Forces + Newton's Laws
(30 seconds)
Which of the following quantities is NOT a vector?
A) Mass
B) Displacement
C) Weight
D) Acceleration
E) Force
Answer: A) Mass
(30 seconds)
The coefficient of kinetic friction:
A) is in the direction of the frictional force
B) is in the direction of the normal force
C) is the ratio of force to area
D) can have units of newtons
E) none of the above
Answer: E) none of the above
(60 seconds)
A 10-kg block sits on a 15 deg inclined surface. What is the component of the gravitational force acting on it that is parallel to the surface?
ANSWER: 8.5 N
----------------------------
mgcos(85) = 8.5 N
OR
mgsin(15) = 8.5 N
(2 minutes)
Two blocks, both hanging vertically, are connected by a massless string that passes over a massless pulley. In the absence of friction, how does the tension force exerted by the string on the 250 N block compare with the tension force exerted by the string on the 350 N block?
A) The force on the 250 N block is smaller by a factor of 0.71.
B) The force on the 250 N block is larger by a factor of 1.4.
C) The force on the 250 N block is always equal to its weight.
D) The force on the 250 N block is equal to the weight of the 350 N block.
E) The two forces are equal.
Answer: E) The two forces are equal.
(60 seconds)
Draw a free-body diagram of a person in an elevator, when the elevator is accelerating downwards. (Ignore air resistance.)
(draw on board)
(30 seconds)
A force of 1 N is:
A) 1 kg/s
B) 1kg × m/s
C) 1kg × m/s2
D) 1kg × m2/s
E) 1kg × m2/s2
Answer: C) 1kg × m/s2
(60 seconds)
A brick slides on a horizontal surface. Which of the following will increase the frictional force on it?
A) Putting a second brick on top
B) Decreasing the surface area of contact
C) Increasing the surface area of contact
D) Decreasing the mass of the brick
E) None of the above
Answer: A) Putting a second brick on top
(60 seconds)
An object sits on an inclined plane. As the angle of the incline increases, how does the normal force acting on the object change (increase, decrease, stays the same)?
Answer: FN decreases
(FN = mgsin(90-theta), where theta is the angle of the incline)
(2 minutes)
A 45 kg child sits on a 3.2 kg tire swing. What is the tension in the rope that hangs from the tree branch?
Answer: 472 N
----------------------------------------------
T = Fg = mg = (45 + 3.2 kg)(9.8 m/s2)
(60 seconds)
You stand on a spring scale on the floor of an elevator. Of the following, the scale shows the highest reading when the elevator:
A) moves upward with increasing speed
B) moves upward with decreasing speed
C) remains stationary
D) moves downward with increasing speed
E) moves downward at constant speed
Answer: A) moves upward with increasing speed
(45 seconds)
Which of the following is a correct statement of Newton’s first law?
A) An object in motion will continue in motion at a constant speed unless acted on by a net external force.
B) An object at rest will stay at rest unless acted on by a net internal force.
C) An object in motion will continue in motion at a constant velocity unless acted on by a net internal force.
D) An object in motion will continue in motion at a constant velocity unless acted on by a net external force.
E) Everything has inertia.
Answer: D) An object in motion will continue in motion at a constant velocity unless acted on by a net external force.
(60 seconds)
A block moves with constant velocity on a horizontal surface. There is a 5 N force acting to the right and a 3 N force acting to the left. Additionally, there is a frictional force exerted by the surface on the block. The frictional force must be:
A) 0 N
B) 2 N, leftward
C) 2 N, rightward
D) slightly more than 2 N, leftward
E) slightly less than 2 N, leftward
Answer: B) 2 N, leftward
(4 minutes)
A 35-kg child is sledding down a 20 deg hill. A drag force of 50 N is acting on the child. What is her acceleration and direction?
Answer: 1.9 m/s2 down the hill
(2 minutes)
A 3-kg block is hanging from the ceiling. Another 8-kg block is hanging from the 3-kg block. Draw a free-body diagram for each block, making sure to label all of the forces.
