Download Unit 2 Objectives: Forces and Laws of Motion

Unit 2 Objectives: Forces and Laws of Motion
Chapters 4-5
Category One: Inertia- Newton’s First Law of Motion
1. What is a force and what is the unit for representing force? Push or pull
(Newtons)
2. State Newton’s First Law of Motion. Inertia (an object in motion …) Give
several examples. Remember the animation I showed with the tug-of-war?
3. Describe inertia. Tendency to keep state of motion Explain what mass and
inertia have in common. More mass = more inertia
4. What is the difference between the weight of an object and the mass of an object?
Mass is amount of “stuff” in an object, weight is how hard earth pulls object
5. Explain what causes friction. Two surfaces in contact rub against each other
How could one reduce the friction an object experiences? Smooth surfaces,
6. What is the weight in Newtons of an object whose mass is 40 kg? 400 N
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? 150 kg
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.
Same direction (add) = 14 N
Opposite direction (subtract) = 2 N
Right angle (Pythagorean theorem) = 10 N
9. Apply the concept of inertia to the importance of using seatbelts. Your body is in
motion, if the car suddenly stops, you have a tendency to remain in motion, a seat
belt keeps you in the car
10. An astronaut happens to have a bowling ball in outer space. She throws the ball.
Since there is no influence of earthly gravity where she is, use the concept of
inertia to describe the motion of the bowling ball. Once pushed, the ball will move
at a constant speed forever
Category Two: Newton’s Second Law of Motion
1. State Newton’s Second Law of Motion, write the formula and give examples.
Acceleration increases with increasing force, but decreases with increasing mass.
Fnet=ma. The harder you throw a ball, the more it will accelerate. The more mass a
ball has, the less acceleration it will have
2. What amount of force was required to give a 40-kg wagon an acceleration of 3
m/s2? 120 N
3. Why the relationship between mass and acceleration is inversely proportional?
If mass increases, acceleration decreases
4. When mass remains constant, what type of relationship will be apparent between
force and acceleration? Both will increase together, so direct relationship
5. What variables influence the force of air resistance on an object in motion?
Speed, weight
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)? Weight depends on mass, falling acceleration doesn’t
7. Define terminal velocity. Air resistance increases with speed until the air
resistance equals the weight. Net force = 0 , a=0 What causes the speed to become
“terminal?” no more acceleration
8. Identify what happens to the acceleration of a freely falling object as it
approaches its terminal velocity. Acceleration decreases (gets closer to zero)
Describe the acceleration of an object that has reached its terminal velocity. a=0
9. A 50- N branch falls from a tree and reaches its terminal velocity. What is the
mass of the branch? 5 kg What is its weight? 50 N What is the force of air
resistance against the branch when it reaches its terminal velocity? 50 N
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
is less so the change in velocity takes more time on the moon
Category Three:
1. State Newton’s Third law of Motion and give examples. Every action has an
equal and opposite reaction.
Action: Billy’s fist punches Bobby’s face
Reaction: Bobby’s face punches Billy’s fist
2. Describe and draw an example of an action and reaction force pair. Same as
above. Any two objects in contact, exert forces on one another. Switch the words
around.
3. Identify the action and reaction force for the following: a man walking feet push
ground, ground push feet, a woman jumping, feet push ground, ground pushes
feet. a dog sitting on the floor dog pushes floor, floor pushes dog , and a child
throwing a ball Child’s hand pushes ball, ball pushes Child’s hand.
4. A 600-N man sits on a chair. What is the support force provided by the chair?
Support force = normal force. FN=Fw. FN=600N
5. A 100-kg man sits on a chair. What is the support force? Normal force equals
weight. 1000 Newtons
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? Add up masses. Multiply
by g. Tension force equals weight. 13,000 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?) action-reaction pairs do not cancel out because they act on different
objects
8. Applying Newton’s Third Law of Motion, diagram a bullet fired from a gun.
Bullet is pushed forward, gun is pushed backward. Forces are equal and opposite
9. An astronaut happens to have a bowling ball in outer space. She throws the ball.
Since there is no influence of earthly gravity where she is, use the concept of
Newton’s 3rd Law to describe the motion of the bowling ball and the astronaut.
Ball goes forward, astronaut goes backward. Same force. Ball accelerates more.
Category Four: Friction
1. Identify what the following symbols represent: FW=weight, FNET=total force,
FN=normal force, FA=applied force, Ff=friction, µK=coefficient of kinetic friction,
µS=coefficient of static friction.
2. What is a Normal Force? Force exerted by a surface to balance the weight of an
object sitting on the surface. Perpendicular to surface. Upward support force.
3. What does the coefficient of friction represent? Ratio of friction to normal force
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’s easier to keep moving
than to start moving
5. Construct a free-body diagram of a book sitting on a desk.
6. Construct a free-body diagram of a 1-kg book being pushed across a table with a
force of 2 N at a constant velocity. up and down = 10 N, left & right = 2N
7. Determine the coefficient of friction for the book in questions #3.
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? Assume Fnet=0, Ff = 0.4 x 400 = 160 N