Forces and Motion

Forces Affect Motion A force is a push or a pull

that changes motion. Forces transfer energy to

an object. The force of gravity

causes objects to have weight.

The SI unit used to measure force is the newton. (N).

One newton is the force needed to change the acceleration of a 1 kg mass by 1 m/sec2.

The force of friction acts to slow the movement of objects.

Gravity and Friction are Forces

• The force of gravity on an object is called weight.

• Mass is the amount of matter and Newton measures the Force of gravity

• Mass does not ever change but weight does.

• Friction acts to slow down objects.

• Friction occurs when an object resists change.

• There are 3 kinds of friction sliding, rolling and fluid.

Types of FrictionSliding friction - when two objects slide over one another; determined by object’s weight and type of surface

Rolling friction -when an object rolls over a surface; a smaller force than sliding friction

Fluid friction - when an object moves through a fluid; a smaller force than rolling friction

Balanced and Unbalanced Forces According to Newton’s

3rd Law, forces always act in pairs.

Forces may be balanced (equal in amount but opposite in direction).

Forces can also be unbalanced (opposite in direction but unequal in amount).

Balanced forces do not cause a change in motion, but unbalanced forces do!

Net Forces

• Net Force is the sum of all the forces actin on an object

• If the forces act in the same direction they are added together to find the net force.

• If the forces are in opposite directions they are subtracted to find the net force.

Universal Forces Gravitational Force - is

the weakest of the universal forces. It depends on the mass of the objects and the distance between them.

Nuclear Forces -The strong force holds the nucleus of an atom together by binding the protons and neutrons. The weak force holds the particles together that make protons and neutrons.

Electromagnetic Forces -may attract or repel. These are much stronger than gravity.

Measuring Motion

Speed Velocity and Acceleration

• Forces cause an object’s motion to change, we need to be able to measure it.

• There are 3 important measurements that we must understand in order to understand how motion affects motion.

• These measurements are: distance, time, and speed or velocity

Speed

• Speed measures how fast something goes.

• Measure in m/s or km/hr

• The unit is always a Distance unit over a unit for time.

• Speed = distance/time

• D = distance• S = speed• T =time

Velocity

• Measures speed and direction

• Velocity = • distance /time but it

must show direction.• Has the same units as

speed

• D = distance• V = velocity• T = time

Acceleration

• Acceleration measures how fast you change velocity.

• Occurs when you speed up,slows down, or changes direction.

• When you speed up acceleration is a positive number.

• When you slow down it is a negative number.

• The unit for acceleration is m/s2

• ∆v = change in velocity

• A= acceleration• T = time

Falling Objects If no other forces act on

an object, gravity causes it to accelerate towards the center of the earth at 9.8 m/sec2.

At terminal velocity, the forces of friction and gravity are equal and opposite on an object. Its speed remains constant.

Projectile Motion

• A projectile is anything that is thrown or shot in a horizontal direction.

Projectile motion is the curved path followed by some moving objects.

Projectile motion is caused by the two forces of a horizontal push and downward gravity.

Newton’s Laws of Motion 1st Law: An object at rest

stays at rest, and an object in motion stays in motion, unless acted on by an outside, unbalanced force.

2nd Law: Force = (mass)(acceleration)

3rd Law: For every force or action, there is an equal and opposite force or action.

Momentum and Circular Motion Momentum is a way to

measure “inertia of motion.” It tells how hard it is to stop or start an object’s movement.

An object moving in a circle has centripetal acceleration.

Centripetal force is the force needed to make an object move in a circle.

Momentum = (mass)(velocity)