Newton’s Third Law
Which pulls more, the apple or the orange?
Whenever one object exerts a force on a second object, the second
object exerts an equal and opposite force on the first
They both exert the same force on each other but in opposite directions.
The orange (on the cart) will accelerate towards the apple because the
force of friction that resists the pull is so low. Note that as well as any
unbalanced forces acting on each object the acceleration of each object
will depend on the inertia (mass) of each in accordance with a = FNET/m
A truck driving down the road runs into a bug flying the other
way.
a) Which object has the greater force exerted on it?
b) Which object will change its motion the most?
FTB
FBT
Both objects have the same small force exerted on them during
the collision according to Newton’s third law. The bug squishes
easily!
The bug obviously changes its motion significantly more than
the truck because of its much lower inertia (mass).
Truck:
aT
=F/
M
:
Bug
- aT = - F /
M
The moon moves around the earth.
a) Which object has the greater force exerted on it?
b) Which object will change its motion the most?
FME
FEM
Both objects have the same large force exerted on them during
the collision according to Newton’s third law.
The Moon obviously changes its motion significantly more than
the Earth because of its much lower inertia (mass).
Earth:
aE
=F/
M
Moon
:
- aM = - F /
M
A boxer hits a piece of paper.
a) Which object has the greater force exerted on it?
b) Which object will change its motion the most?
FGP
FPG
Both objects have the same small force exerted on them during
the collision according to Newton’s third law.
The paper obviously changes its motion significantly more than
the glove because of its much lower inertia (mass).
Glove:
aG
=F/
M
Paper
:
- aP = - F /
M
A bullet is fired from a gun.
a) Which object has the greater force exerted on it?
b) Which object will change its motion the most?
FGB
FBG
Both objects have the same force exerted on them during the
collision according to Newton’s third law.
The bullet obviously changes its motion significantly more than
the gun because of its much lower inertia (mass).
Gun:
aG
=F/
M
:
Bullet
- aB = - F /
M