1. 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!

2. 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

4. 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.

5. Determine the Net ForceA

6. This object must be: Moving At rest Slowing down
We need more information A

7. Determine the Net ForceB
20 N

8. Determine the Net Force
Fnet = 0 N
??? N

9. Acceleration Acceleration is how speed changes over time
the rate of change of speed
A car speeds up by 10 m/sec every second
The car accelerates at 10 meters per second per second
10 m/sec/sec
10 m/sec2
1 sec
10 m/sec
2 sec
20 m/sec
3 sec
30 m/sec

11. 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?

12. Why does the speed change?
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.
Galileo

14. Correctly written, the law should say NET force.
A net force causes acceleration
No acceleration = no net force
Acceleration = net force/mass

15. 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

16. 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?

17. How much net force is needed to accelerate an 500 gram baseball at a rate of 10 m/sec2?

18. What is the mass of an object that requires a net force of 30 N to accelerate at a rate of 5 m/sec2?

19. 1. Dani and Gina are pushing on a 50 kg box
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.
What is the net force if they both push in the same direction?
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?

20. 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 m/s2. This constant is abbreviated: g = 9.8 m/s2
If you drop a ball off a high bridge, how much does it speed up in one second?

21. 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!
2 kg
1 kg
9.8 N
19.6 N

22. Air resistance and terminal speed
Here’s a video about skydiving

23. Homework Skydiving worksheet and:
Page “Solving Problems” # 1, 2, 3, 4, 6, 7, 9, 10