1. Chapter 12- Forces
Physical Science

2. 12.1 Forces Force, Units Measuring Force vectors Combining forces
Balanced Forces – Equilibrium
Unbalanced forces – acceleration
Friction…. Static, sliding, rolling, fluid
Gravity – acceleration
Terminal velocity – Air Resistance
Projectile Motion

3. Force Force (F) - A push or a pull on an object
An ‘influence’ or ‘interaction’
Net Force (Fnet) – Total Force on an object
Forces in opp directions will subtract, forces in same direction will add

4. Combining Forces – Net Force
Multiple forces may be acting on an object at the same time, the Net Force is the combination (resultant) of all these forces.

7. Balanced Forces Forces in each direction cancel each other
Net force of ZERO
If forces are balanced, object is in equilibrium
Object is either AT REST, or AT A CONSTANT VELOCITY

8. Unbalanced Forces Net force is NOT Zero
Unbalanced Forces (net force) causes acceleration

10. Static > Sliding > Rolling
Friction
Static Friction
Resistance to the start of motion
Sliding Friction
Resistance to one object sliding over another object
Rolling Friction
Resistance to one object rolling over another object
About times less than Static/Sliding
Static > Sliding > Rolling
Fluid Friction
Air Resistance
Moving through water

11. Gravity Force that acts between any two masses
Only significant when one of the objects is very large (Earth, Sun, etc.)
Pulls objects together (attractive force)
Force of Gravity is also called weight
Acts downward toward center of Earth
Used balanced by some other force
Support force from ground, chair, etc.
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12. Falling Objects Gravity accelerates objects downward
Air resistance acts opposite to gravity and slows down the acceleration
Air resistance gets greater with velocity
Eventually if falling so long that air resistance balances gravity you will reach terminal velocity
Terminal Velocity – constant velocity of a falling object when the force of air resistance equals force of gravity

13. Terminal Velocity

14. Projectile Motion
Any object that is only under the influence of gravity and air resistance is a projectile
If given some initial forward velocity, projectiles will follow a curved path
Combination of initial forward velocity and downward vertical force of gravity causes it to follow curved path.

15. 12.2 Newton’s 1st & 2nd Laws of Motion
Is a force required for motion?
Aristotle (300 BC)
Proposed that force is required to keep an object moving at a constant velocity
Galileo & Newton (1500’s-1600’s)
Moving objects are that are not experiencing friction or any other force will continue to move indefinitely
Would keep moving forever, if there is no friction
Objects have a natural tendency to stay in motion or at rest

16. Newton’s 1st Law of Motion
Objects at rest will stay at rest, objects in motion will stay in motion unless acted upon by an outside force
Objects have a natural tendency to resist changes in their motion
Inertia – property of matter that resists changes in its motion
Mass is a measure of inertia….. More mass, more inertia

17. Examples of Inertia Seatbelts Rollercoaster Car dashboard
Tennis ball/string
Earth’s rotation
Helicopter

18. Newton’s 2nd Law of Motion
Acceleration of an object is equal to the net fore acting on it divided by the object’s mass
Acceleration is proportional to net force
More force, more acceleration
Acceleration is inversely proportional to net force
More mass, less acceleration
𝑎= 𝐹 𝑛𝑒𝑡 𝑚

19. W = mg Weight & Mass Weight – in Newtons Mass – In Kilograms (kg)
Weight – Force of gravity acting on an object
Mass – How much matter is in an object
A measure of how much inertia an object has
Weight = Mass x acceleration due to gravity
W = mg
Weight – in Newtons
Mass – In Kilograms (kg)
Acc of gravity – in m/s2

20. Gravitational force differs on other planets
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Gravity at equator (Earth=1)
0.38g
0.9g 1g
2.64g
0.93g
0.89g
1.12g

21. .16
.38 10
2.64

23. Why do astronauts float around inside the international space station?
They are falling with the space station around Earth
Continuously falling without ever running into anything.
Gravity is still pulling them

24. Why do satellites orbit Earth?
Satellites are forced to circle the Earth by the pull of Earth’s gravity.
To orbit a satellite must be travelling at a very fast speed!
Around 17,000 mph or faster
Satellites are falling around the Earth
Objects in orbit feel weightless
Not actually weightless

25. 12.3 Newton’s 3rd Law Newton’s 3rd Law
For every action force there is an equal and opposite reaction force
Equal in magnitude, opposite in direction
Forces only exist in pairs (Action/Reaction)
Can’t have an action without a reaction

26. Action & Reaction
Action forces produce the interaction, and the reaction force must be in existence at the same time, have the same size, and be opposite in direction
You apply a 10 N eastward force to wall, wall applies a 10 N westward force to you

27. Stuck in Space??
An astronaut in space…not moving with just a spacesuit on and he’s not tethered to the ship. If he is only a few meters away from his ship and there are no other people to help him how could you get back to his spaceship?
Oh No!!

28. What’s really pushing you?
To accelerate you need an outside force…. You cannot apply a net force to yourself!
Something outside of you needs to push you in order to move/accelerate
The astronaut in the previous slide need his tool to push him.

29. Should always be able to identify each action/reactionx

30. s

31. Astronauts & Reaction Forces

32. How do objects move? What pushes them?
Car??
Rocket??
Person??

33. Are these forces equal?
Bug/windshield
Ball/Bat
Small car/truck

35. P = mv Momentum (p) Inertia in motion’
How much an object in motion wants to stay in motion
How much motion an object has
Momentum = mass x velocity
P = mv
Vector quantity

36. Example Questions
A 100 kg cart is moving with a velocity of 5 m/s, what is its momentum?
500 kg*m/s
A 2 kg bowling ball is rolling with a speed of 5 m/s, what is its momentum?
10 kg*m/s

37. f

38. f

39. Law of Conservation of Momentum
The total amount of momentum in a system must stay constant unless an outside net force acts
Internal forces cannot change momentum
Collisions
If no outside forces…. Momentum is only exchanged between objects never lost
Recoil from gun

40. g

41. 12.4 Universal Forces
There are four fundamental forces that exist in the universe that can act across a distance (push/pull things without touching)
Strong Nuclear Force
Acts on neutrons and protons in the nucleus of an atom to hold them together
Electromagnetic Force
Holds electrons to the positively charged nucleus
Weak Nuclear Force
Responsible for radioactive decay
Gravity
Attractive force that acts between any two masses
Every object in the universe attracts every other object