Forces
• Def – an action exerted on an object to change
the object’s state of motion (resting or moving)
– Units: Newtons (N)
• Net force – total force on an object
Balanced forces
• Net force = 0, no movement
– An object standing still will not move
– An object moving will not stop
Unbalanced Forces
• Net force is greater than zero
– Object will move
Friction
• Force that opposes motion between two surfaces
that are in contact
2 types
– Static – friction between two forces at rest, initial
friction an object must overcome
– Kinetic – friction between two moving objects
Newton’s Laws
• He was born the same
year that Galileo died
• Newton had ideas about
motion, which he called
his three laws of motion
– Also had ideas about
gravity, the diffraction of
light, and forces
• Physics and Math
• Helped create Calculus
First Law
Law of Inertia
• An object at rest will stay at rest and an object in
motion stays in motion unless acted on by an
external force
• Inertia – tendency of an object to stay at rest or
stay in motion
– Mass is a measure of inertia
– Small mass = less inertia
• Seatbelts
1. Explain how this activity demonstrates the law of
inertia.
The penny was at rest, and when you thumped the card from under it quickly, the
penny remained at rest. It simply fell into the cup, rather than flying away with the card.
2. Do you think the texture of the card is important?
The rougher the texture of the card, the less likely the trick will work. The friction
of the card could place a force on the penny, causing it to move with the cloth rather
than staying.
3. What would happen with a quarter rather than a penny?
Second Law
• The force acting on an object equals the object’s
mass times its acceleration
– F=ma
– Force is measured in Newtons (N)
• 1 N = 1 kg x 1 m/s2
2nd Law Problem
1. What is the force necessary for a 1600 kg car to
accelerate forward at 2.0 m/s2 ?
2. Mike's car, which weighs 1,000 kg, is out of gas.
Mike is trying to push the car to a gas station,
and he makes the car go 0.05 m/s2 . How much
force Mike is applying to the car.
Third Law
• For every action there is an equal and opposite
reaction
– Equal force but opposite in direction
• Example: Balloon full of air being released
Write Newton’s 3 laws.
Agenda for Wednesday Feb 19th
1. Review Newton’s laws
2. Bozeman Science vid
3. Gravity
Force of Gravity
• All objects are attracted to each other through
gravitational force
• Bigger mass increases – larger force of gravity
• Distance increases – force of gravity decreases
• Universal Gravitation Equation
F=G(m1m2)
d2
What is the force of gravity when…
What happens to the force of
gravity if….
Objects start close
Then move
farther away
Force of Gravity
• Free fall – only force of gravity is affecting an
object
• Calculate the acceleration at which an object
free falls
– Earth – 9.8 m/s2
Force of Gravity
• Weight is not the same as mass
– Depends on gravity
– Different locations have different gravities and
therefore different weights
• Force of gravity on an object is weight
– weight = mass x free fall acceleration
– w=mg
• Newtons (N)
A person that weighs 50 kg runs and hits a
wall with a force of 50 N. What was the
person’s acceleration as they hit the wall?
With what force did the wall hit the person?
Agenda for Friday Feb 21st
1. Go over Force Problems & Newton’s laws
worksheet
2. Whiteboard questions
3. Energy notes
Quiz Wednesday – newton’s laws
Need a whiteboard, marker,
paper towel, and calculator.
1. Who was the scientist who gave us the Laws of
Motion?
2. Which law explains why we need to wear
seatbelts?
3. Which law says that force is equal to mass times
acceleration (F=MA)?
4. Which law explains how rockets are launched
into space?
5. Which law says that heavier objects require more
force than lighter objects to move or accelerate
them (Throwing a bowling ball requires more
force than throwing a baseball)?
6. In the absence of air resistance, describe the
acceleration of a 1.5 kg book compared to the
acceleration of a 15 kg rock if the objects were
dropped from the same height?
7. What happens to the gravitation force on an
object as it gets closer to another object?
8. There are 2 rocks. Rock A has a mass of 50 kg
and rock B has a mass of 1000 kg. There would
be a greater gravitational force between you and
which rock?
9. If a 50 kg person hits a wall with a force of 25
N, with what force does the wall hit the person
back?
10. What is the weight of a person on earth who
weighs 25 kg?
11. A person kicks a ball with a force of 11 N. It
travels at a rate of 3 m/s2. What is the ball’s
mass?
12. What force is applied to a 2 kg ball to make it
travel at 32 m/s2?
13. What object would have a greater inertia?
A. A car
B. A semi – truck
14. Why?
Gravity
• Read pages 352 – 359
• Answer questions 1-6 on page 359
Projectile Motion
• Vertical and horizontal components
– No affect on each other
– Combined they form a curved path
• Same acceleration downward because of gravity
and will land at the same time
Momentum
• Def. – mass and velocity of an object
– P=mv
– Units: kg x m/s AND direction
• Objects with larger mass have more momentum
• Objects moving faster have more momentum
Calculate Momentum
Calculate the momentum of a 6.00 kg bowling ball
moving at 10 m/s toward the pins.
Calculate the momentum of the following objects:
a. a 75 kg speed skater moving forward at 16 m/s
b. a 135 kg ostrich running north at 16.2 m/s
c. a 5.0 kg baby on a train moving east at72 m/s
d. a seated 48.5 kg passenger on a train that is
stopped
Force and Momentum
• To catch a ball you use a force to stop the ball
– You change the ball’s momentum
– If you increase the time to catch the ball you
reduce the force
Momentum and Collisions
• The total amount of momentum is an isolated
system in conserved
– m1v1 = m2v2
• During a collision, momentum can be
transferred
– Hitting billiard ball