Newton's Laws
(30 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.
D) An object in motion will continue in motion at a constant velocity unless acted on by a net external force.
(30 seconds)
You are driving your car along the road and hit a patch that is so icy that there is effectively no friction between your tires and the road. Which statement is most accurate?
A) If you hit the brakes, you will eventually stop.
B) If you twist the steering wheel, you can turn into the skid.
C) Nothing you do will have any effect; without any friction, you will continue to move in a straight line at a constant speed, as there is no net external force on you.
D) If you hit the accelerator, you will get out of the icy patch faster.
E) Your car will start to spin.
C) Nothing you do will have any effect; without any friction, you will continue to move in a straight line at a constant speed, as there is no net external force on you.
(30 seconds)
A heavy steel ball B is suspended by a cord from a block of wood W. The entire system is dropped through the air. Neglecting air resistance, the tension in the cord is:
A) zero
B) the difference in the masses of B and W
C) the difference in the weights of B and W
D) the weight of B
E) none of these
A) zero
(45 seconds)
Why do raindrops fall with constant speed during the later stages of their descent?
A) The gravitational force is the same for all drops
B) Air resistance just balances the force of gravity
C) The drops all fall from the same height
D) The force of gravity is negligible for objects as small as raindrops
E) Gravity cannot increase the speed of a falling object to more than 9.8 m/s
B) Air resistance just balances the force of gravity
(60 seconds)
Two blocks (A and B) are in contact on a horizontal frictionless surface. A 36-N constant force is applied to A. The mass of A is 4.0 kg and the mass of B is 20 kg. The magnitude of the force of A on B is:
A) 1.5 N
B) 6.0 N
C) 29 N
D) 30 N
E) 36 N
D) 30 N
(30 seconds)
Mass differs from weight in that:
A) all objects have weight but some lack mass
B) weight is a force and mass is not
C) the mass of an object is always more than its weight
D) mass can only be expressed in the metric system
E) there is no difference
B) weight is a force and mass is not
(60 seconds)
A 40-N crate rests on a rough horizontal floor. A 12-N horizontal force is then applied to it. 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
B) 12 N
(60 seconds)
Two blocks (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.
E) The two forces are equal.
(30 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
A) moves upward with increasing speed
(2 minutes)
The figure shows three crates being pushed over a concrete floor by a horizontal force of magnitude 440 N. The masses of the crates are m1 = 30.0 kg, m2 = 10.0 kg, and m3 = 20.0 kg. The coefficient of kinetic friction between the floor and each of the crates is 0.700. (a) What is the magnitude of the force on crate 3 from crate 2?
.
F32 = 147 N
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Entire system:
F - ukFN = (m1+m2+m3)a
F - uk(m1+m2+m3)g = (m1+m2+m3)a
a = 0.473 m/s2
Crate 3:
F32 - uk(m3g) = m3a
F32 = 147 N
(30 seconds)
Acceleration is always in the direction:
A) of the displacement
B) of the initial velocity
C) of the final velocity
D) of the net force
E) opposite to the frictional force
D) of the net force
(90 seconds)
A block slides down a frictionless plane that makes an angle of 30° with the horizontal. The acceleration of the block is:
A) 9.80 m/s2
B) 5.66 m/s2
C) 8.49 m/s2
D) 0 m/s2
E) 4.90 m/s2
E) 4.90 m/s2
(60 seconds)
A 5-kg block is suspended by a rope from the ceiling of an elevator that accelerates downward at 3.0 m/s2. The tension force of the rope on the block is:
A) 15 N, up
B) 34 N, up
C) 34 N, down
D) 64 N, up
E) 64 N, down
B) 34 N, up
(60 seconds)
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
C) 760 N
(4 minutes)
When the three blocks in the figure are released from rest, they accelerate with a magnitude of 0.500 m/s2. Block 1 has mass M, block 2 has 2M, and block 3 has 2M. What is the coefficient of kinetic friction between block 2 and the table?
