CHAPTER 3 SECTION 3Using Newton’s Laws

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

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)

WHAT HAPPENS IN A CRASH? Safety Belts

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

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

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.

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

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

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

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

FORCE AND MOMENTUM

Newton’s 2nd Law Re-written

Vf = final volume Vi = initial volume m = mass

F = force a = acceleration t = time

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)

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

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?

HOMEWORK

Work on the 3.3 Worksheet Use the equations from this section to

answer the calculation problems

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!