Ch 4 – Forces and the Laws of Motion

What is a force?

• A force is a push or pull

• A force causing a change in velocity– An object from rest starts moving– A moving object comes to rest– A moving object changes direction

• The SI Unit for Force is the Newton– 1 N = 1 kgm/s2

2 Types of Forces

• Contact Force– Force that arises from the physical contact of

two objects

• Field Force– Force that can exist between objects, even in

the absence of physical contact between objects

Free Body Diagrams

• These drawings help us by isolating an object and the forces acting on it.

Newton’s First Law

• An object at rest remains at rest, and an object in motion continues in motion with constant velocity unless the object experiences a net external force.

• The tendency of an object not to accelerate is called inertia.

• If there is a net external force, this will cause an acceleration

• Net external force is the sum of all forces acting on an object

Mass is a measurement of Inertia

• The more mass something has, the greater tendency it will have to keep doing what it’s been doing

Equilibrium

• Equilibrium – the state in which there is no change in a body’s motion.

• The net external force acting on a body in equilibrium must be equal to zero– An object stays at rest– An object continues to move at a constant

velocity

• Just because something is moving, DOES NOT mean a force is acting on it!

Example Problem

• An agriculture student is designing a support to keep a tree upright. Two wires have been attached to the tree at right angles to each other. One exerts a force of 30 N north on the tree; the other wire exerts a 40 N force due west. Determine the placement and force in the wire for a third wire so that the tree will have zero net force from the 3 wires.

30 N

40 N

50 N θ

50 N

θ

a2 + b2 = c2

402 + 302+ = c2 c = 50 N

Tan-1 (40/30) =

53.1° W of N

To balance the first 2 wires, we need a 3rd wire that is of equal magnitude and opposite direction of the resultant.

Therefore, the 3rd wire must be 50 N at 53.1° E of S

Newton’s 2nd Law

• The acceleration of an object is directly proportional to the net external force acting on the object and inversely proportional to the object’s mass

ΣF = ma

Net external force = mass x acceleration

How will the acceleration of the elephant and feather compare?

This is the situation WITH air resistance.

NEGLECTING air resistance, how will the acceleration of the elephant and feather compare?

Both have the same acceleration – Gravity!

Although the mass of the elephant is greater, so is the force. The two

proportions end up equaling the same magnitude for acceleration.

 

Newton’s 3rd Law

• For every action there is an equal and opposite reaction

• If two objects interact, the magnitude of the force exerted on object 1 by object 2 is equal to the magnitude of the force simultaneously exerted on object 2 by object 1, and these two forces are opposite in direction.

Action-Reaction Pairs

• A pair of simultaneously equal but opposite forces resulting from the interaction of two objects

• The most important thing to remember is that each force acts on a different object!

• Ex. Hammer on nail,nail on hammer

Newton’s Cradle

Ch 4.4 – Everyday Forces

• Weight – the magnitude of the force of gravity on an object

Weight = mass x gravity

Weight, unlike mass, is dependent on the force of gravity

We usually designate weight on a free body diagram as Fg or mg

Everyday forces, cont.

• Normal Force – a force exerted by one object on another in a direction perpendicular to the surface of contact

• The normal force is always perpendicular to the surface of contact, but not always opposite to the force of gravity

Fn

Fg Fg

Fn

How we abbreviate forces

• Forces are all represented by a capital letter F– The subscript of each F tells us where the

force comes from• Fg – Force of gravity

• Fa – Applied Force

• Fn – Normal Force

• Ff (sometimes also seen as Fk and Fs) – Force of friction

Fun with Friction

• Friction is a force that resists the motion between two objects in contact with one another.

Causes of Friction

• The electrons of the two surfaces in contact with one another

• The surfaces themselves

• Deformation of the surface

Why Friction…Why?!

• Is friction bad? Yes!

• If friction good? Yes!

Forms of Friction

• Sliding Friction

• Rolling Friction

• Friction caused by passing through fluids