1. Chapter 7. NEWTON’S SECOND LAW
A PowerPoint Presentation by
Paul E. Tippens, Professor of Physics
Southern Polytechnic State University
© 2007

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. Objectives: After completing this module, you should be able to:
Write Newton’s second law using appropriate units for mass, force, and acceleration.
Demonstrate your understanding of the distinction between mass and weight.
Draw free-body diagrams for objects at rest and in motion.
Apply Newton’s second law to problems involving one or more bodies in constant acceleration.

4. Newton’s First Law Reviewed
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.

5. 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.

6. 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.

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

8. Acceleration and Force With Zero Friction Forces
Pushing the cart with twice the force produces twice the acceleration. Three times the force triples the acceleration.

9. Acceleration and Mass Again With Zero Friction
Pushing two carts with same force F produces one-half the acceleration. The acceleration varies inversely with the amount of material (the mass).

10. Measuring Mass and Force
The SI unit of force is the newton (N) and the unit for mass is the kilogram (kg).
Before presenting formal definitions of these units, however, we will conduct an experiment by slowly increasing the force on a given object.
Although the force in newtons will become our standard, we begin by using the more familiar unit of force--the pound (lb).

11. Force and Acceleration
4 lb F
a = 2 ft/s2
8 lb
a = 4 ft/s2 F
12 lb
a = 6 ft/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.

12. Force and AccelerationDF Da
= Constant F a DF Da
8 lb
4 ft/s2
= 2 lb
ft/s2
Inertia or mass of 1 slug = 1 lb/(ft/s2)
Mass m = 2 slugs

13. MASS: A Measure of Inertia
a = 6 ft/s2
6 lb
1 slug
a = 3 ft/s2
6 lb
2 slugs
a = 2 ft/s2
6 lb
3 slugs
One slug is that mass on which a constant force of 1 lb will produce an acceleration of 1 ft/s2. Friction forces are ignored in this experiment.

14. F (lb) = m (slugs) a (ft/s2)
Two Systems of Units
USCU system: Accept lb as unit of force, ft as unit of length, and s as unit of time. Derive new unit of mass, the slug.
F (lb) = m (slugs) a (ft/s2)
SI system: Accept kg as unit of mass, m as unit of length, and s as unit of time. Derive new unit of force, the newton (N).
F (N) = m (kg) a (m/s2)

15. Newton: The Unit of Force
One newton is that resultant force which imparts 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?
Remember F = m a
F = ?
a = 4 m/s2
3 kg
F = 12 N

16. 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

17. Example 1: What resultant force F is required to give a 6 kg block an acceleration of 2 m/s2?
a = 2 m/s2
F = ma = (6 kg)(2 m/s2)
F = 12 N
Remember consistent units for force, mass, and acceleration in all problems.

18. Example 2: A 40-lb resultant force causes a block to accelerate at 5 ft/s2. What is the mass?
F = 40 lb
m=?
a = 5 ft/s2
m = 8 slugs
You must recall that the slug is the appropriate mass unit when F is in lb and a is in ft/s2.

19. To find F on 78-kg pilot, assume same acceleration:
Example 3. 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 a of plane.
F = 4.2 x 104 N
m = 3.2 x 104 kg +
F = ma
a = 1.31 m/s2
To find F on 78-kg pilot, assume same acceleration:
F = 98.4 N
F = ma = (75 kg)(1.31 m/s2);

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

21. Consistent Units (Continued . . .)
Acceptable measures of MASS are:
SI units: kilogram (kg)
USCU units: slug (slug)
Unacceptable units are: grams (gm); milligrams (mg); newtons (N); pounds (lb); ounces (oz)
The last three unacceptable units are actually units of force instead of mass.

22. Consistent Units (Continued . . .)
Acceptable measures of FORCE are:
SI units: newton (N)
USCU units: pound (lb)
Unacceptable units are: kilonewtons (kN); tons (tons); ounces (oz); kilograms (kg); slugs (slug)
The last two unacceptable units are not force units—they are units for mass.

