1. Forces at an angle: Saying goodbye to N=mg
You have to get N right to get friction right! Any forces with a “y” component will change N.

2. Inclined planes (ramps, slopes)
If there is no friction, what is a?

3. Inclined planes (ramps, slopes)
What is the normal force?

4. Inclined planes
A 500 N force is applied horizontally to a 20 kg block.
P1. The normal force is
A. mgcos40
B. mgcos sin40
C. mgcos sin40
D. mgcos cos40
E. mgcos cos40

5. Write F =ma to find a up the incline. Include friction with mk

6. Newton’s first law
25mph collision…what happened here?

7. Newton’s first law “Natural state of matter”
Aristotle: Matter comes to rest
Newton’s 1st Law: An object at rest will stay at rest, an object in motion will move with a constant velocity vector, unless a force acts on it.
The first law is really a special case of the second law:
When net force is zero, the velocity vector is constant

8. Newton’s first law
In a closed space ship, can you tell if you are… a) moving at constant speed? b) accelerating?

9. Newton’s first law
What might cause difficulty for this passenger?

10. Newton’s first law
To be able to ascribe accelerations to real forces (which come from other objects), you must be observing the motion from a reference frame that is not accelerating.

11. Newton’s first law
Ball twirled in a circle from a string at constant speed
P2. Is the ball accelerating:
Yes No
P3. If the string breaks, the ball will (neglecting gravity)
curve inward
curve outward
move in a straight line.

12. Newton’s first law
You are sitting in the back right seat as the car is going fast and turning right as shown. Without a seatbelt, you find yourself sliding to the left on the seat.
P4. From Newton’s view this is because
a) friction pulling inward is not enough b) your motion pushes you outward.
c) a force pushes you outward

13. Lecture 7, 1st law and inclined forces
Basic concepts:
objects keep constant velocity (which can be zero) if a is zero
velocity is relative
best to use x along an inclined plane, y perpendicular
Basic problems, skills:
Finding the normal force from adding all y forces to zero
Advanced concepts:
Turning is a form of acceleration
Advanced problems, skills:
Using correct normal force and all components of forces to find acceleration, including for coupled objects