Chapter 5
Newton’s 2nd Law of Motion
Force, mass and acceleration
Newton’s 2nd Law – Force is directly related to the mass and
acceleration of an object (F=ma)
Acceleration = change in velocity / time
Force causes acceleration – Force ~ acceleration
Mass resists acceleration - acceleration is inversely proportional to the mass:
acceleration ~ 1/mass
Keeping the force the constant, if you triple the acceleration, the mass is:
A) Tripled
B) 1/3
C) 1/9
D) 1/2
Newton’s
nd
2
Law
If you double the force on an object, the acceleration will:
A) ½ B) ¼ C) not change D) Double
Acceleration is measured in m/s²
Force is measured in Newtons
Mass is measured in kg
Force = Mass x Acceleration
Mass = Force / Acceleration
Acceleration = Force / Mass
If the man pulls on the box with 400 N,
And the box accelerates at .5 m/s², what
Is the mass of the box?
A) 200 kg
B) 400 kg
C) 800 kg
Newton’s
nd
2
Law
More practice
If a truck can accelerate at 2 m/s²,
What acceleration can it attain of it is
Towing another truck of equal mass?
A) .5 m/s²
B) 1 m/s²
C) 2 m/s²
D) 4 m/s²
How much force is required to move a 4000 kg mass 2 m/s²?
A) 2000 N
B) 4000 N
C) 8000 N
D) 16,000 N
Newton’s
nd
2
Law
Friction
Irregularities of surfaces when objects are in
contact with each other. This is sliding or
rolling friction
Fluid friction –resistance that objects undergo as
they flow through a fluid (not to be confused
with a liquid)
What type of friction is air resistance? A) Sliding B) Rolling
C) Fluid
Friction
When friction is present, an object may move at a
constant velocity even when an outside force is
applied to it. The frictional force balances out
the force applied.
B
A
C
200N
500 N
500 N
300 N
400 N
While your driving 55 mi/hr in your car, you need to apply a constant
Force (gas pedal) to maintain a constant speed - you are overcoming the
Frictional force that opposes motion.
Friction
If the forces on an object are balanced – IT DOES
NOT MEAN IT IS ALWAYS STOPPED!!!!!!
If forces are balanced – it means there is no
acceleration!
A gas pedal on a car is pressed and the car accelerates to 50 mi/hr at
Which point it maintains a constant speed. If the car has to maintain
A constant force of 900 N (by pressing the gas), what is the force
Applied by friction?
A) 0 N
B) less than 900 N
C) 900 N
D) more than 900 N
Friction
A skydiver jumps out of an airplane and
accelerates to 120 mi/hr. Air resistance
(Friction) is equal to the force falling (gravity).
This point at which forces are balanced in
freefall is known as Terminal Velocity.
Is the sky diver accelerating when at Terminal Velocity?
If a 70 kg person jumps out of an airplane, how much FORCE is gravity pulling them down?
A) 70 kg
B) 70 N
C) 700 N
D) 7000 N
When the sky diver reaches terminal velocity, how much force is friction providing?
A) Less than 700 N
B) 700 N
C) more than 700 N
Air Resistance
Which sky diver will achieve a greater Terminal
Velocity?
A)
B)
If person A has 2 times the mass of person B and they both pull their identical parachutes out
At the same time, who will reach the ground first?
Think about the amount of frictional force needed to balance out the forces.
Which will reach the ground first if is thrown from a high flying airplane?
A) 8 kg bowling ball
B) .002 kg penny
C) they will be the same
Falling and Air Resistance
So if there is a cannonball that has a 10 kg mass
and a stone that has 1 kg mass and you drop
them off a tower – why do they hit the ground
By dropping the
at the same time?
Objects off a tower,
Cannon ball’s weight = m x a = 10kg x 9.8m/s² = 98 N of force
Stone’s weight = m x a = 1 kg x 9.8 m/s² = 9.8 N of force
The cannonball has more force towards the ground….
But remember, F =ma rearranged is acceleration = Force / Mass
for the cannonball a = 98 N / 10 kg = 10 m/s²
for the stone
a = 9.8 N / 1 kg = 10 m/s²
The air friction is
Minimal because the
Objects can’t reach
Fast enough speeds
For air friction to
Make a noticeable
Difference.
10 m/s² is gravity’s acceleration
If you add in the effects of friction, the rate of falling changes – the cannonball needs to
apply 98 N of force to make balanced forces whereas the stone needs only 9.8 N to balance
forces. If you were a friction devil – what would be easier to slow down, 98 N or 9.8 N?
Applying Force - Pressure
Pressure = Force / area of application P=F/A
Think of the air in your car tire, it’s measured in
lbs per in². lbs is the force, in² is the area.
Metric Unit of pressure is the PASCAL
Newtons of force per square meter
N/m²
Pressure and Force
The more surface area a force is applied to the
more the force is spread out.
For example, if you stand on one foot, you are
exerting a force in less area than if you are
standing on 2 feet.
Spreading out a person’s weight (Mass x gravity) is a lot easier when there is more surface area
Importance of Surface Area
The force of the hammer is spread out to hundreds of nails
500 N of force from the hammer on 500 nails = 1 N of force on every nail