(draw on board)
(3 minutes)
A 90-kg man stands in an elevator that has a downward acceleration of 1.4 m/s2. The force exerted by him on the floor is about:
A) 0 N
B) 90 N
C) 760 N
D) 880 N
E) 1010 N
Answer: C) 760 N
(45 seconds)
Which of the following is true, according to Newton’s second law?
A) The acceleration of an object is proportional to its mass and inversely proportional to the net force on it.
B) The acceleration of an object is proportional to the product of its mass and the net force on it.
C) The acceleration of an object is proportional to the net force on it and inversely proportional to its mass.
D) The acceleration of an object is equal to the net force on it.
E) The acceleration of an object depends on all the forces on it, both internal and external.
Answer: C) The acceleration of an object is proportional to the net force on it and inversely proportional to its mass.
(60 seconds)
A 40-N crate rests on a rough horizontal floor. A 12-N horizontal force is then applied to it, but the crate does not move. If the coefficients of friction are us = 0.5 and uk = 0.4, the magnitude of the frictional force on the crate is:
A) 8 N
B) 12 N
C) 16 N
D) 20 N
E) 40 N
Answer: B) 12 N
(5 minutes)
Dr. Kipp is flying down a double-black diamond ski run that is inclined at 40° (because she’s awesome). If the coefficient of kinetic friction between her skis and the snow is 0.08, and her mass is 55 kg, what is the magnitude of Dr. Kipp’s acceleration?
Answer: a = 5.7 m/s2
(5 minutes)
Two blocks, both hanging vertically, are connected by a string that passes over a massless, frictionless pulley. One block weighs 100 N and the other block weighs 75 N. What is the tension in the string?
Answer: T = 840 N
---------------------------------------------
m1 = 100 N, m2 = 75 N
T - m1g = m1a1 --> T = m1g + m1a
T - m2g = m2(-a1) --> T = m2g - m2a
m1g + m1a = m2g - m2a
(100)(9.8) + (100)a = (75)(9.8) - (75)a
a1 = -1.4 m/s2
T = 840 N
(3 minutes)
3 forces are acting on an object:
F1 = 20 N in the +x-direction
F2 = 20 N in the +y-direction
F3 = 20 N at an angle of 225 deg
Is the object in equilibrium?
Answer: NO
-------------------------
Fnet,x = 20 + 20cos225 = 5.9 N
Fnet,y = 20 + 20sin225 = 5.9 N
(30 seconds)
A book rests on a table, exerting a downward force on the table. The reaction to this force is:
A) the force of the Earth on the book
B) the force of the table on the book
C) the force of the Earth on the table
D) the force of the book on the Earth
E) the inertia of the book
Answer: B) the force of the table on the book
(3 minutes)
A 24-N force is applied at an angle of 30 deg above the horizontal to a 40-N block initially at rest on a rough horizontal surface. If the coefficients of friction are us = 0.5 and uk = 0.4, what is the magnitude of the frictional force on the block if it begins to move?
Answer: Ff,kinetic = 20.8 N
(6 minutes)
CHALLENGE QUESTION!
Two blocks are connected by a string over a frictionless, massless pulley. Block A sits on a frictionless surface inclined at 37 deg. Block B hangs vertically. Block A = 5 kg. Block B = 7 kg. What is the acceleration (magnitude and direction) of the blocks?
(Hint: each block can have a different coordinate system)
Answer: a = 3.26 m/s2 (Block A uphill, Block B down)
(5 minutes)
A 10-kg block sits on a horizontal frictionless table. It is connected via a string (over a pulley) to another block, that hangs vertically. If the acceleration magnitude of the blocks is 2 m/s2, what is the mass of the hanging block?
Answer: 2.6 kg
(5 minutes)
Four forces are acting on a stationary object:
F1 = 20 N @ 40 deg
F2 = 20 N @ 165 deg
F3 = 15 N @ 270 deg
Find the magnitude and direction of the 4th force.
Answer: F = 5.02 N, theta = -37.14 deg