Download 1 - sciencewithskinner

Name ________________________________________________Date_______________Period__________
Newton’s 2nd & 3rd Laws of Motion (answer sheet)
1. Your weight is the result of the gravitational force of the earth on your body. Describe the
corresponding reaction force.
the gravitational force of your body on the earth
2. If you step off a ledge, you accelerate noticeably toward the earth because of the gravitational
interaction between you and the earth. Does the earth accelerate towards you as well? Explain
Yes, it does. But the mass of the earth is so phenomenal, its acceleration towards you is negligible
3. When a high jumper leaves the ground, what is the source of the upward force that accelerates
her? What force acts on her once her feet are no longer in contact with the ground?
The normal supplied by the floor. Once in the air, only the pull of gravity, represented by her
weight, acts upon her.
A bicycle and a massive truck have a head-on collision.
4. Upon which vehicle is the impact force greater?
The impact force is identical. Action-Reaction forces.
5. Which vehicle undergoes the greater change in its motion?
The bicycle since it has a smaller mass. The same force produces a greater acceleration on an object
with less inertia.
A speeding bus makes contact with a lovebug that splatters onto its windshield. Because of the
sudden impact force, the unfortunate bug undergoes a sudden loss of speed.
6. Is the corresponding impact force that the bug exerts against the bus' windshield greater, the
same, or less than that which it experienced? The impact forces are equal. Action-Reaction:
force of lovebug on windshield equals force of windshield on lovebug
7. Is the resulting change in speed of the bus greater than, the same, or less than that of the
bug? Much less because the bus has a much greater inertia (mass) than the bug.
Suppose you exert 200 N of force on your refrigerator and push it across the kitchen floor at a
constant velocity.
8. How large is the friction force that acts between the refrigerator and the floor? 200 N
9. Does the friction force cancel your applied 200 N-force, thus making acceleration impossible?
Yes, the net F = 0 since the refrigerator is in dynamic equilibrium.
10. Could the friction force be defined as the reaction force to your applied force? No! the friction
force is the force of the floor against the refrigerator. Your applied force is also acting on the
refrigerator. If two forces are acting on the same object they cannot be action-reaction forces.
Name ________________________________________________Date_______________Period__________
11. Since the force that acts on a bullet when a gun is fired is equal and opposite to the force that
acts on the gun, does this imply a zero net force and therefore the impossibility of an
accelerating bullet? Explain. No! the two forces listed are action-reaction forces. They act on
different objects, therefore they cannot cancel.
Consider the two forces acting on the person who stands still, namely the downward pull of
gravity, mg, and the upward support of the floor, N.
12. Are these forces equal and opposite? yes, they have the same megnitude and act in opposite
directions.
13. Do they form an action-reaction pair? That is, are they 3rd law forces? No! they act on the
same object: earth on person, floor on person.
14. Do they cancel each other making acceleration equal to zero? That is, are they 2nd law
forces? Yes! since they have the same magnitude and act in opposite directions on the same
object, they can cencel resulting in an acceleraiton of zero. The person is in a state of static
equilibrium
Two 100 N weights are attached to a spring scale as shown.
15. Does the spring scale read 0 N, 100 N, or 200 N? 100 N
16. Would the spring scale reading change if the left pulley was removed and the left string
attached to a stationary vertical pole? No, the tension in the rope would remain 100 N.
While the left string is attached to a stationary vertical pole, the left 100 N weight is suspended
underneath the original right-hand 100 N weight.
Name ________________________________________________Date_______________Period__________
17. Does the spring scale read 0 N, 100 N, or 200 N? 200 N. A freebody diagram would show that
the upper string connected to the spring scale is now supporting both weights.
An athlete holds a barbell stationary overhead.
18. How does the force he must exert compare to the weight of the barbell? The forces are equal.
The barbell is in a state of static equilibrium.
19. When the barbell was being accelerated upward, how did the athlete's applied force compare
to the weight of the barbell? His applied force would have to be greater than the barbell's
weight. The extra force he would have to apply would equal the product of the barbell's mass
times the magnitude of its upward accceleration.
20. When the barbell is being accelerated downward, how does the athlete's applied force
compare to the weight of the barbell? His applied force would be less than the barbell's weight.
The reduced force he would no longer need to apply would equal the product of the barbell's
mass times the magnitude of its downward accceleration
Suppose two carts, one twice as massive as the other, fly apart when the compressed spring that
joins them is released.
21. How does the force exerted by the spring on the 1m-cart compare to the force exerted by the
spring on the 2m-cart? The forces are equal.
22. How fast does the 2m-cart roll compared to the smaller 1m-cart? It would roll half-as fast since
it experienced half the acceleration when released from the spring
Consider the following 100-N hanging weight.
The tension in the string could be called FSM, that is, the force of the string on the mass.
The mass' weight could be called FEM, that is, the force of the earth on the mass.
23. Are these two forces, FSM and FEM, 2nd Law forces or 3rd Law forces? 2nd Law forces since
they act on the same mass.
24. Can these forces cancel each other? Why or why not? Yes, the weight is in static equilibrium.
25. What is the reaction force for FSM? FMS
26. The magnitudes of the forces FSM and FMS are equal and act in opposite directions. Can they
cancel each other? No, they are action-reaction forces. They act on different objects. They can
NEVER cancel.