1. M. OSTMANN 7th Grade Science Kate Bond Middle School
FORCES CHAPTER 19
M. OSTMANN
7th Grade Science
Kate Bond Middle School

2. WHAT IS A FORCE? FORCES What causes these cyclists to move forward?
What prevents these cyclists from moving as fast as a race car?
WIND
GRAVITY
FORCES

3. FORCE…
IS A PUSH OR PULL THAT ACTS ON AN OBJECT.

4. Measuring Force… is easy.
You can measure weight (force)…
with Spring scales like these.

5. UNITS OF FORCE: NEWTON 1N = 1kg * m/s2
Sir Isaac Newton is the eponymous scientist who discovered how force, mass and acceleration are related.
1 Newton is the FORCE needed to accelerate a 1kg object 1 meter per second every second OR
1N = 1kg * m/s2

6. HOW DO YOU REPRESENT FORCES?
DRAW ARROWS!
Direction of arrow = direction of force
Length of arrow = strength of force
QUESTIONS?
How many forces are represented here?
Which force has the largest magnitude? The smallest?
Is this wood block moving or at rest? How do you know?

7. NET FORCE ADD FORCES IN THE SAME DIRECTION
SUBTRACT FORCES IN THE OPPOSITE DIRECTION
NET FORCE =
PUSHING FORCE OF SURFER = 800 N
SAME DIRECTION
= Wave force + Surfer force
= 1,000 N N = 1,800N
PUSHING FORCE OF WAVE = 1,000 N
OPPOSING DIRCETION
= Weight force – Water force
= 757 N – 757 N = 0 N
UPWARD FORCE OF WATER = 757 N
WEIGHT = 757N
NET FORCE = 1,800 N

8. BALANCED FORCES Skater does not move!!!
NET FORCE = Gravitational force – upward force from the ground = N -680 N = 0 N

9. The Skater is moving in a forward direction.
UNBALANCED FORCE
The Skater is moving in a forward direction.
Net Force = 120N skater N air resistance = 20 N forward force

10. YOU DECIDE… IS THE SKATER MOVING? NO! BALANCED OR UNBALANCED?

11. FRICTION
ALL MOVING OBJECTS ARE SUBJECT TO FRICTION
FORCE THAT OPPOSES THE MOTION OF MOVING OBJECTS.
WITHOUT FRICTION, ALL SURFACES WOULD BE MORE SLIPPERY THAN A SHEET OF ICE
FRICTION ACTS AT THE SURFACE WHERE TWO OBJECTS COME INTO CONTACT.
THERE ARE FOUR MAIN TYPES OF FRICTION:
STATIC
SLIDING
ROLLING
FLUID
Midtown Memphis, 2010

12. STATIC FRICTION Friction force that prevents object from moving.
Acts on objects that are not moving.
Static friction force acts in the direction opposite to applied force.

13. SLIDING FRICTION Force that slows the motion of a sliding object.
Acts on objects that are moving in a sliding motion.
A weaker force than static friction.
Less force is needed to keep an object moving than it takes to start it moving.

14. ROLLING FRICTION
Force that slows the rolling motion of a round object.
100 to 1,000 times less strong than the force of static or sliding motion
This is why round objects are easier to move
Invention of wheel made it easier to move heavy things
Human advancement rooted in principle of frictional force

15. FLUID FRICTION
Force that slows the motion of an object through a fluid
Fluids include water, air, and other viscous fluids such as molasses, mashed potatoes, and honey.
You can feel fluid friction when you stir thick mashed potatoes this batch.
Fluid friction increases as the object’s speed increases.
The faster you stir, the stronger the fluid friction is, and thus, the harder it is to stir.

16. AIR RESISTANCE
IS A TYPE OF FLUID FRICTION THAT DECREASES THE SPEED OF A FALLING OBJECT.

17. GRAVITY THE EARTH HAS THE STRONGEST GRAVITATIONAL FORCE.
An attractive force that pulls objects together.
THE EARTH HAS THE STRONGEST GRAVITATIONAL FORCE.
THIS GOALIE IS PULLED TOWARS THE GROUND DUE TO THE EARTH’S GRAVITATIONAL FORCE.
Acceleration due to gravity = 9.8 m/s2
Veritasium Video 1
Veristasium Video 2

18. FALLING OBJECTS GRAVITY : DOWNWARD FORCE
WHAT FORCES AFFECT THE MOTION OF THESE SKYDIVERS?
AIR RESISTANCE: OPPOSING FORCE
WHICH FORCE IS GREATER? IN WHAT DIRECTION ARE THE SKYDIVERS MOVING?

19. PROJECTILE MOTION G ALL THROWN OBJECTS WILL FOLLOW A CURVED PATH
ONCE THROWN, THE OBJECT FEELS TWO FORCES:
1) GRAVITY
2) AIR RESISTANCE G G
AIR RESISTANCE G G G G
PUSHING FORCE FROM THE SHOOTER G G G

20. PROJECTILE MOTION: A CASE STUDY
THE RED BALL WAS DROPPED
THE YELLOW BALL WAS THROWN G G
WHICH BALL HITS THE GROUND FIRST?
JA JA. TRICK QUESTION.
BOTH BALLS HIT THE GROUND AT THE SAME TIME
WHY???

21. EXTRA CREDIT OPPORTUNITY!!!!!
WHO WROTE THIS SONG?