.
uk = 0.37
-----------------------------------
F = ma for each block:
Block 3: T23 - m3g = m3(-a)
-T23 + m3g = m3a
Block 2: T23 - ukm2g - T12 = m2a
Block 1: T12 - m1g = m1a
Add up the 3 eqns:
m3g - ukm2g - m1g = (m1+m2+m3)a
2Mg - uk(2M)g - Mg = 5Ma
uk = 0.37
(60 seconds)
A crate rests on a horizontal surface and a woman pulls on it with a 10-N force. Rank the situations shown below according to the magnitude of the normal force exerted by the surface on the crate, least to greatest.
A: 10 N applied horizontally
B: 10 N applied at a 45 deg angle
C: 10 N applied vertically
C, B, A
(2 minutes)
A horizontal force of 5.0 N pushes a 0.50-kg block against a vertical wall. The block is initially at rest. If us = 0.60 and uk = 0.50, the frictional force after a period of time is:
A) 0 N
B) 2.5 N
C) 3.0 N
D) 4.0 N
E) 4.9 N
B) 2.5 N
(2 minutes)
A 70-N block sits on a frictionless horizontal surface. It is connected to a 35-N block with a string, over a massless, frictionless pulley. The 35-N block hangs vertically. What is the the acceleration (including direction) of the 35-N block?
3.3 m/s2, downwards
(2 minutes)
A cube has a drag coefficient of 0.8. What would be the terminal velocity of a sugar cube 1 cm on a side in air (ρ = 1.2 kg/m3)? Take the density of sugar to be 1.6 x 103 kg/m3.
A) 1.4 m/s
B) 5 m/s
C) 18 m/s
D) 60 m/s
E) 320 m/s
C) 18 m/s
(5 minutes)
Block B in the figure weighs 711 N.The coefficient of static friction between block and table is 0.25; angle theta = 30 deg; assume that the cord between B and the knot is horizontal. Find the maximum mass of block A for which the system will be stationary.
.
mA,max = 10.5 kg
---------------------------------------
Block B, x: T1 - fs,max = 0
Block B, y: FN - mBg = 0
Block A, x: T2cos(theta) - T1 = 0
Block A, y: T2sin(theta) - mAg = 0
(60 seconds)
The figure shows the same breadbox in four situations where horizontal forces are applied. Rank the situations according to the magnitude of the box’s acceleration, greatest first.
.
(a), (b)/(c)/(d)
(2 minutes)
A 5.0-kg crate is resting on a horizontal plank. The coefficient of static friction is 0.50 and the coefficient of kinetic friction is 0.40. After one end of the plank is raised so the plank makes an angle of 25° with the horizontal, what is the force of friction?
21 N
(2 minutes)
The figure shows four penguins that are being playfully pulled along very slippery (frictionless) ice by a curator. The masses of three penguins and the tension in two of the cords are m1 = 12 kg, m3 = 15 kg, m4 = 20 kg, T2 = 111 N, and T4 = 222 N. Find the penguin mass m2 that is not given.
.
m2 = 23 kg
------------------------------------
T4 - T34 = m4a
T34 - T23 = m3a
T23-T21 = m2a
T21 = m1a
Add eqns together: T4 = (m1+m2+m3+m4)a
(2 minutes)
Calculate the ratio of the drag force on a jet flying at 1000 km/h at an altitude of 10 km to the drag force on a prop-driven transport flying at half that speed and altitude. The density of air is 0.38 kg/m3 at 10 km and 0.67 kg/m3 at 5.0 km. Assume that the airplanes have the same effective cross-sectional area and drag coefficient CD.
2.3
----------------------------
D = (1/2)CDpAv2
Djet = pjetv2jet
Dtrans ptransv2trans
(6 minutes)
In the figure, two blocks are connected over a pulley. The mass of block A is 10 kg, and the coefficient of kinetic friction between A and the incline is 0.20. Angle theta of the incline is 30 deg. Block A slides down the incline at constant speed. What is the mass of block B? Assume the connecting rope has negligible mass. (The pulley’s function is only to redirect the rope.)
.
(Solution on separate paper)