1. Examine the forces exerted on objects by gravity
Standard IV – Objective 2

2. Force = Mass x Acceleration (Newton) Work = Force x Distance (Joule) Power = Work/Time (Watt)

3. Gravity – what is it?
Simply put, it is the force that pulls objects toward Earth
Universally put, it is the mutual force between all objects in the universe

4. What affects gravity? Gravity is affected by distance and mass
The greater the distance between objects, the lower the force of gravity
The greater the mass of the object, the greater its gravity

5. Gravity
All objects have a force that attracts them towards each other - this is gravity
Even you attract other objects to you because of gravity, but you have too little mass for the force to be very strong (gravity only becomes noticeable when there is a really massive object like a moon, planet or star)
When you jump, the gravitational force of the Earth pulls you down, but your gravitational force also pulls the Earth towards you!
You just don’t notice this effect because your mass is so much smaller than Earth’s

6. Mass A measure of the amount of matter in an object
Measured in grams or kilograms
It stays the same for an object no matter where that object is in the universe

7. Weight
The force of gravity on an object at the surface of a planet is its weight
When you step on a scale, you are measuring the force with which Earth is pulling you so your weight varies throughout the universe
Measured in Newtons (1 N = 1 kg x 1 m/s2)

8. The moon has a gravity of just 1.6N/kg (as it is much smaller)
Earth’s gravity pulls objects towards its center - it has a force of 9.8 N/kg
The moon has a gravity of just 1.6N/kg (as it is much smaller)

9. Acceleration due to gravity
9.8 m/s/s (also written as 9.8 m/s2) at Earth’s surface

10. Weight = mass x acceleration due to gravity
An apple weighs about 1 newton
A 50-kg person weighs 50 kg x 9.8m/s2 or 490 newtons (on Earth’s surface)

11. Free fall
When the only force acting on a falling object is gravity, the object is in free fall
An object in free fall accelerates as it falls
A falling object falls 9.8 m/s faster every second that it falls

12. Free fall continues
Therefore, after 3 seconds an object is falling at a speed of 29.4 m/s, after 4 seconds it is falling at 39.2 m/s
All objects fall at the same rate regardless of mass

13. Air resistance
Even though all objects are supposed to fall at the same rate, we know they do not
All objects falling through air experience a type of fluid friction called air resistance
Air resistance (being friction) is opposite to the direction the object is falling and eventually prevents the object from accelerating faster
The maximum velocity any object can reach in free fall is called TERMINAL VELOCITY

14. More air resistance
The greater the object’s surface area, the greater the air resistance
When an object can not accelerate any more because of air resistance, it is said to be at terminal velocity
In Earth’s atmosphere, leaves fall slower than rocks
In a vacuum, they both fall at the same rate

15. Projectile motion
Thrown objects are under the same gravitational acceleration as objects falling straight down so eventually they will fall straight down also
For example, a cannon ball shot horizontally and one dropped vertically from the same height as the cannon will both hit the ground at the same time

16. weight (N) = mass (kg) x gravity (N/kg)
Earth
Gravity on Earth = 9.8 N/kg
Say you have a mass of 100kg on Earth
Can you work out your weight?
Remember: -
weight (N) = mass (kg) x gravity (N/kg)
Your weight on Earth is 100kg x 9.8 N
Your weight = 980 N

17. Moon Say you have a mass of 100kg on Earth
Gravity on moon = 1.6N/kg
Say you have a mass of 100kg on Earth
What is your mass on the moon?
Remember, mass is the amount of stuff there is (you still have two arms, two legs etc… on the moon)
Mass = 100kg on moon
What is your weight on the moon (weight = mass x gravity)?
Your weight on the moon is 100kg x 1.6 N
Your weight = 160N

18. Your mass & weight
See if you can work out your mass and weight on the Earth and on the moon
Remember:
Gravity on Earth = 9.8 N/kg
Gravity on the moon = 1.6 N/kg

19. Gravity
Complete the gravity on other planets worksheet

20. Gravity
Although your mass is always the same wherever you are, you would weigh the most on Jupiter, and the least on Pluto

21. Weight Bingo

22. Questions On Mercury (3) the mass of 10kg =
On Venus (9) the mass of 5kg =
On Earth (10) the mass of 5kg =
On Mars (4) the mass of 4kg =
On Jupiter (23) the mass of 2kg =
On Saturn (9) the mass of 9kg =
On Uranus (8) the mass of 10kg =
On Neptune (11) the mass of 50kg =
On Pluto (0.4) the mass of 90kg =
On Mercury (3) the mass of 30kg =
On Venus (9) the mass of 15kg =
On Earth (10) the mass of 25kg =
On Mars (4) the mass of 100kg =
On Jupiter (23) the mass of 10kg =
On Saturn (9) the mass of 20kg = 30 45 50 16 46 81 80
550 36 90
135
250
400
230
180

23. Questions On Uranus (8) the mass of 16kg =
On Neptune (11) the mass of 11kg =
On Pluto (0.4) the mass of 50kg =
On Mercury (3) the mass of 70kg =
On Venus (9) the mass of 45kg =
On Earth (10) the mass of 12kg =
On Mars (4) the mass of 60kg =
On Jupiter (23) the mass of 5kg =
On Saturn (9) the mass of 52kg =
On Uranus (8) the mass of 95kg =
On Neptune (11) the mass of 54kg =
On Pluto (0.4) the mass of 80kg =
On Mars (4) the mass of 21kg =
On Jupiter (23) the mass of 29kg =
On Venus (9) the mass of 65kg =
128
121 20
210
405
120
240
115
468
760
594 32 84
667
585