1. Chapter 3 Section 3
Using Newton’s Laws

2. What forces are exerted on a rocket?
From what we learned in the last few lessons, what type of forces do you think act on a rocket NASA is about to shoot into space?
Some forces we know so far:
Balanced/unbalanced forces
Static/Sliding Friction
Gravity
Weight
Action/Reaction forces

3. What happens in a crash?
Newton’s 1st Law of Motion explains what happens in a car crash
The 1st law states that a car’s passengers will continue to travel at the same velocity that the car was moving unless a force acts on them. (This is why you fly forward in a crash)

4. What Happens in a Crash? Safety Belts Air Bags
Safety belts slow down your forward momentum by exerting a force on your body
Safety belts loosen a little as they restrain you to increase the time it takes your body to slow down, meaning a smaller acceleration and smaller force exerted on your body
Air Bags
Air bags reduce injury by providing a cushion that reduces acceleration of passengers and prevents them from hitting the dash
When impact occurs a chemical reaction occurs in the air bag to produce nitrogen gas that expands the bag rapidly, the bags also deflate rapidly as the gas escaptes the bag through tiny holes

5. Newton’s 2nd Law & Gravitational Attraction
Newton’s 2nd Law states that an object’s acceleration is the force of gravity divided by the object’s mass
Notice that the object’s mass cancels and the object’s acceleration due to gravity is equal to the strength of gravity
Acceleration = Gravity

6. Air Resistance
Air resistance: a friction-like force that opposes the motion of objects that move through the air
The amount of air resistance on an object depends on the size, shape, and speed of the object, as well as the properties of the air
If there was no air resistance the feather and the pool ball would fall at the same acceleration.

7. Air Resistance- Size and shape
The more spread out an object is, the more air resistance it will experience
A crumpled plastic bag will fall faster than one that is spread out
The downward force of gravity is the same for both bags
The upward force of air resistance on the crumpled bag is leass

8. Air Resistance- speed & terminal velocity
The amount of air resistance also increases as the object’s speed increases
Terminal velocity: the max speed an object will reach when falling through a substance, such as air
Occurs when the upward air resistance force becomes large enough to balance the downward force of gravity resulting in a net force of zero
This means the acceleration of the object is zero and the object falls at a constant speed

9. Air Resistance-Free Fall
Free Fall: when gravity is the only force acting on an object
Ex: If the feather and the pool ball fell in a vacuum they would be in free fall
Ex: Earth is in free fall around the sun (if earth did not have a velocity perpendicular to the gravitational force it would fall into the sun)
Ex: Satallites are in free fall around earth

10. Centripetal Forces
Centripetal force: a force exerted toward the center of a curved path
Anything that moves in a circle is doing so because of a centripetal force accelerating it toward the center

11. Force and Momentum Newton’s 2nd Law Re-written
Vf = final volume Vi = initial volume m = mass
F = force a = acceleration t = time

12. Conservation of momentum
Law of Conservation of Momentum: if not external force acts on a group of objects, their total momentum does not change
This follow’s Newton’s 3rd Law (forces are equal and opposite in direction)
Ex: When a cue ball collides with another ball the momentum from the cue ball is transferred to the other ball (none is lost)

13. Collisions with multiple objects
When the cue ball collides with the group of motionless balls, the cue ball slows down and the rest of the balls begin to move
The momentum that the group of balls gains is equal to the momentum that the cue ball lost
Thus momentum is conserved

14. Rocket propulsion Rocket science follows Newton’s 3rd Law
The momentum of the gas expelled from the bottom of the rocket is conserved by the opposite momentum of the rocket
The total momentum of the rocket and fuel is zero (because they are equal in size and opposite in direction)
By controlling how much gas is ejected the rocket’s momentum can be controlled
So, what forces really are exerted on a rocket?

15. Homework Work on the 3.3 Worksheet
Use the equations from this section to answer the calculation problems

16. Wrap up What did you learn today about momentum?
How does air resistance work?
Why do passengers fly forward in a car crash?
What is terminal velocity?
Have a Great Day!