Newton’s first and second laws
Galileo said that it doesn’t require a
force to keep an object moving… In
fact, there must be a force to cause
an object to stop moving!
 Newton’s First Law: Law of Inertia
An object at rest, stays at rest
— An object in motion, stays in motion
— Unless acted on by a force
—
If an object is at rest:
There are no forces acting on it.
There are forces acting on it.
We need more information to
know.
Determine the Net Force
100 N
A
100 N
200 N
100 N
0N
- 200 N
This object must be:
100 N
A
100 N
Moving
At rest
Slowing down
We need more
information
Determine the Net Force
10 N
70 N
20 N
20 N
B
30 N
0N
40 N
Determine the Net Force
2N
7N
0N
4N
2N
2N
7N
Fnet = 0 N
D
7N
??? N
Acceleration
• Acceleration is how speed changes over time
– the rate of change of speed
• A car speeds up by 10 m/sec every second
1 sec
10 m/sec
2 sec
20 m/sec
3 sec
30 m/sec
• The car accelerates at 10 meters per second per second
– 10 m/sec/sec
– 10 m/sec2
Example
While traveling along a highway a driver speeds
up from 15 m/sec to 24 m/sec in 12 seconds.
What is the car’s acceleration?
What is the acceleration if the same car then
comes to a stop in eight seconds?
Why does the speed change?
Galileo
Remember that Galileo
said that changing
motion requires a
force.
Newton went further:
He said that the
acceleration of an
object is proportional
to the force.
Correctly written, the law should say
NET force.
• A net force causes acceleration
• No acceleration = no net force
• Acceleration =
net force/mass
the SI unit of force: the Newton (N)
• If a = F/m, then F = ma
• The units of force (a Newton) are the units of
mass * acceleration
• Therefore, the units of a Newton are
What is the rate of acceleration of a 2,000-kilogram truck if a
net force of 4,200 N is used to make it start moving forward?
If the engine provides 5,000 N of forward force, how much friction is
pushing the truck back?
How much net force is needed to accelerate an 500 gram
baseball at a rate of 10 m/sec2?
What is the mass of an object that requires a net force of
30 N to accelerate at a rate of 5 m/sec2?
1. Dani and Gina are pushing on a 50 kg box. Dani pushes with 250 N of force and Gina pushes
with 100 N of force.
a. What is the net force if they both push in the same direction?
b. What is the acceleration of the box? (Assume no other forces)
b. What is the acceleration of the box if they push in opposite directions?
Mass, Weight, Gravity, and falling
• Weight is the pull of the Earth on an object. We call that the force of
gravity (heaviness)
• The force of gravity causes unsupported objects to accelerate downward.
• When air resistance is ignored, all objects accelerate at the same rate: 9.8
g = 9.8 m/s2
m/s2. This constant is abbreviated:
• If you drop a ball off a high bridge, how much does it speed up in one
second?
Weight and mass
• One kilogram on Earth weighs 9.8 Newtons. Two kilograms weighs 19.6 N.
• What is the acceleration caused by this force (when unsupported)?
• This is why objects of different mass have the same acceleration when they fall!
1 kg
2 kg
9.8 N
19.6 N
Air resistance and terminal speed
Here’s a video about skydiving
Homework
Skydiving worksheet and:
Page 55-56 “Solving Problems” # 1, 2, 3, 4, 6, 7, 9, 10