Download Chapter 3 - Mrs. Wiedeman

Chapter 3
Forces
Section 1
Newton’s Second Law
Force, Mass and Acceleration





Compare hard thrown ball vs. gently tossed
ball
Force? Velocity? Acceleration?
Throw hard a baseball vs. softball
Who goes faster? Why?
Velocity and Acceleration depend on mass
Newton’s 2nd Law of Motion

Def: acceleration is in the same direction as net
force
Net Force


Net Force = mass x acceleration Fnet = ma
kg x m/s2 = N (Newton)
Practice!


Rocket has 28,913 N of thrust and a mass of
2,350 kg. What is the acceleration?
(frictionless surface)
What is the mass of a rocket that has a thrust
of 105 N and an acceleration of 70 m/s2?
Friction




Roll a ball in the parking lot…
Newton’s 1st law says ball will keep constant
speed
Stops because of friction (negative
acceleration – force is friction)
Def: force that opposes the sliding motion
Microwelds



Def: points where two surfaces come in to
contact
Larger force = stronger microwelds
Break microwelds = need force
Static Friction




Push box = doesn’t
move
Acceleration = 0 so net
force = 0
What’s canceling your
force?
Static Friction: force
prevents two surfaces
from sliding past each
other
Sliding Friction


Def: force that opposes
the sliding motion of
two surfaces
Stop pushing = box
stops
Rolling Friction


Car stuck in mud…
Def: frictional force between rolling object and
surface.
Air Resistance



Falling object pulled by
gravity – opposed by air
resistance
Def: friction like force
opposed motion as moves
through air
No air = objects fall with
same acceleration bowling
ball vs. feather
Terminal Velocity




Def: highest speed a
falling object will reach
When air resistance
balances downward
force
Depends on size, shape,
mass
Sky diver no parachute
vs. with parachute
Section 2
Gravity
What is gravity?





How would you throw a long pass with a
football?
Anything that has mass is attracted by gravity
Gravity: attractive force between two objects
that depends on masses and distance
Stronger force?
More mass/closer together
Gravity





We only feel earth’s gravitational force
1 of 4 Basic Forces
Others:
electromagnetic force – electricity and
magnetism
strong and weak nuclear force – inside nuclei
of atoms
Law of Universal Gravitation

Sir Isaac Newton
Earth’s Gravitational Acceleration





When only force acting on two things is
gravity – they fall at the same rate
Close to earth’s surface acceleration due to
gravity is 9.8 m/s2
Newton’s second law  F = ma
Fgravity(N) = mass (kg) x acceleration of gravity (m/s2)
F = mg
Weight

Def: gravitational force on a object

Calculate your weight in N
Weightlessness and Free Fall



When you free fall gravity pulls everything at same
rate = no gravity
Spaceship in orbit experiences 90% of earth’s
gravity
Ship is in free fall in orbit = no gravity
Projectile Motion




Projectile: anything thrown or shot through air
Gravity causes path to be curved
Horizontal motion – throwing
Vertical motion – gravity
Horizontal and Vertical Distance


Same vertical distance = same mass = same
acceleration = same time
Different horizontal distance
Centripetal Force



Centripetal acceleration: acceleration towards the
center of a curved path.
Newton’s 2nd law – acceleration in direction of force
 centripetal force
Anything moving in circle is because cent. force is
accelerating towards center
Gravity
Section 3
Newton’s 3rd Law of Motion
Newton’s 3rd Law



Def: when one object exerts a force on
another, the 2nd object exerts an equal but
opposite force
Action – Reaction
Jumping on trampoline
Action/Reaction Do NOT Cancel


Forces are equal, but acting on different objects
Act on water – reaction pushes forward = unbalanced
force
Rocket Propulsion

Rocket exerts force on gas backwards – gas exerts
force on rocket forwards
Momentum


How much force is needed to change motion?
Def: product of mass and velocity
Practice!


What is the momentum of a 1,300 kg car
traveling 28 m/s?
What is the velocity of a 0.15 kg baseball that
has a momentum of 6 kg m/s?
Force and Changing Momentum

Catch a foul ball bare handed?
Force and Changing Momentum

a = (vf – vi)
t
Put the two together:

When that baseball hits – calculate the force

F = ma
Force and Changing Momentum

What is the force of a 0.15 kg baseball with a
velocity of 40 m/s when it comes to a stop in 1
second?
Law of Conservation of Momentum



Momentum doesn’t change unless mass,
velocity or both change
Can be transferred – pool balls
Def: group of objects exerts forces only on
each other, their total momentum doesn’t
change
When Objects Collide

1st puck will give more
momentum to the 2nd puck in
the same direction

Moving towards each other
same speed = 0 momentum
Collide = 0 momentum –
moving same speed opposite
direction
