1. Newton’s Laws Friction Free Body Diagrams Frames of Reference
FORCES
Newton’s Laws
Friction
Free Body Diagrams
Frames of Reference

2. The space shuttle Endeavor lifts off for an 11-day mission in space
The space shuttle Endeavor lifts off for an 11-day mission in space. All of Newton’s laws of motion - the law of inertia, action-reaction, and the acceleration produced by a resultant force -are exhibited during this lift-off. Credit: NASA Marshall Space Flight Center (NASA-MSFC).
NASA

3. Newton’s First Law
Newton’s First Law: An object at rest or an object in motion at constant speed will remain at rest or at constant speed in the absence of a resultant force.
A glass is placed on a board and the board is jerked quickly to the right. The glass tends to remain at rest while the board is removed.

4. Newton’s First Law (Cont.)
Newton’s First Law: An object at rest or an object in motion at constant speed will remain at rest or at constant speed in the absence of a resultant force.
Assume glass and board move together at constant speed. If the board stops suddenly, the glass tends to maintain its constant speed.

5. Inertia Newton's first law is often referred to as the law of inertia.
Inertia is the resistance or unwillingness of an object to accelerate (speed up, slow down, or change directions).
The more mass an object has, the harder it is to accelerate. Thus, more mass equals more inertia.

6. How is inertia seen here?

7. How to determine what has a greater inertia?!?
Usually a heavier object will resist change better than a smaller object
Bigger the mass = greater the inertia

8. Understanding the First Law
Discuss what the driver experiences when a car accelerates from rest and then applies the brakes.
(a) The driver is forced to move forward. An object at rest tends to remain at rest.
(b) Driver must resist the forward motion as brakes are applied. A moving object tends to remain in motion.

9. Newton’s Second Law Force Units: Newton’s or N
Second Law: Whenever a resultant force acts on an object, it produces an acceleration that is directly proportional to the force and inversely proportional to the mass.
Force Units: Newton’s or N

10. Measuring Mass and Force
The SI unit of force is the newton (N) and the unit for mass is the kilogram (kg).

11. Acceleration and Force
Pushing a cart with twice the force produces twice the acceleration. Three times the force triples the acceleration.

12. Force and Acceleration
4 kg F
a = 2 m/s2
8 kg
a = 4 m/s2 F
12 kg
a = 6 m/s2 F
Acceleration a is directly proportional to force F and is in the direction of the force. Friction forces are ignored in this experiment.

13. Force and AccelerationDF Da DF Da
= Constant
8 N
4 m/s2
= 2 N
m/s2
The SLOPE of a F vs a graph is the MASS
Mass m = 2 kg

14. Newton: The Unit of Force
One newton is the force required to give an acceleration of 1 m/s2 to a mass of 1 kg.
F (N) = m (kg) a (m/s2)
What resultant force will give a 3 kg mass an acceleration of 4 m/s2?
F = ?
a = 4 m/s2
3 kg
F = 12 N

15. Comparing the Newton to the Pound
1 lb
4.45 N
1 N = lb
1 lb = 4.45 N
A 160-lb person weighs about 712 N
A 10-N hammer weighs about 2.25 lb

16. Example A 40 N resultant force causes a block to accelerate at 5 m/s2
Example A 40 N resultant force causes a block to accelerate at 5 m/s2. What is the mass?
F = 40 N
m=?
a = 5 m/s2

17. To find the force on the pilot, assume same acceleration:
Example A net force of 4.2 x 104 N acts on a 3.2 x 104 kg airplane during takeoff. What is the force on the plane’s 75-kg pilot?
First we find the acceleration of the plane.
F = 4.2 x 104 N
m = 3.2 x 104 kg +
F = ma
a = 1.31 m/s2
To find the force on the pilot, assume same acceleration:

18. A Word About Consistent Units
Now that we have derived units of newtons . We can no longer use units that are inconsistent with those definitions.
Acceptable measures of LENGTH:
SI units: meter (m)
Unacceptable units are: centimeters (cm); millimeters (mm); kilometers (km); yards (yd); inches (in.); miles (mi)

19. Consistent Units (Continued . . .)
Acceptable measures of MASS:
SI units: kilogram (kg)
Unacceptable units are: grams (gm); milligrams (mg); newtons (N); pounds (lb); ounces (oz)

20. Consistent Units (Continued . . .)
Acceptable measures of FORCE are:
SI units: newton (N)
Unacceptable units are: kilonewtons (kN); tons (tons); ounces (oz); kilograms (kg); slugs (slug)

21. Cont. . . First, draw sketch and list given quantities:
Example A 54-g tennis ball is in contact with the racket for a distance of 40 cm as it leaves with a velocity of 48 m/s. What is the average force on the ball?
First, draw sketch and list given quantities:
Given: vo = 0; vf = 48 m/s x = 40 cm; m = 54 g a = ?
Consistent units require converting grams to kilograms and centimeters to meters:
Given: vo = 0; vf = 48 m/s x = 0.40 m;
m = kg; a = ?
Cont. . .

22. Knowing that F = m a, we need first to find acceleration a:
Example (Cont). A 54-g tennis ball is in contact with the racket for a distance of 40 cm as it leaves with a velocity of 48 m/s. What is the average force on the ball?
Knowing that F = m a, we need first to find acceleration a:
F = 156 N
F= (0.054 kg)(2880 m/s2)

23. Weight and Mass
Weight is the force an object applies as a result of gravity pulling it downward. It is directed downward and it varies with gravity.
Mass is a universal constant which is a measure of the matter that makes up an object. It is always constant regardless of location
Because weight is a force created by the downward acceleration of gravity, we can make this substitution.

24. Weight and Mass: Examples
What is the weight of a 10-kg block?
The weight of an object is the force it causes as a result of gravity.
9.8 m/s2 W m
10 kg
W = mg = (10 kg)(9.8 m/s2)
W = 98 N
The weight of an object is also referred to as the force of gravity

25. Example A resultant force of 40 N gives a block an acceleration of 8 m/s2. What is the weight of the block near the surface of the Earth?
W=?
F = 40 N
a m/s2
To find weight, we must first find the mass of the block:
W = mg
= (5 kg)(9.8 m/s2)
Now find weight of a 5-kg mass on earth.
W = 49.0 N

26. Pounds should NEVER be used!
Always Remember!!
In Physics, the use of Newton’s second law and many other applications makes it absolutely necessary to distinguish between mass and weight. Use the correct units!
Metric SI units: Mass is in kg; weight is in N.
Pounds should NEVER be used!
Always give preference to the SI units.

27. Newton’s Third Law Action Reaction Action Reaction
Third Law: For every action force, there must be an equal and opposite reaction force. Forces occur in pairs.
Action
Reaction
Action
Reaction

28. Action and Reaction Forces
Use the words by and on to study action/reaction forces below as they relate to the hand and the bar:
Action
The action force is exerted by the _____ on the _____.
hands
bar
Reaction
The reaction force is exerted by the _____ on the _____.
bar
hands

29. Force on runner = -(Force on board)
Example 6: A 60-kg athlete exerts a force on a 10-kg skateboard. If she receives an acceleration of 4 m/s2, what is the acceleration of the skateboard?
Force on runner = -(Force on board)
mr ar = -mb ab
(60 kg)(4 m/s2) = -(10 kg) ab
Force on Board
Force on Runner
a = - 24 m/s2