Force of Friction practice problems
Honors Physics
1. A couch with a weight of 600 N sits on the floor. Between the couch and floor the coefficients of
static and kinetic friction are .3 & .2 respectively. Find the thresholds of both types of friction.
Static – 180 N; Kinetic – 120 N
2. A 30 kg dog is dragging its butt across the ground. If the coefficient of static and kinetic friction are
.5 and .3 respectively, a) what force must the dog pull with in order to start it moving? b) what
force must the dog pull with in order to keep it moving?
(147 N, 88.2 N)
3. A chair weighing 100 N rests on the carpet. I apply a horizontal force of 70 N. If the coefficients of
static and kinetic friction are .5 and .3 respectively, will I be able to move the chair? If so, what is
the Net Force on the chair? (Yes, 40 N)
4. A kid ties his 5 kg sled to his dog’s harness. The dog pulls horizontally with a force of 240 N. The
snow has been compacted a bit and is now slick with static and kinetic coefficients of friction of .4
and .3 respectively. If the kid has a mass of 50 kg, what is the net force acting on the kid and his
sled? (78.3 N)
5. An old lady accidently walks into a large icy patch in the middle of the sidewalk. She can’t get out
of it on her own, so a good Samaritan comes by and helps her up and pulls her across the ice with a
constant horizontal force of 100 N. If the lady has a mass of 60 kg and the coefficient of friction
between her feet and the ice is .15, a) How much friction is acting between her shoes and the ice,
b) what is the net force acting on her, c) what is her acceleration, and d) how far does she make it
in 2 seconds? (88.2N, 11.8N, .2 m/s2, .4 m)
6. Somebody pass me the mashed potatoes! The kinetic coefficient of friction between the bowl and
the table is 0.34 and you push it with a 15 N horizontal force. If the mass of the mashed potato
bowl is 2.3 kg and you push it for .74 seconds, how fast will it be traveling when it leaves your
hand? (2.4 m/s)
7. A couch with a mass of 75 kg is being pushed across the floor with a force of 500 N to the right.
Adam is being difficult and jumps on the couch with his 50 kg mass. If the coefficients of friction
are .3 and .2, how far does it move in 3 seconds? (9.18 m)
8. Amber and Rachel decide to join forces pushing the 75 kg couch. Amber pushes with a force of 200
N and Rachel pushes with a force of 250 N both horizontally in the same direction. If the
coefficients of friction are .33 and .28, how fast is it traveling from rest after 2 m? (3.61 m/s)
9. A clerk moves a box of cans down an aisle by pulling on a strap attached to the box. The clerk pulls
with a force of 185.0 N at an angle of 25.0° with the horizontal The box has a mass of 35.0 kg, and
the coefficient of kinetic friction between box and floor is 0.450. Find the acceleration of the box.
(1.38)
10. I push a 60 kg couch with a force of 400 N pushing 20o above the horizontal. If the coefficient of
kinetic friction is .3 a) what is the force of friction acting on the couch? B) What is the net force
acting on the couch? C) What is the acceleration of the couch? D) If I push the couch for 3 seconds
from rest, how far does the couch travel? 135N; 241N; 4 m/s2; 18m
11. Dwayne slides his bookbag across the table with a 60 N force.
His bag has a mass of 15 kg. If the coefficients of friction
between the bag and the tabletop are .4 and .29, how far does it
move in 2.5 seconds? (3.6 m)
12. A 4.0 kg block of wood sits on a table. A string is tied to the
wood, running over a pulley and down to a hanging object. The
greatest mass that can be hung from the string without moving
the block of wood is 1.8 kg. Calculate the coefficient of static
friction between the block of wood and the table. (0.45)
15. Once again, Jack is being difficult while Bill and Ralph are trying to move a 75 kg couch. Bill lifts up
on the couch with a force of 200 N while Ralph pushes to the left with a force of 400 N. If Jack has a
mass of 68 kg and the coefficients of friction between the couch and ground are .35 & .28, how fast
will they be moving after 1.2 seconds? (0; prove why)
16. A 925 N crate is being pulled across a level floor by a force F of 325 N at an angle of 25° above the
horizontal. The coefficient of kinetic friction between the crate and floor is 0.25. Find the
magnitude of the acceleration of the crate. (1.0 m/s2)
17. A 75 kg ice skater moving at 10.0 m/s coasts to a halt in 100 m on a smooth ice surface. What is
the coefficient of friction between the ice and skates? (.05)
18. Santa Clause drags his bag across the ground with a force of 600 N at a 30o angle above the
horizontal. If the bag has a mass of 80 kg and the coefficients of friction are .4 and .3, how far does
he drag it in 3 seconds? (21.1 m)
19. Mr. Finau is bad at ice skating…fact. He pushes off of the wall such that he had an initial velocity of
6 m/s. The coefficient of kinetic friction between his skates and the ice is .04. If Mr. Finau has a
mass of 100 kg, is he able to make it across the rink, which is 50 m away? (ie. – how far can he
travel and compare it to 50m) (45.9m)
20. Driving in his car, Mitch slams on the break to stop at the red light. Normally on solid ground, the
coefficient of kinetic friction is 0.7, but on the slushiness of winter, the coefficient of kinetic friction
is 0.34. If the car has a mass of 907 kg and Mitch has a mass of 100 kg, what is the difference in
distance between normal conditions and slushy conditions that Mitch should start breaking if he is
initially traveling at 12.2 m/s? (ie. – find distance separately w/each coefficient of friction then
subtract) (11.5 m)