PhysicsLAB

Worksheet
Distinguishing 2nd and 3rd Law forces

1 Your weight is the result of the gravitational force of the earth on your body. Describe the corresponding reaction force.
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
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?

A bicycle and a massive truck have a head-on collision.
4
Upon which vehicle is the impact force greater?
5
Which vehicle undergoes the greater change in its motion?

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?
7
Is the resulting change in speed of the bus greater than, the same, or less than that of 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?
9
Does the friction force cancel your applied 200 N-force, thus making acceleration impossible?
10
Could the friction force be defined as the reaction force to your applied force?
rifle.gif (1893 bytes)
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.


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?
13
Do they form an action-reaction pair? That is, are they 3rd law forces?
14
Do they cancel each other making acceleration equal to zero? That is, are they 2nd law forces?
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?
16
Would the spring scale reading change if the left pulley was removed and the left string attached to a stationary vertical pole?
17
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. 

Does the spring scale read 0 N, 100 N, or 200 N?

An athlete holds a barbell stationary overhead.
barbell.gif (1848 bytes)
18
How does the force he must exert compare to the weight of the barbell?
19
When the barbell was being accelerated upward, how did the athlete's applied force compare to the weight of the barbell?
20
When the barbell is being accelerated downward, how does the athlete's applied force compare to the weight of the barbell?


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?
22
How fast does the 2m-cart roll compared to the smaller 1m-cart?

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?

24
Can these forces cancel each other? Why or why not?
25
What is the reaction force for FSM?
26
The magnitudes of the forces FSM and FMS are equal and act in opposite directions. Can they cancel each other?
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