Physics please?

Question

Ok, I have a physics question. Now, I know this is easy but it's just not coming to me. You don't have to give me the calculations or anything like that but can you just help me and tell me what to do in order to find it please?

F is for force (I'm sure everyone knows that)
Given F1 = 36 Newtons (25 degrees North of East) and F2 = 42 Newtons ( 15 degrees east of south), determine the force F3 that must be added to the sum of F1 + F2 to produce a net force of zero.

Please I know it's easy but can you tell me how to find it.

Also, I have another question:
The force of gravuty on a textbook is 18 newtons (down):
a) What's the net force on the book if it is held stationary in your hand?
b) Neglecting air resistance, what's the net force acting on the book if you suddenly remove your hand?

Now, I'm aware that for part a) , the net force is 0 because thaere's no force acting on it besides gravity and they cancel out anyways but what about for part b) , can someone help me?

Answer 1

For the first question, you need to figure out the total force F1 + F2, i.e. magnitude and direction. This requires vector arithmetic. To cancel out that force, you need to add the opposite vector, so F3 = - (F1 + F2). Drawing a graph with arrows representing F1 and F2 will help you a lot in visualizing this.

Second question: There are actually two forces acting on the book, gravity plus what is usually called "the normal force" of your hand holding it up. As you mentioned, in part (a) they cancel, so in part (b) if you remove your hand, what's left is ....?

Answer 2

Net Force of Zero Problem: Solve graphically.

Draw a cross showing the vertical axis and the horizontal axis, using graphing paper. Then using a protractor measure 25degrees NE (that's northeast, ok?) and scale a length of 36. That's F1. Then again using a protractor measure 15degrees SE and scale a length of 42. That's F2. I suggest you use a scale of 1cm=5N. Now draw a line parallel to F1 from the tip of F2, and similarly a line parallel to F2 from the tip of F1. Now draw a line connecting the center of the cross and the intersection of the two lines you just drew. Scale that and note the magnitude. That's called
the resultant. It's not the F3 you are looking for, but it's equal to F3 (called the equilibrant) but exactly opposite in direction.

Force of Gravity Problem:

a. You are right. The net force is 0, but not because the force of gravity cancels out (remember there's only one force of gravity) but because your hand is exerting exactly the same force to hold the book stationary. In this case your hand acts as an equilibrant. It is giving a force which is opposite in direction but equal in magnitude to the force of gravity of 18N.

b. If you suddenly remove your hand, tne net force will no longer be 0. It will be 18N, the force of gravity acting downwards, minus 0. (You removed your hand suddenly, right? So suddenly the upward force from your hand of 18N becomes 0.) So the net force becomes? 18N, i.e. 18N downwards minus 0=18N downwards.

Answer 3

As to your first question--I think you need t orephrase it--it's not clear what you are asking, though maybe someonne else will "get it"

On the second:
There are 2 forces actong the book when it's in your hand: gravity (18 N down) and the force you exert with your hand (18 N up) which cancel out. When you let the book go, there is then only 1 force--gravity. So the book falls. I'd watch my toes!

Answer 4

Hi A!
part b) What would happen if you remove your hand I er when you dont hold that book... I guess it falls.
What do you think why?
Thats right because of the gravity guess how many newtons the gravity could be?

Answer 5

try asking your teacher it seems kinda hard

Answer 6

shhhhhh, less talky talky.