1. Physics
Section 4-1 & 4-2
Forces and
Newton’s First Law

2. Force A force is a push or pull on an object with mass.
Forces have the ability to make objects accelerate — speed up, slow down, or change direction.
The derived SI unit of force is the newton (N)
1 N = 1 kg•m/s2
A force is a push or pull on an object with mass.
Forces have the ability to make objects accelerate — speed up, slow down, or change direction.
The derived SI unit of force is the newton (N)
1 N = 1 kg•m/s2
A force is a push or pull on an object with mass.
Forces have the ability to make objects accelerate — speed up, slow down, or change direction.
The derived SI unit of force is the newton (N)
1 N = 1 kg•m/s2
A force is a push or pull on an object with mass.
Forces have the ability to make objects accelerate — speed up, slow down, or change direction.
The derived SI unit of force is the newton (N)
1 N = 1 kg•m/s2
A force is a push or pull on an object with mass.
Forces have the ability to make objects accelerate — speed up, slow down, or change direction.
The derived SI unit of force is the newton (N)
1 N = 1 kg•m/s2
A force is a push or pull on an object with mass.
Forces have the ability to make objects accelerate — speed up, slow down, or change direction.
The derived SI unit of force is the newton (N)
1 N = 1 kg•m/s2
Forces are often measured with a spring scale. Home bathroom scales are a type of spring scale.

3. Force vectors
Force is a vector quantity. The push or pull is in a specific direction.
Several force vectors can act on an object simultaneously, and usually do.
Force is a vector quantity. The push or pull is in a specific direction.
Several force vectors can act on an object simultaneously, and usually do.
Lift

4. Motion and inertia Galileo and others investigated inertia.
Inertia is the tendency of an object with mass to resist changes in motion.
Before Galileo, the Aristotelian view was that all objects naturally come to rest if there is no motive force. Galileo explained that objects in constant motion would remain in motion if nothing disturbed the object.
Galileo and others investigated inertia.
Inertia is the tendency of an object with mass to resist changes in motion.
Before Galileo, the Aristotelian view was that all objects naturally come to rest if there is no motive force. Galileo explained that objects in constant motion would remain in motion if nothing disturbed the object.
Galileo and others investigated inertia.
Inertia is the tendency of an object with mass to resist changes in motion.
Before Galileo, the Aristotelian view was that all objects naturally come to rest if there is no motive force. Galileo explained that objects in constant motion would remain in motion if nothing disturbed the object.
“A body moving on a level surface will continue in the same direction at a constant speed unless disturbed.”

5. Newton’s first law Newton’s first law is called the law of inertia.
Objects at rest remain at rest, and objects in straight line motion remain in motion, if the net force on the object is zero.
Newton’s first law is called the law of inertia.
Objects at rest remain at rest, and objects in straight line motion remain in motion, if the net force on the object is zero.
Newton’s first law is called the law of inertia.
Objects at rest remain at rest, and objects in straight line motion remain in motion, if the net force on the object is zero.
Lex I: Corpus omne perseverare in statu suo quiescendi vel movendi uniformiter in directum, nisi quatenus a viribus impressis cogitur statum illum mutare.
Law I: Every body persists in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed.

6. Equilibrium or acceleration?
All the forces on an object will add together as vectors to give the net force, Fnet.
All the forces on an object will add together as vectors to give the net force, Fnet.
The net force is the vector sum of all forces acting on a single object.
The net force is the vector sum of all forces acting on a single object.
Fnet = ∑F
According to Newton’s First Law, if there is no net force on an object (Fnet = 0), it will either remain at rest (v = 0) or will continue to move at a constant velocity (v = constant). In both cases, acceleration is zero. All forces are balanced, and the object is in equilibrium.
According to Newton’s First Law, if there is no net force on an object (Fnet = 0), it will either remain at rest (v = 0) or will continue to move at a constant velocity (v = constant). In both cases, acceleration is zero. All forces are balanced, and the object is in equilibrium.
According to Newton’s First Law, if there is no net force on an object (Fnet = 0), it will either remain at rest (v = 0) or will continue to move at a constant velocity (v = constant). In both cases, acceleration is zero. All forces are balanced, and the object is in equilibrium.

7. Free Body Diagrams
There are generally two types of force problems. One involves an object in equilibrium. This is when the net force equals zero. When all the forces are in equilibrium, the acceleration is zero. ∴
Fnet = ∑F = 0 N
a = 0
Keep in mind that this is not just for v = 0. It is also whenever v is constant! An object can be moving with constant velocity with no net force.
The second type of problem is when the object has a non-zero acceleration. In these problems:
Fnet = ∑F = ma
a ≠ 0
(For next time: Newton’s Second Law)

8. When solving force problems, you should always draw a diagram showing the forces on every object. This is a free-body diagram.
The easiest problems can be solved with only one free-body diagram, showing all the forces on one object. Never show the forces the object exerts on other objects unless the problem requires it.
There are several classic problems for which free-body diagrams are commonly drawn.
• An object being pulled in different directions.
• An object being pulled along a surface by a rope.
• Stacked blocks.
• An object sitting on, or sliding up or down, an incline.
• An Atwood machine (objects connected by a rope over a pulley).

9. 45° 26.1° 30° 100 N FN = 50 N pull FP = 100 N Ff = 34 N friction
An object being pulled along a surface by a rope.
150 N
Fg = 94 N
An object being pulled in different directions.
FN1
Stacked blocks.
Fg2
FN2
Ff21
friction m2 Ff FP
pull m1
Ff12
Fg2
Fg1
friction

10. a a See the text for example problems for some of these.
An object sitting on, or sliding down, an incline. FN a a Ff FT m1 FT
Fg1 m2 Fg
An Atwood machine.
See the text for example problems for some of these.
Fg2
HyperPhysics has excellent examples of 11 classic problems: