Forcesand

the Motion They Influence

A variety of forces are always affecting the motion of objects

around you

Contact/Noncontact

Force

Gravity

Friction

A force is a vector:With both size and directionRepresent by arrows

right up left

down

By using symbols, the action of vectors (forces) on objects can be read and interpreted.

The length of the vector will show its size and/or strength.

large smallVector can show its impact on an object.

object

Considering the size and direction of all the vectors (forces) acting on an object allows you to predict changes in the object’s motion.

You give it a try!

The force acting on an object when all the forces are combined is the NET FORCE.

If the net force on an object is zero, the forces acting on the object are BALANCED.

A balanced force has the same effect as no force at all.

A balanced force ceates no motion or change in an object’s position.

Only an UNBALANCED force can change the motion of an object.

If one player pushes with a greater force than the other player, the ball will move in the direction of the stronger force.

• Ancient Greek thought it was necessary to apply a continuous force to keep an object in motion.

• Galileo theorized without friction, a moving object will continue moving even if there is no force acting on it.

Early Schools of Thought

Mid 1600sEnglish scientist, Sir Isaac Newton, studied the effects of forces on objects and formulated three laws of motion that are still being used to help describe and predict the motion of objects.

Newton’s First Law of Motion

• Also known as the Law of Inertia

• Objects at rest remain at rest, objects in motion, remain in motion with the same velocity (speed and direction) unless acted upon by an unbalanced force.

Inertia is the resistance of an object to change the speed or direction of its motion.Being closely related to mass, when you measure the mass of an object, you are also measuring its INERTIA.

Newton’s Second Law of Motion

ACCELERATION of an object increases with more force and decreases with less mass.The direction in which an object accelerates is the same as the direction of the force.The equation Force = mass X acceleration (F = ma) describes the relationship between force, mass, and acceleration.

NOTE: If you know 2 of the 3 factors, you can rotate the equation to calculate acceleration. F = ma a = F/m m = F/a

Newton’s Third Law of Motion

Forces always act in pairsWhen one object exerts a force on another object, the second object exerts an equal and opposite force on the first object. (For every action, there is complete opposite reaction.)These force pairs are also known as Action and Reaction Pairs

A moving object has a property called MOMENTUM.

• It is a measure of mass in motion.

• An object’s momentum is the product of its mass and its velocity.

Momentum is similar to inertia because it depends on an

object’s mass.

Unlike inertia, however, momentum takes into account how fast the object is moving.

When you drop any object it falls to the ground faster and faster due to gravity.

Gravity is the force that objects exert on each other due to their masses.It is a universal force because it acts on any two masses anywhere in the universe.

The Force of Gravity

The strength of the gravitational force between two objects depends on two factors: Mass and Distance

The Mass of the Objects Greater mass results in greater force.

• The more mass two objects have, the more the force of gravity the masses exert on each other.

• If one of the object’s mass is doubled, the force of gravity between the objects is doubled.

The Distance Between the Objects

Greater distance results in smaller force.

As distance between the objects increases, the force of gravity decreases. If the distance is doubled, the force of gravity is ¼ as strong as before

Gravity on EarthGravity acts on both masses equally, even though the effects on both masses may be different.

Acceleration due to Earth’s gravity is called g .

g = 9.8 m/s2 at Earth’s surface

Mass and Weight • Mass is a measure of how much matter an

object contains.• It is a property any object has no matter

where it is located.• A balance is used to measure the mass of an

object.

• Weight is the force of gravity on an object.• It depends on the force of gravity acting on that

object.• A spring scale is used to measure how hard gravity

is pulling on an object.

Gravity keeps objects in orbit.Newton hypothesized gravity pulls objects to the ground also pulls the Moon around the Earth.

You can think of an object orbiting Earth as an object that is falling around Earth rather than falling to the ground.