Download 1 Honors Physics, Unit Four, Newton`s Laws, Worksheet (key) 1

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Honors Physics, Unit Four, Newton’s Laws, Worksheet (key)
1. Consider hitting a baseball with a bat. If the force of the bat pushing on the ball is the action force, what is the reaction
force? The force of the ball pushing on the bat is the reaction force.
2. Do action and reaction pairs of forces cancel/balance one another? The action reaction forces are of the same
magnitude but opposite directions. They do not cancel each other out because they act on different objects.
3. (a) As you sit in a chair, does the chair exert an upward force against your butt. (b) How much force does it exert? (c) In
what direction? (a) yes (b) the same amount of force that you exert on the chair (c) upward
4. Consider the forces acting on you as you sit in your seat, the downward pull of gravity and the upward support of your
seat. (a) Are these forces equal and opposite? (b) How do you know? (c) Do they form an action - reaction pair?
(a) yes (b) because you are not moving, so the forces are in static equilibrium (c) yes
5. For each of the following forces, what is the equal and opposite force required by Newton's third law? (a) The force of a
hammer on a nail. (b) the force of gravity (the earth) pulling down on a book? (c) the force of a helicopter blade pushing
down on the air. (d) the force of air resistance acting on a falling baseball. (a) the force of the nail on the hammer
(b) the force of the book pulling up on the earth (c) the force of the air pushing up on the helicopter blade
(d) the force of the baseball pushing down on the air
6. (a) What is the amount of “stuff” in an object, mass or weight? (b) Does an object with mass always have weight?
(c) Does an object with weight have mass? (a) mass (b) Yes, because no matter where it is located in the
universe, some star or planet will be pulling on it. (c) Yes, for an object to have something like a planet or a
star pulling on it, it must have mass.
7. (a) Can an object have mass without having weight? (b) Can an object have weight without having mass?
a star or a planet to pull on the object, it must have mass. (b) No, see letter a.
(a) No, for
8. Why does a 2 kg brick have twice the inertia of a 1 kg brick? Yes, because the 2 kg brick has twice the mass.
9. In the orbiting space station you are handed two identical boxes, one filled with sand the other filled with feathers. How
can you tell which is which without opening the boxes? You can tell by determining which box has the most
inertia. The box with the most mass will have the most inertia.
10. Why is a massive cleaver more effective for chopping vegetables than an equally sharp and less massive knife?
Since the massive cleaver has more mass, it has more inertia. The vegetables would have a more difficult
time stopping the knife.
11. In tearing a paper towel from a roll, why is a sharp jerk more effective than a slow pull? Since an object at rest
wants to stay at rest, a sharp jerk puts less force into the stationary paper roll than a slow pull. Because of
this, the paper roll will more likely stay at rest and the piece of paper towel will tear off of the roll.
12. Since the earth rotates once every 24 hours, the western wall in the classroom moves in a direction towards you at a
linear speed that is about 1000 km/h. When you stand facing the wall, you are carried along at the same speed, so
you don't notice it. But when you jump upward, with your feet no longer in contact with the floor, why doesn't the wall
slam into you at about 1000 km/h? Since you and the wall are both traveling at 1000 km/h, when you jump
straight up, you will fall straight down.
13. Can an object round a curve without any force acting upon it? Explain No. An object must have a net force
applied to it to accelerate. The three ways an object can accelerate is be speeding up, slowing down, or
changing direction. Therefore, an object can not change direction without a force acting on it.
14. To pull a wagon across a lawn with a constant velocity, you have to exert a steady force. Reconcile this fact with
Newton's first two laws, which state that motion at constant velocity requires no net force. The reason you have to
exert a steady force to pull a wagon across a lawn with constant velocity is because there is a constant
steady force of friction acting in the opposite direction that must be overcome. The friction force must equal
the pulling force for the wagon to move at a constant velocity (no net force).
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15. If you jump into the air from the floor you are standing on, how does the force you exert on the ground compare with
your weight? Explain. The force you put into the ground had to be more than your weight or the floor would not
match the force you put into the floor and push you into the air. If the force you put into the floor is the same
as your weight, you would not jump into the air.
16. You find an object that’s not moving and you know it’s being acted on by a force, how do you explain this to your
confused friend? The object is being acted upon by balanced forces (no net force). In this case, the object will
be either at rest or moving at a constant velocity. In this incidence, the object is at rest.
17. When a car moves along the road at a constant velocity, the net force is zero. Why does the engine have to keep
running? The engine has to keep running so the car can match the constant force of friction being put on the
car by the road. If the engine stopped running, the friction would quickly cause the car to stop.
18. (a) What is the net force on an apple that weighs 1 N as you hold it at rest above your head? (b) What is the net force
on it after you release it? (a) zero (b) 1 N downward
19. A cart is pulled to the left with a force of 100 N, and to the right with a force of 30 N. What is the net force on the cart?
