Newton's First Law of Motion states that an object at rest stays at rest and an object in motion stays in motion with the same speed and direction unless acted upon by a force. Mass is related to inertia in that the more mass an object has, the more inertia it has and the more force is required to change its motion. The net force is the overall force acting on an object which is determined by adding up all forces acting in the same direction or subtracting opposing forces.

1. Section 2.1
Newton’s First Law of Motion

2. Newton: Great Scientist or
80’s Rock Star?
 English Physicist
/Mathematician
 1642-1727
 Invented Calculus to
explain what he
observed
 Discovered 3 LAWS
OF MOTION

3. Use the Force, Luke…but only if
you know what it is!
 A force is a push or pull, or any action
that has the ability to change motion.
 Where is the force in these examples?
Action What caused the
force?
Person lifts a box Person pulls object off
the ground
Gun fires bullet Explosion pushes
bullet
Branch falls from tree Gravity pulls branch
to ground

4. Nothing can start or stop moving
unless a force is applied!
O RLY?!?
YEAH RLY!

5. ?Question?
Can there be a change in the motion of
anything without a force being applied?

6. NO!
Okay smarty, I think you got it!

7. Newton’s First Law
An object at rest will stay at rest and an
object in motion will continue in motion
with the same speed and direction
UNLESS acted on by a force.

8. ?Question?
So a space ship is going 75,000 m/s
(or 75 km/s). It takes a left turn at Pluto,
but never changes speed. Was there a
force acting on it??

9. Yes!
In order to change direction,
A force must be applied!

10. Which would you rather do?
Lift (move) a bag with
groceries in it
Lift (move) a semi-
truck

11. Easy choice, right
It is much easier to lift the groceries and
possibly impossible (for you weaklings,
anyway) to lift the truck.
Why is one harder to move than the other?

12. Mass
You remember mass…it’s the measure of
how much matter an object has.
It turns out, the more mass something
has, the more force is required to move it.
Duh!

13. Inertia
Inertia is the property of objects to resist
changes in motion.
If something is stationary, it resists
attempts to move it
If something is moving, it resists attempts
to slow it down (or speed it up)

14. The more mass something has, the more
inertia it has, and this means that more
force is required to increase or decrease
its speed.

15. Which would you rather do?
Part 2
Stop a bag of
groceries that was
thrown at you
Stop a semi-truck
driving toward you

16. Put these in order:
Least to greatest inertia
 Baseball bat (900g)
 Bowling ball (7.2kg)
 Pencil (2.6g)
 Person (100kg)
 Paper (0.1g)

17. The greater an object’s inertia,
the greater the
force needed to change its
motion
Newton’s first law is often referred to
as the law of inertia

18. Units of Force
 SI unit of mass is kilogram (kg)
 SI unit of force is the NEWTON (N)
1 N of force can accelerate a 1kg object by 1
m/s
4.448 N = 1 lb
(yes, weight is a measure of force)

19. What if there are multiple forces
acting on an object?

20. Multiple Forces Add (like in math)
 If two or more forces
are acting on an
object in the same
direction, the net
(overall) force is the
sum of all the forces

22. Opposite forces subtract
 What is the net force
on the ball?
 Which way is it going
to move (if at all)?

23. 2.1 Section Review
1. State Newton’s first law in your own
words.

24. 2. How is mass related to inertia?

25. 3. What is the net force and how is it
determined?

26. Important Vocabulary
 Force
 Newton’s first law
 Newton
 Net force