Transcript Forces

Push or Pull
Investigating Newton’s 2nd Law of
Motion
Sports – Ch. 2, Act. 2
What do you think?
Moving a football one yard to score
a touchdown requires strategy,
timing, and many forces.
What is a Force?
A force is a push or a pull.
Can the same force move a bowling
ball and a ping pong ball?
Yes, but how will they be different?
A Push or Pull
A force can be a push:
A force can be a pull:
How do Forces Act on Objects?
A force can cause an object to speed
up (accelerate).
A force can cause an object to slow
down (accelerate negatively).
A force can cause an object to
change direction.
Force and Mass
Newton determined that the
acceleration of an object is directly
proportional to the force applied to move
it and inversely proportional to the
mass of the object.
What does this mean?
1.
2.
As mass increases and the force is held constant, the
rate of acceleration will decrease.
As the amount of force is increased while keeping the
mass constant, the acceleration will increase.
Newton’s 2nd Law
Newton’s 2nd Law in relation to the
acceleration of objects
a = F/m
Direct Relationship
Inverse Relationship
Slope = mass
Acceleration
Acceleration
More Force = More Acceleration
More Mass = Less Acceleration
Newton’s 2nd Law and the
v vs. t Graph
Newton’s 2nd Law indicates that a
constant force applied to an object
will cause the speed to change at a
constant rate.
The slope (m) of the line
equals the acceleration
of the object.
Time
Force Constant / Increasing Mass
If the force is held constant and the mass
is increased from m1 to m2, what would a
velocity vs. time graph look like?
m1
m2
m2 > m 1
Time
As the mass increases, the rate of acceleration will decrease.
Mass Constant / Increasing Force
If the mass is held constant and the force
is increased from F1 to F2, what would a
velocity vs. time graph look like?
F2
F1
F2 > F1
Time
As the force increases, the rate of acceleration will increase.
An Alternate View
Newton’s 2nd Law of Motion
F = ma
Where:
F = force
m = mass
a = acceleration
The force required to accelerate a 1 kg
object 1 m/s2 is called the Newton.
1 N = 1 kg•m/s2
Mass vs. Weight
Mass is a measure the amount of
matter an object contains, and is
measured in kilograms.
Weight is a measure of the force on
a object due to the acceleration of
gravity.
Fg = mg
Example 1:
Newton’s second Law:
F
=
m
x
a
Sprinter beginning 100 meter
dash
?N
350
70 kg
5 m/s2
Long jumper in flight
800 N
80?kg
10 m/s2
Shot put ball in flight
70 N
7 kg
? 2
10 m/s
Ski jumper going down hill
before jumping
400 N
80?kg
5 m/s2
Hockey player “shaving ice”
while stopping
-1,500 N
100 kg
? 2
-15 m/s
Running back being tackled
? N
-3000
100 kg
-30 m/s2
Example 2a:
Why is the acceleration for the shot putter and
long jumper 10 m/s2?
The key to this answer is the fact that they are in
flight.
While in flight, the shot put and long jumper act as
projectiles.
Gravity is the only force acting on them, and only in the
vertical direction.
Example 2b:
The force on the hockey player while stopping
is negative. Should it be negative.
Yes, because in order to stop, their must be a force
that opposes the hockey player’s motion.
Relative Speed
Positive Acceleration
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Time