7.3 Gravity – a non- contact force

Year 7 Science (NSW Syllabus)

About Lesson

What is gravity?

All objects attract each other. There is a force of attraction between you and your schoolbag, as there is between you and everything around you. Gravity is this force of attraction. The more mass a pair of objects have the stronger the pulling force. This is why you are pulled strongly towards the Earth!

Gravitational fields

If you throw a ball into the air, you know it will fall back to Earths. If an object lies within a region called the Earth’s gravitational field, then a gravitational force will act upon it. This region is called a force field.

Gravity acts through a force field, without direct contact, this is why it is a non-contact force.

Comparing mass and weight

In science mass and weight mean different things.

Falling

For centuries people believed that things fell to the earth at different speeds due to their weight. However this is not the case, the Italian scientist Galileo Galilei, performed experiments with falling objects. He realized that the reason some things fell faster was not because they weighted more but because they had a smaller area than other things. Air is pushed out of the way as an object falls.

An object with smaller surface area experiences less air resistance as it cuts through the air compared to an object with a larger surface area. Without air two objects dropped at the same time will fall at the same speed.

Terminal velocity

Terminal-means END

Velocity means SPEED

Simply put terminal velocity means the speed at which a falling object ends after it is done speeding up. Air resistance increases as an object speeds up until it is EQUAL TO the pull of gravity, at this point the object (or person) no longer speeds up and their speed remains the same.

In free fall, the less surface area of the falling object, the less air resistance and the FASTER the terminal velocity of the object. Similarly, the more surface area, the more air resistance and the SLOWER the terminal velocity of the object. This is why skydivers use a parachute to increase their surface area, which increases their air resistance which means they fall at a much slower speed and avoid hitting the ground too fast and becoming a human Omelette.

7.3 Unit review

Homework

Please complete questions 1-13 6/4/21

Type your answers in the space below.

a) non-contact ✅ b) Pulls ✅ c)attract ✅ d) Surface area ✅
kilograms (kg) ✅
newtons (n)
Air resistance is the force that slows down an object when it falls ✅
1- A force field is a none contact force, eg The earths gravitational force field. The suns gravitational force field. ✅
a – When an object gets near the field it will just fall back to earth like when you throw a ball and it comes back down to earth ✅ b – the field becomes weaker when a object moves further from earth ✅ c- Gravity acts through a force field with no direct contact that’s why its a none contact force ✅
1. a) 1) air resistance 2) gravity ✅ b- n/a c) When they get balanced the speeds the remains constant ( Terminal velocity )
2. The gravity around us stops us from passing it this works because of the force field surrounding us
3. a – When you walk on the moon you will basically just float b – This happens because there is no gravity on the moon
4. Mass is the amount you or an object weighs on the earth though weight is the amount of force gravity uses to push you back down to earth
6. No there isn’t gravity on the moon because the moon doesn’t have a force field to push you back down to the moon you’ll just float away
7. a – b-
- Teachers Answers
  1. a. Gravity is a non-contact force.
    b. Gravity pulls objects toward the Earth c. All objects naturally attract each other d. Objects of the same mass fall at different speeds due to their surface area.
  2. Kilograms (Kg)
  3. Newtons (N)
  4. Air resistance slows down an object as it falls.
  5. 1- A force field is a non-contact force, eg The earths gravitational force field. The suns gravitational force field.
  6. a.When an object gets near the field it will just fall back to earth like when you throw a ball and it comes back down to earth b. the field becomes weaker when a object moves further from earth
    c. Gravity acts through a force field with no direct contact that’s why its a none contact force
  7. a. Air resistance and gravity b.
  - Force diagram
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 c) When they get balanced the speeds the remains constant ( Terminal velocity ) 
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  1. Gravity pulls objects towards the centre of another object. This is why objects on earth fall toward the ground, they are being pulled toward the earths centre.
  2. a)The moon is much smaller than earth only 1/6th of the mass of Earth. Remember that larger objects have stronger gravity. So on the moon you will feel a lot lighter and you could even jump 6 times higher on the moon! b) There is less gravity on the moon than on Earth
  10.Mass is the amount of matter or ‘stuff’ in a object. Weight is the force of gravity acting on this matter, more matter, more gravity.
  1. Ignoring air resistance all 3 objects will fall at the SAME speed. Even a heavier shot put ball falls at the same rate as a cricket ball.
  2. There is gravity on the moon as objects that are very close to the moon fall toward it. Like an astronaut jumping on the surface of the moon.
  13.a. Remember mass is the amount of ‘stuff’ someone has inside them. This could be water, fat, sugar, anything. So a persons mass could change if they consume food a lot of food. It can change if they exercise and lose weight and it can change if they grow taller. b. Now a person’s mass can stay the same if they move from Earth to the moon. But because the moon has less gravity than Earth, the person has less weight on the moon.
  
  Hope these help Zaara! Let me know if you want me explain any of them further with a comment 🙂