Newtons 3rd law
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While a football is in flight, what forces act on it? What are the action–reaction pairs
while the football is being kicked and while it is in flight?
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A book is given a brief push to make it slide up a rough incline. It comes to a stop and
slides back down to the starting point. Does it take the same time to go up as to
come down? What if the incline is frictionless?
Question 1
Three blocks are in contact with each other on a frictionless, horizontal surface, as in Figure
1. A horizontal force F is applied to m1. Take m1 = 2.00 kg, m2 = 3.00 kg, m3 = 4.00 kg, and F
= 18.0 N. Draw a separate free-body diagram for each block and find
(a) the acceleration of the blocks,
(b) the resultant force on each block, and
(c) the magnitudes of the contact forces between the blocks.
Question 2
Question 3
The lower block in the FIGURE below is pulled on by a rope with a tension force of 20 N. The
coefficient of kinetic friction between the lower block and the surface is 0.30. The coefficient
of kinetic friction between the lower block and the upper block is also 0.30. What is the
acceleration of the 2.0 kg block?
Question 4
A block of mass m = 2.00 kg is released from rest at h = 0.500 m above the surface of a table,
at the top of a 30.0° incline as shown in Figure below. The frictionless incline is fixed on a table
of height H = 2.00 m.
(a) Determine the acceleration of the block as it slides down the incline.
(b) What is the velocity of the block as it leaves the incline?
(c) How far from the table will the block hit the floor?
(d) How much time has elapsed between when the block is released and when it hits the floor?
(e) Does the mass of the block affect any of the above calculations?
Question 5
A 3.00-kg object is moving in a plane, with its x and y coordinates given by x = 5t2- 1 and y =
3t3 + 2, where x and y are in meters and t is in seconds. Find the magnitude of the net force
acting on this object at t = 2.00 s.
Question 6
A bag of cement of weight Fg hangs from three wires as shown in Figure P. Two of the wires
make angles θ1 and θ2 with the horizontal. If the system is in equilibrium, show that the
tension in the left-hand wire is
Question 7
Two objects are connected by a light string that passes over a frictionless pulley, as in Figure
P. Draw free-body diagrams of both objects. If the incline is frictionless and if m1 = 2.00 kg, m2
= 6.00 kg, and θ=55.0°, find
(a) the accelerations of the objects,
(b) the tension in the string, and
(c) the speed of each object 2.00 s after being released from rest
Question 8
Figure gives, as a function of time t, the force component Fx that acts on a 3.00 kg ice block
that can move only along the x axis. At t = 0, the block is moving in the positive direction of
the axis, with a speed of 3.0 m/s. What are its (a) speed and (b) direction of travel at t = 11 s?
Question 9
A shot putter launches a 7.260 kg shot by pushing I along a straight line of length 1.650 m and
at an angle of 34.10° from the horizontal, accelerating the shot to the launch speed from its
initial speed of 2.500 m/s (which is due to the athlete’s preliminary motion).The shot leaves
the hand at a height of 2.110 m and at an angle of 34.1o, and it lands at a horizontal distance
of 15.90 m. What is the magnitude of the athlete’s average force on the shot during the
acceleration phase?
(Hint: Treat the motion during the acceleration phase as though it were along a ramp at the
given angle.)
Solutions