1. Building Science Champions
Gravity and Friction

2. In thought Write a paragraph describing the picture.
Words to include in your paragraph:
Gravity
Free fall
Wind resistance
Speed
Acceleration
Motion
Velocity
Terminal Velocity

3. Objectives
Describe friction and identify the factors that determine the friction force between two surfaces
Explain how mass differs from weight
State the universal law of gravitation
Describe the effects of gravity and air resistance on an object in free fall

4. Key terms Force Contact force Noncontact force Net force Friction
Fluid friction
Rolling friction
Static friction
Sliding friction
Gravity
Free fall
Projectile
Air resistance
Terminal velocity

5. Types of force Force – is a push or a pull
Non-contact
Force – is a push or a pull
Some forces are only applied when objects are in contact with one another; while others are applied when objects do not touch
Contact force is a push or pull on one object by another that is touching it – someone playing a guitar
Are the following pictures contact or non-contact force
Noncontact force is a force that one object can apply to one another object without touching it – electric force
Using the force
Contact Force

6. Strength of forces SI unit for force is the newton (N)
What is the net force of the squares below?
SI unit for force is the newton (N)
Forces can be added together if they are working in the same direction
Forces can be subtracted if they are working against one another
5 N
5 N
10 N
15 N
15 N
10 N
10 N
10 N
5 N
0 N
10 N
5 N

7. Net Force and Direction of forces
The net force is the sum of all the forces added together
Vector analysis – is the analysis of the different forces on an object resulting in the calculation of net force
Net force is given with a direction – in the example below 27 N right
10 N
17 N

8. Friction Surfaces may seem smooth but they have many irregularities
When two surfaces rub the irregularities of one surface get caught on those of the other surface
Friction – the force one surface exerts on another when the two rub against each other
Friction

9. Nature of friction
The strength of the force of friction depends on two factors: the types of surfaces involved and how hard the surfaces push together.
Rough surfaces produce greater friction than smooth surfaces
Objects with greater mass and weight produce greater friction than lighter mass and weight
Friction releases energy in the form of heat

10. Types of friction
Fluid Friction – friction that occurs when an object moves through a fluid
Rolling Friction – friction that occurs when an object rolls over a surface
Sliding Friction – friction that occurs when an object slides over a surface.
Static Friction – prevents surface from sliding past one another

11. Fluid Friction Sliding Friction
Static Friction – prevents you from sliding. If the box doesn’t move when force is applied it is because there is not enough force to overcome the static friction
Fluid Friction

12. Gravity Gravity – is the force that pulls objects towards Earth
Gravity – is an attractive force that exists between all objects that have mass
Gravity is calculated on Earth as 9.8 m/s/s or 9.8 m/s2

13. Free Fall
Free fall – when the only force acting on a falling object is gravity
Objects in free fall accelerate at a constant rate of speed – 9.8 m/s/s
1 sec – 9.8 m/s
2 sec – 19.6 m/s
3 sec – 29.4 m/s
The mass of an object does
not affect the rate of speed at
Which the object falls
Flashphoto

14. Projectile motion Projectile – an object that is thrown or shot
Flash photo of a projectile
Projectile – an object that is thrown or shot
An object that is dropped and one that is thrown horizontally are both in free fall
Horizontal motion does not interfere with free fall and both objects will hit the ground at the same time
Both cannon balls will hit the ground at the end of 6 seconds. B will travel a further distance.

15. Air resistance
Air resistance – a type of fluid friction that all objects encounter while falling through the air
Friction is the direction opposite to motion, so air resistance is an upward force
This why some objects fall faster than others
Eventually air resistance will equal out the force of gravity – when this happens the object is no longer increasing in velocity
Terminal velocity – the greatest velocity that an object reaches

16. Weight
Weight – is the measure of force of gravity on an object, and mass is the amount of matter in that object
Weight = mass (kg) x acceleration due to gravity
Weight = mass x 9.8 m/s/s

17. Universal gravitation
The law of universal gravitation states: that the force of gravity acts between all objects in the universe
The moon has a gravitational effect on Earth, the Earth has a gravitational effect on the moon
Gravitational pull on objects compared to the gravity rate on Earth

18. Gravitational force & Mass
When the mass of one or both objects increases, the gravitational force between them also increases
F = G*m1*m2 / r2
F = Force
G = Gravity
M1 = Mass of first object
M2 = Mass of second object
r = Distance between the objects
1 kg
1 kg
2 kg
2 kg
Gravitational force increases if the mass of at least one objects increases.

19. Gravitational Force & Distance
The attraction between objects decreases as the distance between objects increases
Force
Distance

20. Review Weight and mass are not equal Weight = mass * gravity
All objects have a gravitational force/attraction to one another
There are multiple types of friction

21. References
Anderson, M. et all (2012) Physical Science. McGraw-Hill: Columbus
Frank, D.V et al (2001). Physical Science. Prentice Hall: New Jersey