Forces and Newton’s Laws of Forces and Newton’s Laws of MotionMotion

ForcesForces

The natural state of an object –its behavior if The natural state of an object –its behavior if free of external influences - is uniform free of external influences - is uniform motion with constant velocity.motion with constant velocity.

““At rest” has no special significance in At rest” has no special significance in Galileo’s view of motion.Galileo’s view of motion.

Influences that lead to deviations from Influences that lead to deviations from uniform motion are called forces.uniform motion are called forces.

Newton’s First Law of MotionNewton’s First Law of Motion

Consider an object with no force acting on it. Consider an object with no force acting on it. If it is at rest, it will remain at rest;If it is at rest, it will remain at rest;

If it is moving, it will continue to move in a If it is moving, it will continue to move in a straight line at constant speed. straight line at constant speed.

Consider the crash test dummy who Consider the crash test dummy who continues in motion after the car has hit a continues in motion after the car has hit a wall. Always wear your seat belt! It’s just wall. Always wear your seat belt! It’s just good physics!good physics!

Properties of forcesProperties of forces

A force is a push or pullA force is a push or pull A force acts on an object. Forces do not act A force acts on an object. Forces do not act

in isolation.in isolation. A force requires an agent. An agent is A force requires an agent. An agent is

something that pushes or pulls. It can be an something that pushes or pulls. It can be an inert objectinert object

A force is a vector with magnitude and A force is a vector with magnitude and direction. The size or strength of the force direction. The size or strength of the force is its magnitude.is its magnitude.

FF

Properties of forces cont.Properties of forces cont.

A force can be either a contact force or a A force can be either a contact force or a long range force. long range force.

Contact forces are forces that act on an Contact forces are forces that act on an object by touching it at a point of contact object by touching it at a point of contact (ex. Bat must hit ball to hit it)(ex. Bat must hit ball to hit it)

Long range forces are forces that act on an Long range forces are forces that act on an object without physical contact (ex. Gravity object without physical contact (ex. Gravity or a magnet)or a magnet)

ForcesForces

In general the object exerts a force back on In general the object exerts a force back on the agent.the agent.

A force is really an interaction between two A force is really an interaction between two objectsobjects

Drawing force vectorsDrawing force vectors

Drawing force vectorsDrawing force vectors FFN is the normal force. Normal means perpendicular.N is the normal force. Normal means perpendicular.

Include what is being acted on, by what. (object and agent)Include what is being acted on, by what. (object and agent)

Fg on the

box by the

earth

FN

More Force DrawingsMore Force Drawings

Forces can represent a push or pull. Either Forces can represent a push or pull. Either way they are depicted the same, acting on a way they are depicted the same, acting on a particleparticle

Combining ForcesCombining Forces

The mathematical sumation is called the The mathematical sumation is called the superposition of forces. The net force is the superposition of forces. The net force is the resultant.resultant.

FFNET NET is not a new force – The original forces is not a new force – The original forces

are replaced by Fare replaced by FNET.NET.

What is the net force?What is the net force?

Net forceNet force

Stop to ThinkStop to Think

Two of the three forces exerted on an object Two of the three forces exerted on an object are shown. The net force points directly to are shown. The net force points directly to the left. Which is the missing third force?the left. Which is the missing third force?

CC

Balanced ForcesBalanced Forces

• Forces are balanced if the object isForces are balanced if the object is• Not moving orNot moving or• At constant velocityAt constant velocity

Unbalanced ForcesUnbalanced Forces

Forces are unbalanced if Forces are unbalanced if – The object starts movingThe object starts moving– The object changes directionThe object changes direction– The object stopsThe object stops

If there is an unbalanced force an object will If there is an unbalanced force an object will accelerateaccelerate

InertiaInertia

Inertia is an objects tendency to resist a Inertia is an objects tendency to resist a change in motionchange in motion

Remember the crash test dummyRemember the crash test dummy