Download Newton*s 2nd Law and the Force of Gravity

ND
2
NEWTON’S
LAW AND
THE FORCE OF GRAVITY
The student will be able to state
Newton’s Second Law and apply it in
qualitative and quantitative terms to
explain the effect of forces acting on
objects.
SPH4C
Findlay
HOW WILL THE MARBLE ROLL WHEN IT
LEAVES THE TRACK?
Review: Newton’s First Law
1. Which object has the most inertia?
a)
b)
c)
d)
A Feather
A textbook
Mr. Findlay
An Elephant
2. Fuzzy dice are hanging from the rear -view mirror of a car
that is travelling forward at constant speed. The dice are:
a)
b)
c)
d)
Angled toward the back of the car
Angled toward the front of the car
Hanging straight down
It cannot be determined
Review: Newton’s First Law
3. Fuzzy dice are hanging from the rear -view mirror of a car
that is travelling forward and speeding up. The dice are:
a)
b)
c)
d)
Angled toward the back of the car
Angled toward the front of the car
Hanging straight down
It cannot be determined
4. Fuzzy dice are hanging from the rear -view mirror of a car
that is travelling forward and slowing down. The dice are:
a)
b)
c)
d)
Angled toward the back of the car
Angled toward the front of the car
Hanging straight down
It cannot be determined
REVIEW: NEWTON’S FIRST LAW
5. A block of weight 4.0 N is suspended from the ceiling by a
piece of string. What is the magnitude of the tension in the
string?
a)
b)
c)
d)
4.0 N
Less than 4.0 N
Greater than 4.0 N
Zero
6. A block of weight 4.0 N is being lifted at constant velocity by a
piece of string. What is the magnitude of the tension in the
string?
a)
b)
c)
d)
4.0 N
Less than 4.0 N
Greater than 4.0 N
zero
NEWTON’S 2 ND LAW OF
MOTION AND WEIGHT
SPH4C
Findlay
Newton’s Second Law of Motion
 If the net force on an object is not zero, the object
will be accelerated in the direction of the net force.
𝐹𝑛𝑒𝑡
𝑎=
𝑚
 i.e. The more massive the object, the harder it is to
change its motion (the higher its inertia).
𝐹𝑛𝑒𝑡 = 𝑚𝑎
Example
 A weightlifter lifts a 165 -kg weight by exerting a force of 1 .8
kN [up]. The force of gravity on the object is 1 .6 kN [down].
Draw a FBD of the weight. What is the net force on the
weight? What is the acceleration of the weight ?
1.8 kN
1.6 kN
𝐹𝑛𝑒𝑡 = 𝐹𝐴 + 𝐹𝑔
𝐹𝑛𝑒𝑡 = 1.8 kN + −1.6 kN
𝐹𝑛𝑒𝑡 = 0.2 kN [up]
𝐹𝑛𝑒𝑡
𝑎=
𝑚
200 N
𝑎=
165 kg
𝑎 = 1.2 N/kg or 1.2 m/s 2
Example Using Equations of Motion
 A car is travelling at 25 m/s [forward] when the driver slams
on the brakes and stops the car in 3.0 s. Calculate
a)
a)
the acceleration of the car
𝑣𝑓 − 𝑣𝑖
𝑣𝑖 = +25 m/s
𝑎=
Δ𝑡
𝑣𝑓 = 0 m/s
0 − 25 m/s
Δ𝑡 = 3.0 s
𝑎=
= −8.3 m/s 2
3.0 s
𝑎=?
the net force needed to cause that acceleration if the mass of the
car is 1200 kg.
𝑎 = −8.3 m/s 2
𝑚 = 1200 kg
𝐹𝑛𝑒𝑡 = ?
𝐹𝑛𝑒𝑡 = 𝑚𝑎
𝐹𝑛𝑒𝑡 = 1200 kg −8.3 m/s 2
𝐹𝑛𝑒𝑡 = −10 000 N
Weight (The Force of Gravity)
 The formula 𝐹𝑛𝑒𝑡 = 𝑚𝑎 can also be used to calculate the
weight, 𝐹𝑔 , of an object, which represents the magnitude of
the force of gravity acting on it.
 If the acceleration, 𝑎, is due to the acceleration due to
gravity, 𝑔 = 9.8 m/s 2 [down],
𝐹𝑔 = 𝑚𝑔
 Where the mass, 𝑚, is measured in kg.
 Weight is how much force of gravity is pulling down on an
object.
 Weight is therefore measured in Newtons.
 The direction is always [down].
Example
 What is the weight of a 140 -kg person?
𝑚 = 140 kg
𝑔 = 9.8 m/s 2 [down]
𝐹𝑔 = ?
𝐹𝑔 = 𝑚𝑔
𝐹𝑔 = 140 kg 9.8 m/s 2
𝐹𝑔 = 1372 N
Practice
1. Calculate the weight of the objects of given mass below.
a) Oxygen gas molecule: 5.356 x 10 -26 kg
5.25 x 10 -25 N
b) Penny: 0.00235 kg
0.023 N
c) Tennis ball: 0.057 kg
0.56 N
d) Student: 61 kg
598 N
e) Newborn Elephant: 105 kg
1030 N