P5 Inertia and Newton's Third Law (page 212) Flashcards
Inertia is what?
the tendence for motion to remain unchanged.
(This tendency to continue in the same state of motion is called Inertia).
Until acted upon by a resultant force, what will objects do at rest?
they stay at rest. (this is newtons first law)
When objects are moving at a steady speed will they stay moving at a steady speed?
Yes
(this is newton’s first law)
An object’s inertial mass measures what?
how difficult it is to change the velocity of an object.
Inertial mass can be found using what Newton’s law?
Newton’s second law. of F = ma (see page 211).
Regarranging this gives m = f ÷ a, so inertial mass is just the ratio of force over acceleration.
Newton’s Third Law involves what?
Equal and Opposite Forces.
What do Newton’s Third Law say?
When two objects interact, the forces they exert on each other are equal and opposite.
Give an example of Newton’s Third Law when you push a shopping trolley?
If you push something, say a shopping rolley, the trolley will push back against you, just as hard.
And as soon as you stop pushing, so does the trolley. (clever).
If the forces are always equal , how does anything ever go anywhere?
Explain this with 2 scaters from diagram on p.212.
Its important to remember that the two forces are acting on different objects.
When skater A pushes on skater B (the ‘action’ force), se feels an equal and opposite force from skater B;s hand (the ‘normal contact’ force). Goth skaters feel the same sized force, in opposite directions, ad so accelerate away from each other.
Akater A will be accelerated more than skater B, though because she has a smaller mass - remember a = F ÷ m
Give an example of newton’s Third Law in an equilibrium situation, where a man is pushing against a wall? (see diagram 2 on page 212)
As the man pushes the wall, there is a normal contact force acting back on him. These two forces are the same size, as the man applies a force and pushes the wall, the wall ‘pushes back’ on him with an equal force.
It can be easy to get confused with Newton’s Third Law when an object is in equilibrium.
A book resing on the ground is in equilibrium why?
the weight of the book is equal to the normal contact force.
(But this is NOT Newton’s Third Law because the two forces are different types, and both acting on the book)
(Newton’s thrid law can trip people up, so make sure ou understand exactly what the forces are acting on and how that results in movement (or lack of it).
A car moves at a constant velocity along a road, so that it is in equilibrum. Give an example of a pair or forces that demonstrate Newton’s Third Law in this situation (1 mark)
Any one from: e.g.
the gravitational force of the Earth attracts the car and the gravitational force of the car attracts the earth (1 mark).
the car exerts a normal contact force down against the ground and the normal contact force from te ground pushes up against the car (1 mark).
the car (tyres) pushes the road backwards and the road pushes the car (tyres) forwards (1 mark).
