Download 28. Unit 11 Study Guide

28
Force Study Guide
Balanced/Unbalanced Forces
1. How do unbalanced forces acting on an object affect the object’s motion?
~ Unbalanced forces ALWAYS change the motion of an object.
2.
A person throws a ball up into the air. The diagram above shows the ball’s position in the air at 1
second intervals. What is happening at the point the ball is at its highest position with regards to
gravity and the force of the throw?
~At the highest position, the ball is not moving because the force of the throw up and the force of gravity
pulling down are balanced, so there is no motion.
3. Decide whether each of the following is an example of Balanced or Unbalanced forces:
Balanced or Unbalanced: A tug of war game in which no one wins.
Balanced or Unbalanced: A roller coaster going down the first drop.
Balanced or Unbalanced: A car hitting a wall.
Balanced or Unbalanced: A person pushing a shopping cart.
4. Jenny has two identical marbles. She rolls one marble across a concrete surface with a force of 2 N. At the
same time, she rolls the other marble across an icy surface with the same force.
There is a significantly greater frictional force between the first marble and the concrete surface
than between the second marble and the icy surface. How will the increase in friction affect the
distance the first marble travels?
~More friction will cause the first marble to slow down and stop BEFORE the second marble.
5. Julio and Luis have to move a box containing a 35 kg television from one room of their house to another.
Julio has the ability to push with a force of 250 N while Luis can only push with a force of 100 N.
If both boys pushed the box in the same direction with all the force they could, what would be the
net force on the box, assuming no friction?
~250 N + 100 N = 350 N
6.A boat is moving east with a force of 30 N, the wind is blowing west with a force of 7 N, and the fluid
friction from the water is pushing against the boat with a force of 5 N. What is the net force of the
boat as it travels east?
~30 N east – (7 N west +5 N west) = 30 N east – 12 N west = 18 N east
Friction
7. Decide whether each is an example of Increasing or Decreasing friction:
Increasing or Decreasing: waxing skis
Increasing or Decreasing: adding grease to gears on a bike
Increasing or Decreasing: throwing sand on an icy driveway
Increasing or Decreasing: oiling a squeaky door
8. In the picture above, what does arrow A represent?
~The force of friction between the tires and the road. Friction goes OPPOSITE the motion of the object.
9. How does increasing the amount of pressure pushing down on an object affect the friction between the object
and the surface?
~Increasing pressure increases the amount of friction.
10. Newton's first law of motion states that an object will keep a constant speed and direction unless acted upon
by an unbalanced force.
To test this statement, Martin rolled a ball on a long, level street. The ball did not bump into any
object, but it eventually came to a stop. How is this possible?
~ Friction is the unbalanced force that causes the ball to slow and eventually stop.
11. Which picture below illustrates the least amount of friction between the hands? ~Less force = less friction
A
B
C
D
Gravity
12. The force that causes two objects with mass to be pulled toward each other is called:
~gravity
13. An object has a mass of 2 kg on Earth. What will happen to its mass and weight on the moon, which has less gravity?
~Gravity only changes weight; mass is always constant. On the moon the object will have the same mass
(2 kg) but it will weigh less on the moon because the moon has less gravity.
14. Four objects are orbiting a star along the same path.
The objects are the same distance from the star and have
the masses shown to the right.
The gravitational force of attraction is weakest
between the star and which object?
~Object Y because Object Y has the smallest mass (0.1 kg).
Gravity depends on distance (all the objects are the same
distance from the sun) and mass (less mass = less gravity).
15. True or False: Weight is the force of gravity acting on an object, while mass is the amount of matter in an object.
16. What is the law of universal gravitation?
~Any two objects in the universe, without exception, are attracted to each other. (gravity is everywhere)
17. Four weather satellites are orbiting the Earth at a height of 825 kilometers. Some characteristics of the satellites are
collected in the table below.
satellite length (m) altitude (km) mass (kg)
I.
5.5
825
692
II.
9.8
825
610
III.
4.4
825
840
IV.
5.6
825
903
The Earth’s gravity exerts the greatest for on which
satellite? How do you know this?
~Satellite IV; The length of the satellite has
no effect on the amount of
gravity. The altitude is the same
for every satellite, so this does not
make a difference. The greater
the mass, the more gravity;
Satellite IV has the greatest mass
(903 kg), so it would have the
greatest gravity.
18.
The distance between two objects affects
the force of gravity between them.
Based only on distance, the gravity
between the sun and which planet would
be the weakest?~Mars, because Mars is
the furthest away. The greater the distance, the less gravity.
19. What two factors affect the force of gravity? Describe how each factor affects gravity.
~Distance: more distance means less gravity; less distance means more gravity.
~Mass: more mass means more gravity; less mass means less gravity.
Magnets
20 . Mark opened a box and pulled out the longest, widest magnet. Well, he only tried to pull that bar out. A
smaller magnet stuck to the end of it. He shook the bigger magnet to make the small one fall off. It didn’t.
How would the poles of the magnets line up when Mark picked up the small magnet?
~Opposite poles attract each other (North pole of one magnet attracts the South pole of another magnet).
21. A magnet is placed near a pile of steel paper clips. What will most likely occur?
~The magnet will provide an unbalanced force that pulls the paper clips toward the magnet.
22. Where is the magnetic field of a magnet the strongest?
~at the poles (ends) of the magnet
Inertia
23. The tendency of an object to resist change in its motion is known as
~inertia (newton’s first law: an object at rest stays at rest, an object in motion stays in motion, unless
acted upon by an unbalanced force.
24. Last-minute Louis frantically tried to finish his homework while his mom drove him to school. He placed
his book on his lap and was reaching for a pencil when his mom suddenly slammed on the brakes.
Although Louis stayed buckled in his seat, his book flew forward and crashed into the front
windshield. Name and describe the force most responsible for the book flying forward.
~The book will continue moving forward due to inertia.
25. A teacher was discussing with the class that a bowling ball has more inertia than a basketball. What physical
property is most likely being compared between the balls to make the bowling ball have more inertia?
~The bowling ball has more MASS than the basketball, so the bowling ball will have more inertia. More
mass = more inertia.
26. A student walking quickly while carrying an orange on his lunch tray stops suddenly. What will most likely happen
to the orange? How do you explain this?
~The inertia of the orange caused it to continue to move forward, even though the student and the tray
stopped moving.
27. Which box would you expect to have the most inertia and how do you know?
~Box D because it has the most mass (400 kg); the size of the box doesn’t matter; more mass = more
inertia.