1. Types of forces
And
Free Body Diagrams

2. What types of forces
do you already know?

3. Forces are usually divided into two types.
Different types of forces
Forces are usually divided into two types.
Contact forces occur because of physical contact between objects.
Examples: pushing open a door
pulling on a rope
Field forces – Act at a distance through space. The
presence of an object effects the space around it, creating a
region of space around the object called a field.
Example: gravitational field
magnetic field around a magnet

4. Force in Newtons is a VECTOR.
Vector Vs Scalar
Force in Newtons is a VECTOR.
There is a direction, and positive and negative numbers relate to movements on a plane. (Y+ as up, Y- as down)
There is a scalar unit for acceleration = Gs.
Each G is one Gravity. We are all under one G right now.

6. Contact Forces Normal force 𝐅 𝐧
Force that prevents an object from falling through the surface of another body
Always acts perpendicular to the surface
Always equals the forces applied to the surface
(or surface will break!) Fn
Called the ‘normal’ force because it is always normal (perpendicular) to the surface. mg

7. Contact Forces Normal force 𝐅 𝐧
Force that prevents an object from falling through the surface of another body
Always acts perpendicular to the surface
Always equals the forces applied to the surface
(or surface will break!)
Ffr Fn
mg sin q
mg cos mg Fn mg F Fn mg

8. Contact Forces Friction force Ffr
Friction is a force that is created whenever two surfaces move or try to move across each other. 
Friction always opposes the motion or attempted motion of one surface across another surface.
Friction is dependent on the texture/roughness of both surfaces.
Friction is also dependent on the force which presses the surfaces together. 
Friction caused by microscopic roughness in surfaces and, in ultra-smooth surfaces, by molecular attraction
motion
friction

9. Contact Forces Air resistance / Drag
When an object moves through air or any other fluid, the fluid exerts a friction-like force on the moving object. The force is called drag.
Drag depends upon the speed of the object, becoming larger as the speed increases. (UNLIKE FRICTION!)
Drag also depends upon the size and the shape of the object and the density and kind of fluid. (UNLIKE FRICTION!)
Friction caused by microscopic roughness in surfaces and, in ultra-smooth surfaces, by molecular attraction
B/c drag increases with speed, object moving through the air reach a terminal velocity – a maximum speed at which Fg = Fdrag so there is no more acceleration.

11. Contact Forces Air resistance / Drag
When an object moves through air or any other fluid, the fluid exerts a frictionlike force on the moving object. The force is called drag.
Drag depends upon the speed of the object, becoming larger as the speed increases. (UNLIKE FRICTION!)
Drag also depends upon the size and the shape of the object and the density and kind of fluid. (UNLIKE FRICTION!)
Friction caused by microscopic roughness in surfaces and, in ultra-smooth surfaces, by molecular attraction
Without drag, raindrops would fall 340 m/h.
With drag, they only fall 17 m/h.

12. Contact Forces Tension
the force that the end of the rope exerts on whatever is attached to it.
Direction of the force is along the rope.
What is the relative force along the two yellow arrows?
Why?
physics T2 T2 T1

13. Contact Forces Spring Force Force due to the elasticity of a material
Depends on the elasticity of the spring
Direction is opposite displacement

14. Contact Forces: Think PaIR SHARE
Type of Force
Direction
Normal
Friction
Drag
Tension
Spring

15. Contact Forces: Think PaIR SHARE
Type of Force
Direction
Normal
Perpendicular to surface, opposite applied / gravitational forces
Friction
Opposite motion
Drag
Tension
Along the rope & opposite motion
Spring
Opposite displacement

16. Field Forces Field Forces Relative Strength Action Distance
Gravitational Force
attraction between objects due to their masses
10-45
Infinite – but decreases with square of distance
Electromagnetic Force
between charges
10-2
Strong Nuclear Force
keeps nucleus together 1
Very short!
Weak Nuclear Force
arise in certain radioactive processes
10-8
Very very short!

17. Field Forces Field Forces Relative Strength Action Distance
Gravitational Force
attraction between objects due to their masses
10-45
Infinite – but decreases with square of distance
Electromagnetic Force
between charges
10-2
Strong Nuclear Force
keeps nucleus together 1
Very short!
Weak Nuclear Force
arise in certain radioactive processes
10-8
Very very short!
At the atomic level – all contact forces are result of repulsive electromagnetic forces – the repulsion of atoms’ electric fields

18. How to Solve Force Problems
Draw a free body diagram – label all the forces acting on one object.
Add up the forces
Apply Newton’s second law: F = ma.

19. How to draw a force diagram
3. Identify forces that act on the system
Label them on diagram
How to draw a force diagram
1. Choose ONE body to be isolated
dog or the cart?
dog
decision: cart fr
2. Make a simple sketch of the system – point system
net
4. Find out the net force by adding the force vectors
5. Apply Newton’s second law

20. Weight on the moon
Divide your weight by to get your approximate mass in kilograms.
Weight = Mass x Acceleration of Gravity
On Earth, that’s 9.8m/s² but on the moon it’s a measly 1.62m/s²
How much do you weigh on the moon?