1. FORCE and Newton’s Laws of Motion

2. Definitions of Force Force is a push or a pull acting on an object.
Force is a vector (magnitude & direction)
Force can be contact or long range (aka non-contact or field).

3. Types of Forces Contact Forces Friction (Ff) Tension (FT) Normal (FN)
Spring (FS)
Air Resistance (FR)
Applied (FA)
Long Range (Field) Forces
Gravity or weight (FW or Fg)
Electrical (FE)
Magnetic (B)

5. More on Force… Forces can change the motion of an object
SI unit = newton (N)
Net force is the vector sum of all forces acting on ONE object. Since force is a vector, the net force will have magnitude and direction.

6. More on Net Force Forces in same direction: add
Net force in same direction as forces.
See picture 2 below
Forces in opposite directions: subtract. Net force in direction
of larger force.
See picture 3 below
Equal forces in opposite directions : subtract. Net force is zero
See picture 1

7. What is the net force acting on each object below?

8. Balanced forces Net force = 0
Balanced forces do NOT cause any change in an object’s motion.

9. Balanced forces
Net force = 0

10. Unbalanced force Net force ≠ zero
Causes an object to change its motion
Many unbalanced forces causing motion: video

11. Balanced and unbalanced forces, cont.
Two tugboats are moving a barge. Tugboat A exerts a force of 3000 Newtons on the barge. Tugboat B exerts a force of Newtons in the same direction. What is the combined force on the barge?
Draw arrows showing the individual and combined forces of the tugboats.

12. Balanced and unbalanced forces, cont.
Now suppose that Tugboat A exerts a force of 2000 Newtons on the barge and Tugboat B exerts a force of 4000 Newtons in the opposite direction. What is the combined force on the barge?
Draw arrows showing the individual and combined forces of the tugboats.

13. Balanced and unbalanced forces, cont.
Could there ever be a case when Tugboat A and Tugboat B are both exerting a force on the barge but the barge doesn't move? Draw arrows showing the individual and combined forces in such a situation.

14. Newton’s 1st Law
Galileo’s Law of Inertia: An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force.
Concerned with the force on ONE object
The motion of an object changes only when there are unbalanced forces (Net force is NOT zero)

15. Newton’s 1st Law, cont.
What two kinds of motion can an object have if the forces are balanced?
An object at rest stays at rest
An object in motion stays in straight line motion at a constant speed (constant velocity)

16. Equilibrium: Net Force = Zero
Balanced Forces said another way…
Equilibrium: Net Force = Zero
Static Equilibrium
Net force = zero
No motion
Kinetic Equilibrium
Constant speed in a straight line (constant velocity)

17. Newton’s 1st Law Summary
- Also known as the Law of Inertia - Inertia is an object’s resistance to change in motion - More mass = more inertia!

18. Don’t let this be you. Wear seat belts!
Newton’s 1st Law & You
Don’t let this be you. Wear seat belts!
Because of inertia, objects (including you) resist changes in their motion. When the car going 80 km/hour is stopped by the brick wall, your body keeps moving at 80 m/hour.

19. Newton’s 2nd Law Net Force equals mass times acceleration. Fnet = ma
Fnet = net force (N)
m = mass (kg)
a = acceleration (m/s2)

20. Newton’s 2nd Law and Acceleration due to gravity (g)
On Earth’s surface:
g = 9.8 m/s2
On the moon’s surface:
g = 1.6 m/s2

21. Newton’s 2nd Law and Weight
Symbol = FW or Fg Weight is the force of gravity on an object. Unit – Newton (N) Formula FW= mg

22. Examples
1. What acceleration will result when a 12 N net force applied to a 3 kg object?
2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine the mass.
3. How much force is needed to accelerate a 66 kg skier 1 m/sec2?
4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec2?

23. Check Your Understanding
1. What acceleration will result when a 12 N net force applied to a 3 kg object?
2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine the mass.
3. How much force is needed to accelerate a 66 kg skier 1 m/s2?
4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/s2?
Given
FNet = 12 N
m = 3 kg
Unknown
a = ?
Equation
FNet = ma
a = FNet/m
Substitute
a = 12 N /3 kg
Solve
4 m/s2
Given
FNet = 16 N
a = 5 m/s2
Unknown
m = ?
Equation
FNet = ma
m = FNet /a
Substitute
m = 16 N /5 m/s2
Solve
3.2 kg
Given
m = 66 kg
a = 1 m/s2
Unknown
FNet = ?
Equation
FNet = ma
Substitute
FNet = 66 kg(1 m/s2)
Solve
66 N
Given
m = 1000 kg
a = 9.8 m/s2
Unknown
FNet = ?
Equation
FNet = ma
Substitute
FNet = 1000 kg(9.8 m/s2)
Solve
9800 N

24. Newton’s 3rd Law Forces are always in balanced pairs
For every action force there is an equal and opposite reaction force.
Forces are always in balanced pairs
Two objects acting on each other
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25. Newton’s 3rd Law, cont.
According to Newton, whenever objects A and B interact with each other, they exert forces upon each other. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body.

26. Newton’s 3rd Law, more examples
The finger pushes on the wall and the wall pushes on the finger.
The rocket pushes on the gases and the gases push on the rocket.
The hot gases push against the inside tube of the rocket and escape out the bottom of the tube. As the gases move downward, the rocket moves in the opposite direction.