# Unit 3 Review Sheet Answers

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```					Answers
Category I

1. Force: A push or pull. Units: Newtons; lbs
2. An object at rest, remains at rest or an object in motion continues its motion until
it is acted on by an unbalanced force. See your which is which worksheet or your
quiz that you got back today for examples.
3. Inertia: the tendency of an object to resist a change in motion. Inertia is
dependent upon mass. The more mass an object has, the more inertia.
4. Weight is dependent on gravity and it changes from location to location. Mass
remains constant no matter where you are and is not dependent upon gravity.
5. Friction is caused by two surfaces rubbing/touching as they move past each other.
Reduce friction by making the load lighter, lower the coefficient of friction by
making the surfaces slide past each other easier, such as greasing the surface.
6. Fw= mg = 40 (10) = 400 N
7. 150 kg – Mass doesn’t change!!!!! (tricky question)
8. Vectors moving in the same direction are added when determining the net force.
Vectors opposing each other, such as with the applied force and friction, are
subtracted. Forces at right angles to one another use the Pythagorean theorem.
9. You and the car each have inertia. You have the same speed as the car when
inside of it. The seatbelt’s job is to keep you in place so that your own inertia
doesn’t send you flying forward into the windshield or dashboard.

Category II

1. The amount of force needed to make an object change its motion depends on the
mass of the object and the acceleration required. F= ma.
2. As you increase the mass of an object, the object will decelerate as long as you
are pulling with a constant force (Remember making the car heavier in the lab,
but pulling with the same amount of force each time…the car got slower!)
3. The more mass an object has the slower it will move when the force is constant.
This should say indirectly proportional here: one goes up as the other goes down.
4. Forces cause objects to accelerate. More force = greater acceleration The reverse
is true also.
5. Air resistance is caused by friction between the object and the air.
6. Here’s an example: F=ma F=(5)(10) = 50 N                   F=ma F=(3)(10) = 30N
Gravity acts the same on an object despite its mass. The product of the mass
and the gravitational constant make the larger object have a greater force.
7. Terminal velocity: The greatest speed achieved by a falling object. It results
when the weight of the object is equal to the air resistance underneath the object.
8. Objects falling at terminal velocity are falling at a constant speed. When an
object is moving at a constant speed its acceleration is always zero!
9. Take the weight =50N and divide by gravity to get the mass. = 5 kg. Air
resistance is equal to that of the weight when it reaches terminal velocity.
10. The hang time is greater on the moon because gravity is 1/6 of what gravity here
on Earth is. Since the gravitational field on the moon is not as strong, the object
‘hangs’ in the air for longer.

Category III

1. For every action (or force) there is an equal and opposite reaction (or force).
2. See your which is which worksheet for examples.
3. Action = Man walking pushing his feel down on the ground; Reaction =The
ground pushing back up on his feet.
Action = A woman pushes down with her feet, Reaction = The ground pushes
back on her feet.
Action = Dog pushes down with his feet as he sits on the ground Reaction =
ground pushes back on the dog’s feet.
Action = Child’s hand applies a force to the ball; Reaction = the ball applies a
force back to the child’s hand.
4. The support force is equal to the force of the man’s weight = 600 N.
5. Support force = 100 kg x gravity = 1000 N
6. 300 kg + 1000 = 1300 kg = mass Multiply the mass times gravity to get the
weight. The weight will equal the tension force according to Newton’s 3rd Law =
13, 000 N.
8. Will do in class.

Category Four: Friction

1.    Fw = Weight Force, FN = Normal Force, Fa = Applied Force Ff = Frictional Force,
see notes for last two.
2.   See quiz from today. Pretend like it is the book and the answer is there.
3.   Will draw on board before test tomorrow!
4.   = 20/10 = 2. Yes this was a poor choice of numbers since it should be less than
one.
5.   Fp = .4(400) =160 N

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