acceleration and force

Question

if you have a hammer hitting a stake into the ground why will the acceleration of the hammer be less than the acceleration of the stake....

 

a.  the force of the hammer is greater than the force of the stake

b.  the mass of the hammer is greater than the mass of the stake

c.  the mass of the stake is greater than the mass of the hammer

d.  none of these

 

 

thanks

Answer 1

Force is mass times acceleration. F=ma. The force is the same (presuming no energy is lost converting from the hammer to the stake, which is not precisely true, but usually too small to significantly affect the result). Therefore mass of hammer times acceleration is equal to mass of stake times acceleration. If the acceleration of the hammer is less than the acceleration of the stake (given in problem), the mass of the hammer must be greater than the mass of the stake to maintain the equality m*a (for hammer) = m*a (for stake).

 

Hope this helps

 

Mike

Answer 2

Newton's 3rd Law

NIII: If body A exerts a force on body B, then body B exerts an equal and opposite force on body A.

If you punch a tissue. Why doesn't the tissue punch you back with the same force?

It does but the tissue has very little inertia (mass)so cannot exert a large force, hence you cannot exert a large force on it.

Now punch a wall!

Force isn't a thing, but part of an interaction, between one thing and another. NIII states that there are always equal and opposite forces in a single interaction.

Important NB: In NII we consider the forces acting ON a body, in NIII we consider the forces acting IN an interaction.

Examples:

Leaning against a wall. The wall exerts a force back, hence you don't fall over.
Hammer hitting a stake (The hammer exerts a force on the stake, driving it into the ground. BUT the stake also exerts a force on the hammer causing the hammer to slow down and even rebound.
Why if a ball dropped on Earth is pulled down towards the Earth, the ball doesn't also pull the Earth upwaqrds towards it ?

It does! But because the Earth is very much more massive than the ball the force has a much larger effect on the ball, while the effect on the Earth is not noticable.