23. Consistent Units (Cont.)
When we say that the acceptable units for force and mass are the newton and the kilogram, we are referring to their use in physical formulas. ( Such as F = m a)
The centimeter, the millimeter, the milligram, the mile, and the inch may be useful occasionally in describing quantities. But they should not be used in formulas.

24. Problem Solving Strategy (For the Simpler Problems.)
Read problem; draw and label sketch.
List all given quantities and state what is to be found.
Make sure all given units are consistent with Newton’s second law of motion (F = m a).
Determine two of the three parameters in Newton’s law, then solve for the unknown.

25. Example 4. A 54-gm 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 gm a = ?
Consistent units require converting grams to kilograms and centimeters to meters:
Given: vo = 0; vf = 48 m/s x = 0.40 m; m = km; a = ?
Cont. . .

26. Example 4 (Cont). A 54-gm 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 = ma
F = 156 N
F= (0.054 kg)(2880 m/s2);

27. F = m a so that: W = mg and m =
Weight and Mass
Weight is the force due to gravity. It is directed downward and it varies from location to location.
Mass is a universal constant which is a measure of the inertia of a body.
F = m a so that: W = mg and m = W g

28. Weight and Mass: Examples
What is the weight of a 10-kg block?
W = mg = (10 kg)(9.8 m/s2)
9.8 m/s2 W m
10 kg
W = 98 N
What is the mass of a 64-lb block?
64 lb
32 ft/s2
W = mg

29. Mass is Constant; W Varies.
32 lb
16 ft/s2
49 N
4.9 m/s2
Earth
98 N
9.8 m/s2
Earth
64 lb
32 ft/s2
m = = 2 slugs W g
m = = 10 kg W g

30. Description of Objects
Objects described by mass or weight:
W (N) = m (kg) x 9.8 m/s2
W (lb) = m (slugs) x 32 ft/s2
Conversions made by Newton’s 2nd Law:
W = mg
m = W g

31. Inconsistent Common Usage
In the United States, objects are often referred to by their weight at a point where gravity is equal to 32 ft/s2.
You might hear: “An 800-lb force pulls a 3200-lb car.”
W = 3200 lb
800 lb
This car should be called a 100-slug car.
Thus, when an object is described as a _?_-lb object, we remember to divide by g to get mass.

32. Inconsistent Usage (Cont.)
Even metric units are used inconsistently. Mass in kg is often treated as if it were weight (N). This is sometimes called the kilogram-force.
A chemist might be asked to weigh out 200 g of a certain element. Also, you hear about a 10-kg load as if it were weight. F
10 kg
The kilogram is a mass - never a force - and it doesn’t have direction or vary with gravity.

33. 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.
USCU units: Mass is in slugs; weight is in lb.
Always give preference to the SI units.

34. Example 5. 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?
To find weight, we must first find the mass of the block:
W=?
F = 40 N
a m/s2
W = mg
= (5 kg)(9.8 m/s2)
Now find weight of a 5-kg mass on earth.
W = 49.0 N

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

36. Acting and Reacting 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

37. 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

38. Review of Free-body Diagrams:
Read problem; draw and label sketch.
Construct force diagram for each object, vectors at origin of x,y axes.
Dot in rectangles and label x and y compo- nents opposite and adjacent to angles.
Label all components; choose positive direction.

39. Example of Free-body Diagram
300
600
4 kg B By A A B Ay
600
300 Ax Bx
W = mg
1. Draw and label sketch.
2. Draw and label vector force diagram.
3. Dot in rectangles and label x and y compo-nents opposite and adjacent to angles.

40. Applying Newton’s Second Law
Read, draw, and label problem.
Draw free-body diagram for each body.
Choose x or y-axis along motion and choose direction of motion as positive.
Write Newton’s law for both axes:
SFx = m ax SFy = m ay
Solve for unknown quantities.

41. n + Diagram for Cart: F x W
Example 7: A cart and driver have a mass of 120 kg. What force F is required to give an acceleration of 6 m/s2 with no friction?
1. Read problem and draw a sketch.
Diagram for Cart: n W F x +
2. Draw a vector force diagram and label forces.
3. Choose x-axis along motion and indicate the right direction as positive (+).

