Download Newton`s Second Law

Chapter 3: Forces
The amount of force needed to change an
object’s motion depends upon the object’s mass
and the acceleration used during the change.
Newton’s Second Law
F = force, units are Newtons (N)
m = mass, units MUST be kg
a = acceleration, units MUST be m/s2
Examples:
1. How much force is needed to accelerate a 1000
kg truck at a rate of 3 m/s2 ?
F = ma
F = (1000 kg)(3 m/s2)
F = 3000 N
2. A force of 10 N is used to push a 10 kg box.
What is its acceleration?
F = ma
10 N = (10 kg)a
a = 1 m/s2
3. Applying a force of 10 N to an object causes
the object to accelerate at a rate of 5 m/s2.
What is the mass of the object?
F = ma
10 N = m(5 m/s2)
m = 2 kg
Types of forces
Friction: a force that occurs when two touching
objects move past each other. Frictional force is
always in the opposite direction to the motion.
Fapp
Ff
Ff
Fapp
Force
More force
Fapp
Microwelds form
where bumps come
into contact.
If the applied force is
not big enough to break
the microwelds, the
object will not move.
More downward force
(weight) pushes the bumps
closer together.
This causes the static
friction to increase.
Types of forces
Gravitational: occurs
between any two objects
and depends only the
masses of the objects
and the distance between
them
Fg
F
Fg
g
• The three Fg vectors show how the masses of the Earth, the
moon and the astronaut attract each other.
1. Which one is the most significant to the astronaut?
The vector between the astronaut and the Moon
2. To the Moon?
The vector between the Moon and the Earth
Force of gravity = weight
Close to the Earth’s surface, all objects are
accelerated by gravity at a rate of 9.8 m/s2
0s
5m
1s
Fg
20 m
2s
Fg = mg
g = 9.8 m/s2
Projectile motion
vy
vy = 0
vx
vx
vx
vy
There is no acceleration in the x direction: vx is constant
Gravity changes vy. In the first half of the throw, the
upward motion of the ball is slowed down by gravity.
In the second half of the throw, gravity accelerates the
ball’s vy.