desk is pushing back up on the book with an equal force of 5 N.
This force from the desk is called the contact force. The contact force acts up from any surface to support an object. The contact force is always equal to the weight of the object when the surface is not moving.
Sometimes the weight of the object is larger than the contact force. If this happens, the surface will break, or the object will sink into the surface.
Can you think of any other examples where the weight of an object is larger than the contact force?
Show Answer
Examples: A heavy truck stuck in soft mud, or a person standing on thin ice. In both cases, the object’s weight exceeds the supporting contact force.
Questions
5. The diagram shows a box on a desk. Copy this diagram.
On your diagram:
- a add an arrow to show the weight of the box. Label this arrow W.
- b add an arrow to show the contact force from the desk. Label this arrow C.
6. A large rock rests on the ground. The weight of the rock is 8000 N.
Write down the size of the contact force from the ground.
Show Answer
8000 N — the contact force equals the weight of the rock if the surface is not moving.
7. An elephant is standing on four feet. The weight of the elephant is 40000 N.
Calculate the contact force from the ground on each of the elephant’s feet.
Show Answer
10000 N — the weight is equally distributed, so 40000 N ÷ 4 feet = 10000 N per foot.
8. A car travels into soft mud. The contact force needed to support each wheel is 24000 N.
- a At first, the contact force from the mud on each wheel is 20000 N. Explain why the wheels will start to sink.
Show Answer
The contact force is less than the weight. The ground cannot support the car fully, so the wheels sink.
- b The contact force from the mud increases with depth. Explain what will happen to stop the wheels sinking.
Show Answer
As the car sinks, the mud pushes back harder. When the contact force matches the weight, the wheels stop sinking.
Weight and mass
Weight is the force of gravity on an object. It is measured in newtons, N.
Mass is the quantity of matter in an object. It is measured in kilograms, kg.
On Earth, the force of gravity is 10 N on every 1 kg of mass.
Writing this as an equation:
weight (N) = mass (kg) × 10 (N/kg)
or, using letters: W = m × 10
The force of gravity that pulls on 1 kg tells you the strength of gravity. On Earth, this is 10 N.
As 10 N acts on 1 kg, you say this as “10 newtons per kilogram”, or 10 N/kg. For example, if a person has a mass of 45 kg, their weight on Earth is:
45 × 10 = 450 N
You can use the equation to calculate mass if you know the weight. For example, a computer games console has a weight of 28 N. The mass of the console is:
28 / 10 = 2.8 kg
The strength of gravity on Earth is 10 N/kg.
The strength of gravity on the Moon is lower than on Earth. This means that objects weigh less on the Moon.
Questions
9. The strength of gravity is 10 N/kg on Earth.
a. Calculate the weight of an adult who has a mass of 75 kg.
Show Answer
Answer: $75 \times 10 = 750\,N$
b. Calculate the mass of a car that has a weight of 8500 N.
Show Answer
Answer: $8500 \div 10 = 850\,kg$
10. Use the information in the diagram of the planets on the previous page to answer these questions.
a. Give the location where your weight would be greatest.
Show Answer
Answer: Jupiter
b. Name the planet where you would have the same weight as on Earth.
Show Answer
Answer: Saturn
c. Calculate the weight of a 25 kg mass on Mars.
Show Answer
Answer: $25 \times 3.7 = 92.5\,N$
d. Explain how your mass on Earth would compare with your mass on Mercury.
Show Answer
Mass stays the same regardless of the planet. Only weight changes with gravity.
11. When you stand on scales you see your mass in kg. Explain whether it is your mass or your weight that makes the scales work.
Show Answer
Scales measure weight, but they convert it into mass assuming Earth’s gravity (10 N/kg).
12. In 1969, a spacecraft carrying people went from the Earth to the Moon. [...]
Show Answer
More power is needed to escape Earth's stronger gravity. Returning from the Moon requires less energy.
Think Like a Scientist
Linking weight and mass
In this investigation, you will find the weights of some masses and draw a graph of your results.
- force meter
- clamp stand
- mass hanger and masses
- Start by hanging the force meter from the clamp stand. Leave enough space to hang the masses, remembering that the spring will extend.
- Hook the mass hanger to the force meter. Record all your results using the kg unit for mass. Remember that weight = mass × gravity.
- Using the force meter, carefully measure the weight. Remember that this result is in newtons (N).
- Increase the mass by adding one mass at a time. Use the force meter to measure and record the weight after every increase.
- Record the weights in a table. Remember to put the units in the column headings and not in the table itself.
- Measure the weights as accurately as possible. Being accurate means being close to the true value.
- Your results should go from low to high.
- Draw a line graph of your results. Put mass in kg on the horizontal axis and weight in N on the vertical axis.
As the mass gets bigger, the weight gets …
When the mass doubles, the weight …
Show Answer
As the mass gets bigger, the weight gets bigger. When the mass doubles, the weight also doubles.
Show Answer
The actual weight of 1.0 kg is about 9.8 N, not exactly 10 N. The equation uses 10 N/kg as an estimate for easier calculations.
Explain why you can use the value of 10 instead of these more accurate values.
Show Answer
Using 10 N/kg is a rounded value that makes calculations quicker and easier. The difference is small enough that it does not affect most school-level experiments significantly.
- For each of these statements about your experiment, decide how well you think you did.
- I worked safely, taking care not to drop any masses or knock the clamp stand over.
- I took the reading from the force meter as accurately as possible.
- I continued to record actual results, even when I thought I could see a pattern developing.
- I wrote down or drew my results clearly, so that someone else could understand them.
- I made my graph accurate and clear.
- Write down one thing that you did really well.
- Write down:
- one thing that you could do better next time
- how you will try to improve next time.