1. Newton’s Second Law of Motion
VIDEO

3. Force
It’s what causes “things” to accelerate
Unbalanced Force
1000 kg

4. If all forces are balanced there is no acceleration in any direction.
Balanced Forces
If all forces are balanced there is no acceleration in any direction.
(Either Zero Motion or Constant Velocity)

5. Unbalanced Forces Causes Acceleration
Adding Forces
Subtracting Forces

6. Force equals mass times acceleration.
Newton’s Second Law
Force equals mass times acceleration.
F = ma
Acceleration: a measurement of how quickly an object is changing velocity.

8. Mayweather V. Tyson
Mike Tyson
Mayweather

9. Solving Newton’s Second Lawm F a
F = ma
F: force (N)
m: mass (kg)
a: accel (m/s2)
1 N = 1 kg ·m/s2

10. a F m Problem #1 “I DO” m = 40 kg a = 4 m/s2 Unknown: FORCE F = ma
What force would be required to accelerate a 40 kg mass by 4 m/s2?
GIVEN:
m = 40 kg
a = 4 m/s2
Unknown: FORCE
Equation
Substitute
F = ma
F = (40 kg)(4 m/s2)
Solve
F = 160 N m F a

11. a F m Problem #2 “WE DO” m = F = ma a = F= Unknown: Solve
What force would be required to accelerate a 80 kg mass by 12 m/s2?
GIVEN:
m =
a =
Unknown:
Equation
Substitute
F = ma F=
Solve
F = (___)(____)
F =_____N m F a

12. a F m Problem #3 “WE DO” a = F ÷ m Unknown: a = (30 N) ÷ (4.0 kg)
A 4.0 kg shotput is thrown with 30 N of force. What is its acceleration?
GIVEN:
m = 4.0 kg F = 30 N
Unknown:
Equation:
Substitute:
a = F ÷ m
a = (30 N) ÷ (4.0 kg)
Solve:
a = m F a

13. INDEPENDENT PRACTICE Please use GUESS format
5. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine the mass.
6. How much force is needed to accelerate a 66 kg skier 3.6 m/s2?

14. What is the difference between weight and mass?
Amount of matter
(kg)
WEIGHT
Mass and gravity
(N)

15. Weight- the force of gravity on an object
W = mg
UNITS
W: weight (N)
m: mass (kg)
g: acceleration due to gravity (m/s2)
GRAVITY =9.8(m/s2)

16. g W m Problem #6 “WE DO” M= 56.8kg W = m X g G = 9.8 m/s2
Mrs. J. mass is 56.8 kg. What is her weight?
GIVEN:
M= 56.8kg
G = 9.8 m/s2
Unknown: Weight
Equation:
Substitute:
W = m X g
W = (56.8 kg) X (9.8 m/s2)
Solve: W = N m W g

17. g W m Problem #7 “YOU DO” M= ______kg W = ________ G = 9.8 m/s2
A person with a mass of 90 kg has a weight of _____?
GIVEN:
M= ______kg
G = 9.8 m/s2
Unknown: Weight
Equation:
Substitute:
W = ________
W = (___ kg) X (9.8 m/s2)
Solve: W = ______ N m W g

18. Problem #8 “YOU DO”
A person with a mass of 50 kg has a weight of N.
GIVEN:
Unknown:
Equation:
Substitute:
W = ________
W = (___ kg) X ( )
Solve: W = ______ N m W g

19. Which rock would hit the ground first?

20. 2 ROCKS with different masses accelerate to the ground at the same rate (On Earth = 9.8 m/s2).
However, because of the 2nd Law we know that they don’t hit the ground with the same force.

21. Air Resistance
Air resistance acts in the opposite direction to the motion of an object through air.
If the object is falling downward, air resistance acts upward on the object.
What forces are acting against the Parachutist ???
The size of the air resistance force also depends on the size and shape of an object.

22. Balance between Gravity and Friction
Q: If the friction of the air and the pull of gravity were in perfect balance, what
would the skydiver be doing?
A: The skydiver would be going a constant velocity (moving with no acceleration) down towards the ground until it hit the ground.
The skydiver is colliding with the
molecules of gas in the air!
Which slows down the fall of the skydiver!

23. How it works Speed: Air Resistance 0 m/s 25 m/s 28 m/s 20 m/s 10 m/s
Constant Velocity
How it works
Notice that as the skydiver leaves the plane, the force of gravity accelerates him faster towards the earth; however, the moment he jumps out of the plane, another force begins to oppose gravity – Air Resistance, (Friction).
This friction with the air begins to slow down his rate of acceleration. Meaning, he’s still speeding up, just not as fast.
Once the force of air friction equals the force of gravity, the skydiver steadily descends towards earth at a constant velocity.
Accelerating
Accelerating
Air Resistance
30 m/s
0 m/s
Speed:
0 m/s
25 m/s
28 m/s
20 m/s
10 m/s
Constant Velocity
Gravity

24. Terminal Velocity
An object reaches its terminal velocity when the force of air resistance = the force of gravity

25. Restate Newton’s Second Law in your own words
Which would require a greater force to
accelerate? WHY?
A hockey puck on ice or a hockey puck on uncut grass?
An empty suitcase or a suitcase full of bricks?
3. If the mass of a rock is doubled, what happens to its acceleration if the force does NOT change?

26. What is Friction??
Friction is the force that opposes the sliding motion of two surfaces

27. Static Friction
Static friction is the frictional force that prevents two surfaces from sliding past each other.
A cardboard box filled with books and want to move it.
It’s too heavy to lift, so you start pushing on it, but it doesn’t budge.

28. Static Friction
If the box doesn’t move, then it has zero acceleration.
Is this balanced or unbalanced force?
According to Newton’s second law, if the acceleration is zero, then the net force on the box is zero.

29. Sliding Friction You ask a friend to help you move the box.
Pushing together, the box moves. Together you and your friend have exerted enough force to push the box
Sliding friction is the force that opposes the motion of two surfaces sliding past each other.

30. Rolling Friction
As a wheel rolls over a surface, the wheel digs into the surface, causing both the wheel and the surface to be deformed.
Rolling friction is the frictional force between a rolling object and the surface it rolls on.