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Chapter 3-Forces
Section 3.1 Newton’s 2nd Law
I. Force, Mass, and Acceleration
A. The greater the force exerted on an object the greater the object’s
acceleration.
B. The greater the mass an object has the greater its acceleration
C. Force, mass, and acceleration are ALL related!!
II. Newton’s 2nd Law-the net force acting on an object causes the object to
accelerate in the direction of the net force.
A. Formula:
Acceleration (a)=net force (F)
Mass (m)
A= F
M
B. Units:
1. a=m/s2
F=Newton (N) or kg x m/s2
m=kg
C. Ex: A student pedaling a bicycle applies a net force of 200 N. The
mass of the rider and the bicycle is 50 kg. What is the acceleration of the
bicycle and the rider?
D. Using Newton’s 2nd law
1. Can solve for F or m
F=ma
M=F
A
2. Ex: A tennis ball has the acceleration of 5500 m/s2 and the mass
of 0.06 kg. What is the force?
III. Friction-the force that opposes motion between two surfaces that are touching
each other.
A. Depends on two factors
1. Kind of surface
2. Force pressing the surfaces together
B. What causes friction?
1. Microwelds form between two surfaces
a. Where two surfaces stick together
2. The stronger the force, the stronger the microwelds will be
C. Three types of Friction
1. Static Friction-the friction between two surfaces that are not
moving past each other
a. Ex: Pg 71 figure 5
2. Sliding Friction-the force that opposes the motion of two
surfaces sliding past each other
a. Caused by microwelds breaking and forming again.
3. Rolling Friction-the friction between a rolling object and the
surface it rolls on
a. Less than static or sliding friction
D. Air Resistance-the force opposing gravity
1. Depends on:
a. Speed, size, and shape of an object
b. The greater the surface area=greater air resistance an
object has
E. Terminal Velocity-the highest velocity that a falling object will reach
1. Parachute
Section 3.2 Gravity
I. The Law of Gravitation-when any two masses exert an attractive force on
each other.
A. Depends on:
1. mass of the objects
2. Distance between the objects
B. Four types of forces
1. Gravity
2. Electromagnetic
3. strong nuclear
4. Weak nuclear
II. Gravitational Acceleartion
A. Gravitational Acceleration (g) = 9.8m/s2
B. When an object is affected by the force of gravity it is said to be in free
fall
C. All objects fall with the same acceleration no matter the mass/inertia
D. Formula:
F=mg
III. Weight-gravitation force exerted on an object
A. Formula:
Weight = mass x 9.8 m/s2
B. Unit = N (Newton)
C. Ex: How much does a person with a mass of 70 kg weigh on Earth?
D. Losing weight
1. The farther you are from the Earth, the lighter you are
2. Weight and mass are NOT THE SAME!!
IV. Weightlessness and Freefall
A. To be weightless you need to be free from the effects of gravity
B. In free fall everything is falling at the same rate
1. Ex: Elevator
V. Projectile Motion
A. Projectiles follow a curved path because of gravity and inertia
B. Follows a horizontal and a vertical motion
VI. Centripetal Force-an unbalanced force action on an object toward the center
A. Centripetal acceleration-acceleration toward the center of a curved or
circular path
B. Ex: Moon orbiting the Earth
1. Caused by gravitational force and the moon’s straight line of
motion
Section 3.3 The Third Law of Motion
I. Newton’s 3rd Law of Motion- (action-reaction)-in every action force there is an
equal and opposite reaction force.
A. Action and Reaction
1. Ex: jumping on a trampoline
B. Even though forces are equal they are not balanced because they act
on different objects
1. Ex: Swimming
C. Rocket propulsion
1. Action-force of the rocket engine pushing the air
2. Reaction-force of the air against the rocket engine
II. Finding Planets with Newton’s Laws
A. Orbit of the planets is caused by gravitational force between the planets
1. Determined by mass and distance
B. Momentum-the product of an object’s mass and velocity
1. Unit=kg x m/s
2. Calculation
Momentum (P)=mass (m) x velocity (v)
3. If two objects have the same velocity but different masses, the
object with the larger mass will have more momentum
4. Force and changing momentum
a. Can find the net force of an object by taking the change in
momentum and dividing by time
5. Law of Conservation of Momentum-momentum can be
transferred but not created or destroyed
a. Ex: playing pool
6. Colliding objects
1. Depends on the momentum of the different objects
2. If an object hits another from behind, the 2nd object
receives the momentum in the same direction
3. If objects are moving toward each other with the same
speed and mass, the total momentum is Zero