PhysicsLAB: Action-Reaction #2

CP Workbook
Action-Reaction #2

Refer to the following information for the next thirteen questions.

Nellie Newton holds an apple weighing one newton at rest on the palm of her hand. The force vectors shown are the forces that act on the apple.

To say the weight of the apple is 1 N is to say that a downward gravitational force of 1 N is exerted on the apple by

the earthher hand

Nellie's hand supports the apple with normal force N, which acts in a direction opposite to W. We can say N

equals Whas the same magnitude as W

Since the apple is at rest, the net force on the apple is

zerononzero

N is equal and opposite to W. We ____ say that N and W comprise an action-reaction pair.

cancannot

The reason is because action and reaction always

act on the same objectact on different objects

and here we see N and W

both acting on the appleacting on different objects

In accord with the rule, "If ACTION is A acting on B, then REACTION is B acting on A," if we say action is the earth pulling down on the apple, reaction is

the apple pulling upon the earth
Nellie's hand pushing up on the apple

To repeat for emphasis, we see that N and W are equal and opposite to each other

and comprise an action-reaction pair
but do not comprise an action-reaction pair

Another pair of forces is N [shown] and the downward force of the apple against Nellie's hand [not shown]. This force pair ____ an action-reaction pair.

isis not

Suppose Nellie now pushes upward on the apple with a force of 2 N. The apple

is still in equilibriumaccelerates upward

and compared to W, the magnitude of N is

the same
twice
not the same, and not twice

Once the apple leaves Nellie's hand, N is

zerostill twice the magnitude of W

and the net force on the apple is

zeroonly Wstill W - N, a negative force