70 N to the left
20. If the net force on a sliding block is tripled, by how much does the acceleration increase? 3 times
21. If the mass of a sliding box is tripled while a constant force is applied, by how much does the acceleration decrease?
3 times
22. If the mass of a moving object is tripled just as the net force on it is tripled, how is the acceleration affected?
no effect
23. What is the acceleration of a 39,690 kg jumbo jet just before takeoff if each of its four engines produces 281.57 kN of
Force? 28.4 m/s2
24. What is the mass of a kiwi fruit thrown with an acceleration of 142 cm/s 2? The thrower applied a force of 0.35 N to the
kiwi fruit. 0.25 kg
25. You hold an apple over your head. (a) Identify all the forces acting on the apple and their reaction forces. (b) When
you drop the apple, identify all the forces acting on it as it falls and the corresponding reaction forces. (c) Identify the
forces on the apple when it hits the ground. (d) What force causes the apple to stop? (a) the force of the apple on
your hand – the force of your hand on the apple, the force of the apple on the earth – the force of the earth on
the apple (b) the force of the apple on the air – the force of the air on the apple, the force of the apple on the
earth – the force of the earth on the apple (c) the force of the apple on the ground – the force of the ground
on the apple, the force of the apple on the earth – the force of the earth on the apple (d) The upward force of
the ground on the apple caused the apple to stop.
26. (a) If a Mack truck and a Honda Civic have a head-on collision, upon which vehicle is the impact force greater?
(b) Which vehicle experiences the greater acceleration? Explain each answer. (a) The impact forces of both are
the same according to Newton’s third law of motion. (b) According to Newton’s second law (F = m a), the
Honda experiences the greater acceleration than the Mack truck because it has less mass than the Mack
truck.
27. Which team wins in a tug-of-war; the team that pulls harder on the rope, or the team that pushes harder on the
ground? Explain. The team that pulls harder on the rope, by definition, will be the team that pushes harder on
the ground. The reason for this is that, for the team to pull harder on the rope, it must push harder on the
ground to give itself leverage to pull harder. Therefore, the team that does both of these will win.
28. A horse pulls a heavy wagon with a certain force. The wagon, in turn, pulls back with an opposite but equal force on
the horse. (a) Doesn't this mean the forces cancel one another, making acceleration impossible? (b) Why or why not?
(a) No. (b) The reason for this is that these opposite forces are acting on different objects, one acting on the
horse, the other acting on the wagon.
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Answers to Conceptual Physics Worksheets on Pages 4 and 5 of this Worksheet Key
Pages 11 and 12 from the Conceptual Physics Worksheet (Page 5 of this Worksheet Key)
1. The rock will continue moving away in a straight line at constant speed.
Inertia
2. D
3. 100 km/hr 100 km/hr directly at your feet below your hand is the same as if the bus were at rest
Once the pencil is moving at 100 km/hr relative to the road, it will want to continue moving at 100 km/hr
relative to the road whether it is held by the passenger or dropped by the passenger.
4. The force due to gravity on an object is its weight. The quantity of matter in an object is its mass. The
amount of space an object occupies is its volume.
5. 49 N
10 kg
20 kg, 196 N
6. Since 1 kg weighs 9.8 N, melon – 9.8 N
apple – 0.1 kg
Uncle Harry – 882 N
7. The girl isn’t hurt because the block and the books on her head are at rest and will want to remain at rest
due to Newton’s first law. The hammer into the nail on top of the books doesn’t deliver enough force on
the books and block to have them move enough to hurt the girl.
If the block were less massive, the
situation would be more dangerous because the combined mass of the blocks and book would be less.
Therefore, less mass would equal less inertia. This would mean that the hammer on the block would
cause the block and books to move easier and possibly hurt the girl.
Pages 27 and 28 from the Conceptual Physics Worksheet (Page 4 of this Worksheet Key)
1. arrow in opposite direction for all examples. (a) Ball bumps head. (b) Bug hits windshield. (c) Ball hits
bat. (d) Nose touches hand. (e) flower pulls on hand (f) bar pushes athlete downward (g) balloon
surface pushes compressed gas inward
2. Draw on picture (should have 12 forces).
3. (a) the Earth (b) has the same magnitude as W (c) zero (d) cannot, act on different objects, both acting
on the apple (e) apple pulling up on the earth (f) do not comprise an action reaction pair (g) is
(h) accelerates upward, not the same – but twice (i) zero, only W
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1. A 70 kg box falls out of an airplane. It experiences 200 N of air drag. What is its acceleration?
m = 70 kg, g = 9.8 m/s2, Fdrag = 200 N, ay = ?
