Newtons Third Law
Now this is an interesting law.The best way to understand this is by experiment.
I want you to slap yourself on your cheek.
Not too hard.
Go on! Do it!
Do not be a sissy!
This means that BOTH your hand and your cheek felt a force each, although there was only ONE SLAP.
This single event of a slap involved two separate objects and hence two separate forces. But your intention was to only make your cheek sting.
Some Facts
1. cheek felt a force : F1F1 acts ON cheek
F1 is exerted BY hand
2. hand also felt a force : F2
F2 acts ON hand
F2 is exerted BY cheek
3. F1 and F2 are of the same size.
Their effects may be different.
Don’t be fooled into thinking that the cheek was hit harder.
It only feels harder because the cheek is more sensitive due to more sensitive nerve endings!
Stuff we do not notice.
We often think that for an object to apply a force, that object must move.
Not so.
In the SLAP, the cheek did NOT move towards the hand but it DID APPLY a force on the hand.
Newtons Third Law
If object X applies a force onto object Y, then object Y simultaneously applies the same size force back onto object X.
This is often called Action-Reaction pair of forces.
In order to identify Action-Reaction Forces in a situation,it is usually easier when you first identify the Action-Reaction Objects first.
Examples
1. An insect is splattered against the windscreen of a fast moving, large truck.
1.1.What applied the force ON the insect?
the truck
1.2. What applied the force ON the truck?
the insect
1.3. What can be concluded about the size (magnitude) of these two forces?
They are of the same size.
1.4. Why do the two forces not cancel each other out?
Although the forces are of the same size, they act on DIFFERENT objects.
One force acts on the insect, and the other force acts on the windscreen.
Cancellation can only occur if two equal sized forces act in opposite directions on the SAME object.
1.5. Name the law used to come to the conclusion in 1.3.
Newton's Third Law
1.6. Why is the insect splattered but the windscreen is not damaged?
Although the forces are of the same size, the insect is not strong enough to widstand the force, and is splattered.
The windscreen is made strong enough to easily widstand this force.
2. A MAN pushes a TROLLEY rightwards with a force of 50N across a flat floor. The friction force between the trolley and the floor is 5N.
45N rightwards
2.2. What is the size of the force that the MAN exerts on the TROLLEY?
50N rightwards
2.3. What is the size of the force that the TROLLEY exerts on the man?
50N leftwards
Don't get confused with resultant force and Newton 3 pairs of forces.