Download Unit 3 Objectives: Forces and Laws of Motion

Unit 2 Objectives: Forces and Laws of Motion
Chapters 4-5 Happy Studying Ladies and Gentleman!!!!
Category One: Inertia- Newton’s First Law of Motion
1. What is a force and what is the unit for representing force? A force is a push or a
pull and always occurs in pairs. The unit is a Newton which breaks down into a
kg x m/s2.
2. State Newton’s First Law of Motion. An object at rest will stay at rest or an object
in motion will continue that motion until acted on by an unbalanced force. Give
several examples. A pencil sitting on a desk, the sheet being pulled out from
under a full set of china dishes or a car rolling down the road until it hits
something or there is enough friction between the tires and the surface to stop it.
3. Describe inertia. The tendency of an object to resist a change in motion.Explain
what mass and inertia have in common. The more mass you have the more of an
ability you have to resist a change in your motion.
4. What is the difference between the weight of an object and the mass of an object?
Weight is dependent upon gravity and Mass is not dependent upon anything.
5. Explain what causes friction. Friction is caused by little microscopic bumps
between the surface of two objects moving against each other. How could one
reduce the friction an object experiences? You could lubricate the surface
between two objects to reduce friction.
6. What is the weight in Newtons of an object whose mass is 40 kg? Fw = mg
Fw=40(10)= 400N
7. The gravitational attractive force on the moon is 1/6 that of earth giving objects an
acceleration of 1.67 m/s2 (in other words, on the moon g= 1.67 m/s2). What
would be the mass of a 150-kg TV set on the moon? The mass would be the same
as that on earth 150 kg because mass doesn’t change silly:0)
8. Using the concept of combining vectors, determine the net force when a 6 N and
8 N interact with each other directly against each other, at right angles, and when
the forces are applied in the same direction. 6-8 = 2N , at right angles use the
paythagorean thereom, when applied in the same direction 6+8= 14N
9. Apply the concept of inertia to the importance of using seatbelts. A seatbelt
restrains your mass. Your mass is directly related to inertia. The more mass you
have, the more inertia you have as well. The car’s breaks only act on the car. The
seatbelt is needed to act on you in order to restrain your inertia.
Category Two: Newton’s Second Law of Motion
1. State Newton’s Second Law of Motion, write the formula and give examples. An
object’s mass will resist acceleration when a force is applied to try to change the
object’s postion.
2. What amount of force was required to give a 40-kg wagon an acceleration of 3
m/s2? Fnet = ma Fnet = 40(3) = 120 N.
3. Why the relationship between mass and acceleration is indirectly proportional?
When force is held constant, and the mass is increased then the acceleration will
always decrease. When mass is increasing and acceleration is decreasing this
indicates there is an indirect relationship between the two variables.
4. When mass remains constant, what type of relationship will be apparent between
force and acceleration? The greater the force, the greater the acceleration of the
object if its mass remains constant. So there is a direct relationship.
5. What variables influence the force of air resistance on an object in motion? The
mass of the object, the surface area of the object.
6. How can an object experience more gravitational force but having the same
gravitational acceleration of an object with less mass (assuming no air
resistance)? An object can have more gravitational force if it has a larger mass.
Fw = mg Fw = 10 kg (10m/s2)= 100N Fw = mg Fw= 15 kg(10 m/s2)= 150 N
Both objects in the example above have a different mass but have the same
gravitational acceleration:0)
7. Define terminal velocity. The greatest speed reached by a falling object. What
causes the speed to become “terminal?” When the Weight of the falling object is
equal to the air resistance pushing up.
8. Identify what happens to the acceleration of a freely falling object as it
approaches its terminal velocity. Acceleration gradually decreases as on object
reaches terminal velocity. Describe the acceleration of an object that has reached
its terminal velocity. Acceleration is zero once an object reaches terminal
velocity.
9. A 50- N branch falls from a tree and reaches its terminal velocity. What is the
mass of the branch? Fw=mg 50N =m(10) m= 50/10 = 5kg What is its weight?
The problem told us this information. It is 50 N. What is the force of air
resistance against the branch when it reaches its terminal velocity? The air
resistance must be equal to the weight of the falling object in order to reach
terminal velocity, so the answer is 50N.
10. Suppose you buy some new basketball shoes and test them out by jumping
straight up off the ground. Use Newton’s 2nd Law to explain why your hang time
is much greater while jumping on the Moon than when on the Earth. Acceleration
due to gravity on the moon is much less than that here on earth. Because it is not
as strong, the object’s mass can more easily resist the acceleration of the moon,
there fore your hangtime will be longer. The person’s mass is unchanged in each
location.
Category Three:
1. State Newton’s Third law of Motion and give examples. For every action or force
there is an equal and opposite reaction or force.
2. Describe and draw an example of an action and reaction force pair. My feet are
pushing down on the floor. The floor pushes back up on my feet.
3. Identify the action and reaction force for the following: a man walking, a woman
jumping, a dog sitting on the floor, and a child throwing a ball. All action/reaction
pairs here are between the object’s feet and the floor. The feet push down on the
floor and the floor pushes back up on the feet. The child throws a ball example:
The child’s hand pushes on the ball, the ball pushes back on her hand.
4. A 600-N man sits on a chair. What is the support force provided by the chair?
The support force is equal and opposite in direction to the force of the man’s
weight = 600 N.
5. A 100-kg man sits on a chair. What is the support force? The support force is
equal and opposite in direction to the man’s weight pushing down on the chair =
1000 N- You have to take his mass 100 kg and multiply by gravity to get the
weight here:0).
6. Three 100-kg triplets step onto a 1000-kg elevator. What is the tension force on
the cable that is supporting the triplets and the elevator?The tension force on the
cables is equal in force and opposite in direction to the weight of the triplets and
elevator combined. Fw = mg Fw = (300 +1000)(10) = 13000 N.
7. Explain how motion is achieved if force pair interactions are always opposite and
equal. (i.e.- How is the horse able to move the cart when forces on each are
equal?) Force pairs must act on the same object in order to move it. There are two
objects being acted on at the same time w/ a horse and cart. The horse pulls
forward on the cart and the cart pulls in the opposite direction on the horse. The
horse moves the cart because there is only one force acting on each of the objects
involved. I will draw a picture of this tomorrow on the board.
8. Applying Newton’s Third Law of Motion, diagram a bullet fired from a gun.
I will diagram this for you tomorrow.
Category Four: Friction
1. Identify what the following symbols represent: FW, FR, FN, FA, Ff, µK, µS. Fw =
Weight force, Fn= Normal Force, Fa = applied force, Ff= frictional force, µK =
coefficient of kinetic friction, µS = coefficient of static friction
2. What is a Normal Force? The force perpendicular to the surface.
3. What does the coefficient of friction represent? The amount or percentage of
resistance between two surfaces that are touching.
4. Why is the coefficient of kinetic friction always less than the coefficient for static
friction for the same object under the same conditions? It always takes more force
to get something moving than it does to keep it moving.
5. Construct a free-body diagram of a book sitting on a desk. See description for #5
6. Construct a free-body diagram of a 1-kg book being pushed across a table with a
force of 20 N at a constant velocity. Draw a box w/ 1 kg in the center, and 20N
going to the left and right. Both Fp and Ff are equal due to the object moving at
constant velocity.
7. Determine the coefficient of friction for the book in questions #6. u = Ff/Fn
u=20N/10N = 2.0
8. A 40-kg TV is sitting on an entertainment stand. If the coefficient of friction is
0.4, what is the force needed to slide the TV to the other side of the stand for
dusting purposes? Ff =u(Fn) Ff = .4(400) = 160 N