Download 1. Why must an object at rest have either no force or at least two

Newton’s Laws of Physics (Lyzinski)
1. Why must an object at rest have either no force or at least two forces acting on it?
2. A 50 kg wood block is being dragged at an angle of 20o above horizontal across a wood floor,
where the coefficient of friction is 0.5. The dragging force is 750N. Find the acceleration of the
block.
3. A 20 kg object is at rest. A force of 180N is required to set the block in motion. Once the object is
in motion, it will maintain a constant velocity as long as a 150 N acts upon it. Find the coefficients
of static and kinetic friction between the object and the floor.
4. A 3.0 kg toy is pulled by a force of 24 N. If the toy starts from rest, and if the ground is assumed
frictionless, how far will it travel in the first 5.0 s?
5. A 40 kg sprinter starts from rest and 2.0 s later is running at a speed of 8.0 m/s. What is the
average net horizontal force acting on her? What exerts this force?
6. A 70 kg hockey player coasts along the ice on steel skates. If the coefficient of kinetic friction is
0.010, what is the force of friction? How long will it take him to coast to a stop, if he is traveling at
1.0 m/s?
7. A boy pulls a 50 kg crate across a level floor with a force of 200 N. If the force acts at an angle of
30° up from the horizontal, and the coefficient of kinetic friction is 0.30, determine
a.
b.
c.
the normal force exerted on the crate by the floor
the horizontal frictional force exerted on the crate by the floor
the acceleration of the crate
8. An 8.0 g bullet traveling at 400 m/s passes through a heavy block of wood in 4.0 x 10–4 s, emerging
with a velocity of 100 m/s. Ignore any motion of the wood.
a) with what average force did the wood oppose the motion of the bullet?
b) how thick is the block of wood?
9. A 0.22 caliber rifle shoots a bullet of mass 1.8 g with a muzzle velocity (exit velocity) of 500 m/s.
If the barrel is 25 cm long, what is the net force exerted on the bullet while it is in the barrel?
10. A child’s wagon experiences a frictional force of 73 N whenever it is in motion, regardless of the
load it is carrying. An applied horizontal force of 128 N causes the wagon to accelerate at 5.0
m/s2. The same applied force, with a child on the wagon, causes it to accelerate at 1.0 m/s2.
What is the mass of the child?
11. A 10 kg box is pushed with a 50 N horizontal force. The coefficient of friction between the box
and the ground is 0.4. Determine the acceleration of the box.
12. A 45 N force is applied horizontally to a 10 kg toy causing it to accelerate at 0.58 m/s2. Determine
the coefficient of friction.
13. A gardener pushes down along the handle of a lawn mower of 20 kg mass with a force of 150 N.
The handle makes an angle of 60° with the ground. Calculate the instantaneous acceleration of
the mower if the frictional force between its wheels and the ground at that instant is 25 N.
14. A boy with a mass of 30 kg pulls a cart with a mass of 100 kg towards himself by a rope. With
what force does he have to pull on the rope to accelerate the cart at 2.0 m/s2? With what force
must his feet push on the ground to keep him from moving towards the cart? If there is no
friction between his feet and the ground, what is his acceleration?
15. An F-14 Tomcat reaches speeds of 74 m/s from rest in a distance of 305 meters from the deck of
an aircraft carrier. The Tomcat has a mass of 27,000 kg at takeoff. Determine the necessary force
for takeoff. (FYI: this force is supplied by the twin jet engines and the tow-line, which is
powered by steam)
16. The same airplane from the previous problem comes to rest while landing in 2 seconds. It is
landing at 67 m/s and then comes to rest on the dock of the aircraft carrier. What is the force of
the Toe-hook acting on the plane as it brings it to rest?
17. A 9-month-old boy is pushing down on his Fisher-Price Popcorn Popping walker toy. The toy has
a mass of 4 kg and is moving with a constant velocity of 2.3 m/s. The boy is pushing down with a
force at an angle of 45o with respect to the ground. There is a frictional force of 30 N acting on
the toy. Determine the force of the boy acting on the handle.
18. A train of three cars ( each of mass 500 kg) is being pulled across a surface whose coefficient of
kinetic friction is 0.3. If the tension in the last coupling is 1600 Newtons, find…
a) the acceleration of the train.
b) the force that the train is pulled with.
c) the tension in the other coupling.
19. Three boxes, each of mass 70 kg, are pushed into each other. The force between the first box (to
which the external force is applied) and the second box is 400N. If the surface they are pushed
on is frictionless, find …
a) the acceleration of the blocks.
b) the external force that they are pushed with.
c) the force between the 2nd and 3rd boxes.
20. A mass of 50 kg is pushed by two forces. One is 56N [N] and the other is 90N [SE]. Find the
acceleration of the block. (think “VFA”)
2)
3)
4)
5)
6)
7)
a = 11.7606 m/s2
0.918, 0.765
100 m
160 N
10.2 sec
1.12 m/s2
8) -6000N, 10 cm thick
9) 900N
10) 44 kg
11) 1.08 m/s2
12) 0.4
13) 2.5 m/s2
14)
15)
16)
17)
18)
19)
20)
200N, 200N, -6.7 m/s2
242,280.3 N
-904,500 N
42.4 N
0.26 m/s2, 3200 N, 4800 N
2.857 m/s2, 600N, 200N
1.28 m/s2 [E 7o S]