42. n + ay = 0 Diagram for cart: F x W SFx = max; F = ma
Example 7 (Cont.) What force F is required to give an acceleration of 6 m/s2?
4. Write Newton's Law equation for both axes.
Diagram for cart: n W F x +
m = 120 kg
ay = 0
SFx = max; F = ma
SFy = 0; n - W = 0
The normal force n is equal to weight W
F = (120 kg)(6 m/s2)
F = 720 N

43. Example 8: What is the tension T in the rope below if the block accelerates upward at 4 m/s2? (Draw sketch and free-body.)
10 kg
SFx = m ax = 0 (No info) T a
SFy = m ay = m a
T - mg = m a
a = +4 m/s2
mg = (10 kg)(9.8 m/s) = 98 N T mg +
m a= (10 kg)(4 m/s) = 40 N
T = 138 N
T - 98 N = 40 N

44. Example 9: In the absence of friction, what is the acceleration down the 300 incline?+
300
mg sin 600 W
mg cos 600
600 mg
SFx = m ax
a = (9.8 m/s2) cos 600
mg cos 600 = m a
a = 4.9 m/s2
a = g cos 600

45. First apply F = ma to entire system (both masses).
Example 10. Two-Body Problem: Find tension in the connecting rope if there is no friction on the surfaces.
2 kg
4 kg
12 N
Find acceleration of system and tension in connecting cord.
First apply F = ma to entire system (both masses). n
SFx = (m2 + m4) a
12 N
12 N = (6 kg) a
(m2 + m4)g
a =
12 N
6 kg
a = 2 m/s2

46. Example 10 (Cont.) The two-body problem.
2 kg
4 kg
12 N
Now find tension T in connecting cord.
Apply F = m a to the 2 kg mass where a = 2 m/s2. n
SFx = m2 a T
m2 g
T = (2 kg)(2 m/s2)
T = 4 N

47. Example 10 (Cont.) The two-body problem.
Same answer for T results from focusing on 4-kg by itself.
2 kg
4 kg
Apply F = m a to the 4 kg mass where a = 2 m/s2.
12 N n
m2 g T
SFx = m4 a
12 N - T = (4 kg)(2 m/s2)
T = 4 N

48. First apply F = m a to entire system along the line of motion.
Example 11 Find acceleration of system and tension in cord for the arrangement shown.
First apply F = m a to entire system along the line of motion.
2 kg
4 kg
SFx = (m2 + m4) a
Note m2g is balanced by n. n
m2 g T
m4 g
+ a
m4g = (m2 + m4) a
(4 kg)(9.8 m/s2)
2 kg + 4 kg
a = =
m4g
m2 + m4
a = 6.53 m/s2

49. To find T, apply F = m a to just the 2 kg mass, ignoring 4 kg.
Example 11 (Cont.) Now find the tension T given that the acceleration is a = 6.53 m/s2.
To find T, apply F = m a to just the 2 kg mass, ignoring 4 kg.
2 kg
4 kg n
m2 g T
m4 g
+ a
T = (2 kg)(6.53 m/s2)
T = 13.1 N
Same answer if using 4 kg.
m4g - T = m4 a
T = m4(g - a) = 13.1 N

50. T +a a = 4.20 m/s2 2 kg 5 kg SFx = (m2 + m5) a m2 g m5 g
Example 11. Find the acceleration of the system shown below. (The Atwood machine.)
2 kg
5 kg
First apply F = ma to entire system along the line of motion.
SFx = (m2 + m5) a T
m2 g
m5 g +a
a = 4.20 m/s2

51. Summary
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.
Newton’s Second Law: A resultant force produces an acceleration in the direction of the force that is directly proportional to the force and inversely proportional to the mass.
Newton’s Third Law: For every action force, there must be an equal and opposite reaction force. Forces occur in pairs.

52. Summary: Procedure N = (kg)(m/s2) Read, draw and label problem.
Draw free-body diagram for each body.
Choose x or y-axis along motion and choose direction of motion as positive.
Write Newton’s law for both axes:
SFx = m ax SFy = m ay
Solve for unknown quantities.

53. CONCLUSION: Chapter 7 Newton’s Second Law of Motion