1. Forces
Chapter 4

2. What is a Force? Push or a Pull
Must have direction as well as size - vector
Can be at a distance or in contact
Gravity, friction

3. 4 main Forces in Physics Gravity Electromagnetic Weak nuclear
Strong nuclear

4. Complications of Forces
Force is often doing 2 things
Example – dragging a desk across the room
Pulling over, but also pulling up
Want to split force into the two parts

5. Back to Forces -history
Aristotle – natural and violent motion
Copernicus – Earth moves!
Galileo – Forces are push or pull – friction causes things to slow down, stop

6. Newton’s Laws
Newton comes up with 3 laws that help understand forces and motion
1st Law – “An object in motion will stay in motion; an object at rest will stay at rest unless acted on by an outside force”

7. Newton’s 1st Law Does this make any sense? Examples? Counter Examples?
Sometimes called law of inertia
Definition of inertia – tends to do what it is doing
Measure of inertia – mass

8. Equilibrium All forces cancel out Net Force is 0
Needed to apply 1st law (no net outside force)
Ex: - skydiver

9. Friction – What is it?
2 surfaces rubbing together causing motion to be lost - Velcro
Tug of war
Hard to demonstrate 1st law with friction present
Can be from solid, liquid, or gas
static vs. sliding friction

10. Newton’s 2nd Law
Here Newton connects the idea of inertia (mass) with those of Force and Acceleration
2nd Law – “The acceleration a force produces is directly proportional to the size of the force and indirectly proportional to the mass of the object”
or put in terms of math …F = ma

11. Newton’s 2nd Law: F=ma
Means it is harder to accelerate something with a lot of mass
Means something with little mass accelerates with little force

12. Weight – example of 2nd Law
Amount of force that gravity places on an object
Is a force
Units: Newton = ?

13. Weight = gravity * mass W = mg; example of F = ma
Calculating Weight
Weight = gravity * mass
W = mg; example of F = ma
9.8(N) = 1(kg)*9.8(m/s2)

14. 2nd Law Problems
If you push a 55 kg box across an ice covered pond with a force of 125 Newtons, what is it’s acceleration?
A rock has a weight of 225 N. What is its mass?
A 65 kg sled has a .20 coefficient of friction. If it is being pulled with a force of 250 N, what is its acceleration?

15. Newton’s 3rd Law
Forces always exist in pairs; if one object exerts a force on a second one, the second one will exert an equal and opposite force on the first one
If A acts on B, then B acts on A

16. Newton’s 3rd Law Examples: Problem: Smart horse Tug-of-war
hand on the wall
Pulling on a rope
Problem: Smart horse
Tug-of-war