Force and Motion

What is a force? A force is a push or a pull

Types of Forces Electrical forces (balloon

with hair)

Magnetic forces (magnet pulling a paperclip)

Force of Gravity (apple falling from a tree and hits the ground due to gravity)

Net force (the total amount of push or pull acting on an object) measured in Newtons (N)

Remember IFSMU I: Identify Variables F: Formula S: Substitute variables (#’s) into formula M: Do the math U: Add the correct units. IF SMU (If you) do your math

problems this way you will get points IF SMU don’t … you won’t

Example of net force

You and your brother are working together to move a box. You are both moving in the same direction. You are pulling at 30 N to the left and your brother is pushing at 20 N to the left.

What is the net force?

Net Force answer I: Force=30 N left, 20 N left F: Add S: Net Force= 30 N left + 20 N left M: 50 U: 50 N to the left

Net Force Example

You and your brother are fighting over the TV remote. You are pulling at 10 N to the left and he is pulling at 8 N to the right.

Who wins?

TV Remote Net Force I: Force=10 N left, 8 N right F: Subtract S: 10 N left – 8 N right M: 2 U: 2 N left You Win!

Main Points

A force is a push or a pull A force always acts in a certain

direction Forces in nature include: electrical,

magnetic, and gravity. Weight is a force.

Newton’s Law of Gravity

Gravity Force of attraction between all

objects in the universe An object with a LARGER MASS will

have a LARGER FORCE of GRAVITY. So…a smaller mass will have a

smaller force of gravity

Main points on Gravity Isaac Newton discovered the law of gravity

All objects in the universe move toward each other because of the force of gravity between them.

The amount of gravitational force between two objects depends on their mass.

The force of gravity between two objects decreases as the distance between them increases.

Friction, Speed, and Velocity What is friction? Friction is a force that opposes the

motion of an object

Types of Friction

Sliding friction (book moving across a desk)

Air resistance (object falling in air is pushed by air resistance)

Rolling friction (wheel of bicycle and the road)

Is friction helpful or not helpful?

Depends! Helpful because on earth friction between

the road and bicycle wheel, your feet and the floor, cars, and trains all keep in motion from friction…without friction you would not be able to move the bike, car, train, or walk on the road.

Not helpful metal parts touching in a machine can cause too much friction = heat and energy and parts will break or overheat.

Main points of friction

To stop moving object, a force must act in the opposite direction

Friction is a force that opposes the motion of an object

There are different types of friction Friction makes motion possible Friction can sometimes be a

problem

Speed What is speed? Speed is the distance traveled per

unit of time Speed has two parts: distance and

time Speed= distance/time Motion is a change in position

Types of speed Instantaneous

speed is the speed at any given instance or point in time

Average speed =total distance/total

time

Velocity What is velocity?

Velocity describes the speed (distance and time) and the direction of a moving object

Moving from place to place involves distance, time, and direction so it is measured in velocity

Main points of speed and velocity Motion is a change in position

Average speed=total distance/total time for the trip

Speed at any instant is called instantaneous speed

Velocity describes the speed (distance and time) and direction of a moving object

Newton’s Laws of Motion Newton’s three

laws of motion can be used to explain the movement of all objects in the universe.

What is Newton’s First Law of Motion? Inertia: tendency of an object to

stay at rest or in motion

An object will stay at rest unless acted on by an unbalanced force. An object in motion will stay in motion at the same speed and in the same direction unless acted on by an unbalanced force

Newton’s First Law of Motion

If your car is traveling at 45 km/h then you are traveling at 45 km/h

If your car suddenly stops because of a wall…you are still traveling at 45 km/h until you are stopped by another force (your seatbelt or possibly the ground).

What is Newton’s Second Law of Motion?

Force= mass X AccelerationThe force (push or pull) of an object

is equal to its mass (amount of matter) times its acceleration (change in object’s velocity over time).

Formulas:F=ma, a=f/m, m= f/a

Newton’s 2nd Law of Motion If you apply the same force on a

small stone as you do a boulder…the small stone will accelerate much faster than the boulder.

Units to Remember

Force(F) measured in Newtons (N)

Mass (m) measured in kilograms (kg)

Acceleration (a) measured in meters/seconds2 (m/s2)

Math Problem: f=ma, a=f/m, m=f/a

A marble has a mass of 0.05 kg and is

dropped from a building at a force of

0.49 N. What is the acceleration of the

marble? (units for a= m/s2)

Math Problem: f=ma, a=f/m, m=f/a

I: F=.49N, m= 0.05 kg, a= ? F: a=f/m S: a=.49 N/ 0.05 kg M: a= 9.8 U: a= 9.8 m/s2

Math Problem: f=ma, a=f/m, m=f/a

A bowling ball has a mass of 7.0 kg and

is dropped from a building at a force of

68.6 N. What is the acceleration of the

bowling ball?

Math Problem: f=ma, a=f/m, m=f/a

I: F=68.6 N, m= 7.0 kg, a=? F: A=f/m S: A=68.6 N/7.0 kg M: A= 9.8 U: A= 9.8 m/s2

Why is the acceleration the same for the bowling ball and marble?

Because falling objects don’t accelerate through their whole fall. Eventually, the force of air resistance pushing up against the object equals the force of gravity pulling down on the object.

At this point the final or terminal speed is reached.

So a heavy object (bowling ball) and a light object (marble) dropped from the same height will hit the ground at the same time.

What is Newton’s Third Law of Motion?

Equal and Opposite For every action, there is an equal

but opposite reaction Newton’s 3rd law includes two parts:

the action force and the reaction force

All forces act in pairs

Example

Hitting a baseball: bat exerts force on ball (action) and ball exerts equal and opposite force on bat (reaction)

Calvin and Hobbs Interpretation