Name:_________________________________
Unit Review
Forces, Friction, and Newton’s Laws
1. What is centripetal acceleration?
ACCELERATION BY CHANGING DIRECTION
2. What type of friction involves the air?
FLUID FRICTION
3. How do static friction and kinetic friction differ from each other?
STATIC FRICTION INVOLVES NO MOTION, KINETIC (SLIDING) FRICTION
HAS 2 THINGS IN CONTACT MOVING PAST ONE ANOTHER.
4. Identify the type of friction in each situation described below…
a. Two students are pushing a box at rest. _________STATIC ____
b. The box pushed by the students is now sliding. ___SLIDING (KINETIC)___
c. The students put rollers under the box and push it forward.
SLIDING FRICTION
5. Why is friction necessary to drive a car on a road?
IF YOU HAVE NOT FRICTION, YOUR WHEELS WOULD JUST SPIN
6. How could you increase friction on an icy road?
PUT DOWN SAND, KITTY LITTER, OR SALT ON THE ROAD.
ADD CHAINS TO TIRES.
7. The force pulling a truck downhill is 2000 N. What is the size of the static friction
acting on the truck if the truck doesn’t move? 2000 N
8. The combination of all forces acting on an object is the ___NET FORCE___.
9. Static friction is usually greater than __SLIDING (KINETIC) FRICTION_______.
10. When the net force on an object is greater than zero, the object will accelerate in
the direction of the ___GREATER___ force.
11. Identify the following examples of friction as helpful or harmful.
___HARMFUL___
Friction between bone joints.
____HELPFUL________
Friction between a dentist’s drill and your tooth.
____HELPFUL________
Friction between a saw blade and a piece of
lumber.
_____HELPFUL_________
Friction between your hands as you rub them
together for warmth.
_____HARMFUL_________
Friction between motor parts.
12. Suggest why placing wheels under a heavy box reduces the necessary force
required to push it along at a constant speed. REDUCES FRICTION
13. Analyze the following situations, and indicate whether the forces are balanced
or unbalanced.
_UNBALANCED________
A. a skydiver accelerating downward
__UNBALANCED________
B. a cannonball fired parallel to the ground
__BALANCED____________
C. a motorboat coasting at a constant speed.
_BALANCED______________
D. a bike leaning against a tree
14. What is the force needed to accelerate a 1357 kg car forward at an acceleration
of 8.0 m/s2.
F = M X A = 1357 KG X 8 M/S2 = 10 856 N
15. Calculate the acceleration of a 82 Kg couch that is pushed across the floor with
a force of 21 N.
A = F/M = 21 N/82 KG = .256 M/S2
16. How much mass would a dog have if it exerts a force of 769 N on the earth?
M = F/A = 769N/9.8 M/S2 = 78.5 KG
17. Using the concept of inertia, why is volleyball not played with a ball that has a
mass similar to a bowling ball?
INERTIA OF BOWLING BALL – TENDENCY TO STAY MOVING – IS
GREATER THAN THE FORCE EXERTED BY ARMS – NOT ENOUGH TO
CHANGE THE MOTION OF THE BALL
18. Identify which of Newton’s 3 laws of motion specifically applies in each of the
following situations.
_____3_______ a. You feel a force against the sole of your foot as
you take a step forward.
____1_______ b. A meteor moving in a straight line path changes
direction when it flies by Earth.
___2_______ c. A full grocery cart that is pushed starts moving
and increases speed, but the same push
increases the speed even more when the cart is
empty.
____1______ d. a falling book lands on the floor with a precollision speed of 9 m/s.
19. Why is a backward facing car seat safer for an infant than a seat that faces
forward during a collision or an abrupt stop. BABY CONTINUES IN
STRAIGHT LINE MOTION AND PUSHES BACK INTO THE SEAT. IF THE SEAT
WERE TURNED AROUND, BABY CONTINUES FORWARD AND BENDS IN
HALF, THUS CAUSING INTERNAL INJURIES.
20. What is Newton’s third law? Give two examples.
FOR EVERY ACTION THERE IS AN EQUAL AND OPPOSITE REACTION.
BALLOON GOING UP – AIR RUSHING OUT
ROCKET GOES UP – EXHAUST GOES DOWN
ANSWERS FOR THE BACK
1. B
2. A
3. SAME
4. B
5. A
6. A