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Newton’s Laws of Motion

Chapter 4

Force

FORCE! Newton stated that the change in

velocity of an object is caused byFORCES.

! When the velocity of an object isconstant, or if the object is at rest, itis said to be in equilibrium.

FORCE

! Contact forces: forces that result fromphysical contact between two objects.

! Examples: friction, a push or a pull

! Field forces: forces that can act at adistance.

! Examples: magnetism, gravity

FORCE DIAGRAMS! Force is a vector.

! Force diagrams – showforces vectors asarrows

! Free body diagrams –shows only the forcesacting on a singleobject

Newton’s First Law

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Newton’s First Law(Law of Inertia)

! A body at rest will remain at rest, a bodyin motion will remain in motion, travelingwith a constant velocity in a straight line,unless an unbalanced force acts on it.

! INERTIA = a measure of a body’s abilityto resist changes in velocity.! INERTIA ! MASS

! (the greater the mass of a body, the less itwill accelerate under the action of an appliedforce)

It’s easier to push a Volkswagenthan a Mack Truck

But, once it’s moving, it’sharder to stop the truck

That’s because theMack Truck has more

mass, and therebymore inertia.

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Newton’s Second

and Third Laws

Newton’s Second Law

!F = ma

! The acceleration of an object is directlyproportional to the resultant force acting on itand inversely proportional to its mass.

! The direction of the acceleration is in thedirection of the resultant force.

! The SI unit of force is the NEWTON.! 1N = 1 kg•m/s2

! Weight: w = mg

Any accelerationrequires a force.

Q

Acceleration can be…

An elevator accelerates upwards. IfBart steps on the scale, it will read…

a. Zero

b. His weight

c. More than b

d. Less than b

Now the elevator travels upward with aconstant velocity. If Bart steps on the

scale, it will read…

a. Zero

b. His weight

c. More than b

d. Less than b

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Newton’s Third Law

! If two bodies interact, the forceexerted on a body 1 by body 2 isequal in magnitude, but opposite indirection to the force exerted onbody 2 by 1.

! OR….

! For every action there is anequal and opposite reaction.

Implicit in these laws are thefollowing ideas.

• If no unbalanced force acts on abody, its acceleration must be zero.

• An unbalanced force is one whosevector sum does not equal zero.

Implicit in these laws are thefollowing ideas.

! If an unbalanced force acts on abody, it must accelerate. It willcontinue to accelerate for as long asthe force(s) are unbalanced.

Implicit in these laws are thefollowing ideas.

! If a body has no acceleration, thevector sum of all the forcesacting on it must be zero.

What is thevector sum of allthe forces acting

on this point?

Zero

A 1 kg rock is thrown at 10 m/s straight upward.Neglecting air resistance, what is the net force thatacts on it when it is half way to the top of its path?

1. -9.8 N 2. -9.8/2 N 3. 0 N 4. -9.8/4 N