1. Forces
Unit 9 Lecture

2. Newton’s 1st Law of Motion
An object at rest will stay at rest, and
an object in motion will stay in motion
UNLESS acted on by an unbalanced force
In other words: An object will keep doing what it’s doing until a force changes its motion

3. Forces A force is a push or a pull on an object
All changing motion is caused by ONE or MORE forces.
The SI unit for force is Newton (N).

4. Kinds of forces we will talk about
Frictional Force
Air Resistance
Gravitational Force
Tensional Force
Applied Force
Electrical Force
Normal Force
Spring Force
Magnetic Force

5. Newton’s Second Law of Motion
An object will accelerate in the same direction as the net force on an object!
Force = mass x acceleration
F = m x a

6. Net Force Net force = F1 + F2 + F3 + …
The net force on an object is the sum of all the forces on the object in each direction.
Net force = F1 + F2 + F3 + …

7. Practice Problem
Bob the squirrel is mowing his lawn. A lawn mower is pushed across the ground with a force of 25 N. The ground provides 14 N of resistance. What is the net force on the lawn mower in the x-direction (along the ground)?
Net Force = F1 + F2 + F3……
F1 = + 25 N
F2 = - 14 N (negative because it is resisting)
Net Force = (+25N) + (-14N)
= 25N – 14N
= 11N

8. TRY IT!
If the mass of a helicopter is 4,500 kg. and the net force on it is 18,000 N, what is the helicopter’s acceleration? Given: Finding? m = 4,500 kg a = ? (acceleration) F = 18,000 N F = m x a a = F_ m = 18,000 N 4,500 kg a = 4 m/s2 F a m

9. Gravity Gravity is the attractive force between two objects
Gravity’s strength depends on the masses of the objects and the distance between them

10. Force of Earth’s Gravity
F = mg
m is the mass of the object gravity is acting on
g is the gravitational acceleration constant: g = 9.8m/s2
The “F” on Earth is weight.

11. Let’s try one!
On earth, what is the weight of a television having a mass of 75 kg?
F = mg where ‘F’ is weight
m= 75 kg g = 9.8 m/s2 weight =?
weight = (75 kg) (9.8 m/s2)= 735 kg x m/s2
weight = 735 N

12. Try one by yourself…..
Find the acceleration due to gravity on Mars if a person with a mass of 60.0 kg weighs 22.2 N on Mars?

13. Normal Force Fnormal + Fgravity = 0 N
When gravity acts on an object, but the object does not fall, there is a normal force pushing up on the object
Fnormal + Fgravity = 0 N

14. Balanced Forces If an object is not accelerating, either:
1. no forces are acting on it, or
2. all the forces balance out
Two forces are balanced if they have BOTH:
equal sizes
opposite directions

16. Unbalanced Forces Every force has a magnitude and a direction
If the net force (sum of the magnitude in each direction) is not zero, the object accelerates
Forces in the same direction add together.
Forces in opposite directions are subtracted.

17. Force Diagrams
Force diagrams use different sized arrows to represent the Forces acting on an object
Including: sizes, directions, and type of force
The object is usually represented by a box

19. EXAMPLE 1
Draw the forces affecting a textbook on the table
BOOK
BOOK

20. EXAMPLE 2
Draw the forces affecting a textbook being pushed across the table at a constant velocity
BOOK
BOOK

21. Special Forces

22. Newton’s Third Law
For every action force, there is an equal and opposite reaction force
Action and Reaction forces do not cancel
-they can act on different objects or spread in different directions

23. Air Resistance
Air resistance is a force that opposes the motion of objects that move through air
Air resistance depends on an object’s size, shape, and speed
Ignore air resistance in this class unless you are told to include it

24. Friction Friction is the force that opposes motion.
Sliding friction – opposes the motion of 2 surfaces as they slide against each other

25. Static Friction
Static friction – prevents 2 surfaces from sliding against each other at all
Rolling friction – slows down the motion of an object rolling on a surface
Static friction keeps a wheel from spinning in place and makes it turn instead

26. Projectile Motion

27. The “nitty-gritty” of projectiles
The only forces that acts on a projectile is gravity.
So, the only acceleration is in the downward direction.

28. Types of Projectiles
Drops- no initial velocity in the horizontal or vertical direction
Throw vertically- initial velocity in the vertical, but not in the horizontal
Launch- initial horizontal and vertical velocity