(+
Fw = m g = (70 kg)(9.8 m/s2)
Fw = 686 N
Fdrag = 200 N
∑Fy) Fw – Fdrag = m ay
ay = (Fw – Fdrag) / m = (686 N – 200 N) / 70 kg = 6.94 m/s2 = ay
Fw = 686 N
2. Dr. G throws his car keys across the floor. While the keys have a mass of 0.05 kg and are accelerating at –2 m/s2,
What is the force of friction on the keys?
m = 0.05 kg, g = 9.8 m/s2, ax = –2 m/s2, Ff = ?
Fw = m g = (0.05 kg)(9.8 m/s2)
Fw = 49 N
FN must = 49 N because the keys are not floating or going down into the floor
Ff
FN = 49 N
Fw = 49 N
Therefore the sum of forces in the y direction = 0
(∑Fx) Ff = m ax = (0.05 kg)(–2 m/s2) = –0.1 N = Ff (negative because acting in same direction as acceleration
and opposite to the movement of the car keys)
3. A car located on a level highway has a mass of 400 kg. The friction force opposing the car’s motion is 750 N. What
acceleration will a force of 2,250 N produce on the car?
Fw = m g = (400 kg)(9.8 m/s2)
Fw = 3,920 N
FN must be 3,920 N because the car remains on the road.
Ff = 750 N
Therefore, (∑Fy) = 0
m = 400 kg, Ff = 750 N, Fa = 2,250 N, ax = ?
FN = 3,920 N
Fa = 2,250 N
Fw = 3,920 N
(∑Fx) Fa – Ff = m ax
ax = (Fa – Ff) / m
ax = (2,250 N – 750 N) / 400 kg = 3.75 m/s2 = ax
4. (a) What is the mass of a small rocket that weighs 14.7 N? (b) It’s fired from a high platform, but the engine fails to
burn properly. The rocket gains a total upward force of only 10.2 N. What is the rate and direction of the rocket’s
acceleration? (c) A new engine is added making the new upward force 20.4 N. What is the new rate and direction of
the rocket’s acceleration?
Fup = 10.2 N, Fw = 14.7 N, m = ?, arate/direction = ?
(a) Fw = m g
m = Fw / g = 14.7 N / 9.8 m/s2 = 1.5 kg = m
Fup = 10.2 N
(b) (+ ∑Fy)
Fup – Fw = m ay
ay = (Fup – Fw) / m = (10.2 N – 14.7 N) / 1.5 kg
= –3 m/s2 (up) = ay
(b)
Fw = 14.7 N
Fup = 20.4 N (new engine)
(c) (+ ∑Fy)
Fup – Fw = m ay
ay = (fup – Fw) / m = (20.4 N – 14.7 N) / 1.5 kg
= +3.8 m/s2 (up) = ay
Fup = 20.4 N
(c)
Fw = 14.7 N
5. A wagon with a precious little girl in it has a mass of 20 kg and is being pulled by a force of 25 N at an angle of 40o
above the horizontal. The friction force against the little girl and her wagon is 8 N. What is the acceleration of the little
girl and her wagon?
m = 20 kg, Fa = 25 N, Ff = 8 N, θ = 40o, ax = ?
FN Fay
Fay = Fa sin θ = (25 N)(sin 40o) = 16.1 N = Fay
Fax = Fa cos θ = (25N)(cos 40o) = 19.2 N = Fax (force pulling sled and little girl)
θ
Fax
Ff
(+
∑Fx)
Fax – Ff = m ax
ax = (Fax – Ff) / m
ax = (19.2 N – 8 N) / 20 kg = + 0.56 m/s2 = ax
Fa
Fw
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6. Harry holds a block from falling by pushing it horizontally against a vertical wall. If the block weighs 3 N and the
coefficient of friction between the block and the wall is 0.46. What force must Harry exert to just hold the block against
the wall (just enough to not allow the block to slide down the wall)?
FF
FW = 3 N, µ = 0.46, FA = ?
FN
FN is always perpendicular to the contact surface between the wall and the
FA
box. Since the block is not moving into or away from the wall, F N = FA.
If the block is held up against the wall just to the point where it is not moving,
then FW = FF.
equations: FF = µ FN
can assume that
1. FW = FF
FW = µ F N
FW
FW = FF and FN = FA
FW = µ F A
FA = FW / µ = 3 N / 0.46 = 6.5 N = FA
7. An artillery shell has a mass of 8,000 g. It is fired from the muzzle of a large gun with a speed of 700 m/s. The gun
barrel is 3.5 m long. What is the average force on the shell while it is in the gun barrel?
m = 8 kg, vf = 700 m/s, vi = 0 m/s, d = 3.5 m, Fx = ?
vf2 = vi2 + 2 a d
l
3.5 m
l
a = vf2 / 2 d = (700 m/s)2 / 2 (3.5 m)
ax = 70,000 m/s2
Fave = m a = (8,000 g) (1 kg / 1,000 g) (70,000 m/s) = 560,000 N = Fave