Chapter 10 – Forces

PowerPoint notes

Examples of Forces

A soccer player kicks the ball into the goal.

A shotput hits the ground.

An archer’s arrow hits the bullseye.

A paintball capsule hits its target.

Why does each object move as it does? What causes an object to start moving, stop moving, or change direction? The answer is: In each of these activities, a FORCE is applied to the object.

What is a Force?

You say the first object is exerting a force on the second object. In science, the meaning of force is simple and specific. Force – a pull or a push

Some things do the pulling.

Some things are pulled upon.

Some things are pushed upon.

Some things do the pushing.

Like velocity and acceleration, forces are described by how strong they are AND which direction they are applied. If you pull a door, you exert a force in a different direction than if you push it.

Unbalanced Forces

Suppose you push a heavy object by yourself. You exert a force on it.

Suppose you have a friend help you push a heavy object. The total force is the sum of your force and your friend’s force. When two forces act in the same direction, they add together.

These arrows show how forces can combine
> + > = >  
< + > = 0
< + > = <
NET FORCE  - When forces are added together or subtracted from each other. Ex: Unbalanced forces can cause an object to move. Ex: Unbalanced forces can change an object’s motion.  UNBALANCED FORCES – When opposing forces are unequal  

Balanced Forces

Different forces being exerted on an object do not always cause it to move.  Neither one of these two individuals seem to be winning this tug of war. That is because they are both exerting the same force in opposite directions.


Balanced Forces - equal forces acting on an object in opposite directions.  Example: When you add equal and opposite forces, the net force is zero.  < + > = 0

Newton’s First Law of Motion

The Greeks noticed that objects had natural resting places. Example: A rock falls to the ground. Example: A puff of smoke rises into the air.

Early in the 1600’s, Galileo Galilei, an Italian astronomer, suggested that once objects began to move, they resisted changes in their motion. This resistance is called Inertia. Inertia - The tendency for an object to resist changes in motion.

The English mathematician Sir Isaac Newton expanded on Galileo’s ideas with his three laws of motion. Newton’s first law is called the Law of Inertia. It reads as follows: Newton’s 1st Law:“An object at rest will remain at rest and an object moving at a constant velocity will continue moving at constant velocity unless acted upon by an unbalanced force.”

Inertia is obvious in many situations. A good example is that of crash test dummies. Like a human in a quickly stopping vehicle, crash test dummies continue to move forward even after the car has abruptly stopped


Which is easier to move, a bag of packing peanuts or a bag of coins? Obviously, the coins will be harder to move. Even if they both occupy the same volume, the coins have more mass.

The amount of inertia in an object depends on its mass. Presidential limosine  = 6,800 kg Batmobile = 2,300 kg Which would be harder for the towing company to tow? The presidential limousine – because it has much more mass than the Batmobile.

Newton’s Second Law of Motion

Newton’s 2nd Law: “The net force on an object is equal to the product of its acceleration and its mass.” This one is a bit harder to understand. Perhaps if we use a child’s red wagon to demonstrate, it will be more understandable.

In this picture, you can see that the mass of the man and the force he exerts on the wagon makes it easy to accelerate the wagon.

In this picture, you can see that the mass of the wagon is far more than these men, which makes it much more difficult to accelerate the wagon.

You can increase the acceleration of the wagon by decreasing the mass or inertia of the wagon or increasing the mass or inertia of the force.

You can decrease the acceleration of the wagon by increasing the mass or inertia of the wagon or decreasing the mass or inertia of the force.

Friction and Gravity

What if you push an eraser across your desk and then stop? It stops. What if you lift an eraser above your desk and then let go? It falls.

From Newton’s First Law of Motion, we know that unbalanced forces are acting on the eraser. Gravity makes the eraser fall. Friction makes the eraser stop. Two other forces do indeed act upon the eraser. Gravity and friction affect nearly all motion.


When a fireman slides down a pole, the pole may seem quite smooth, but both his hands and the surface of the pole have irregularities. When surfaces rub against each other, the irregularities of one surface get caught against the irregularities of the other surface.

Friction acts in a direction opposite to the direction of motion. Without friction, an object could keep going forever. Eventually friction will cause an object to come to a stop.

The strength of the force of the friction depends on two factors: The types of surfaces involved  How much pressure is exerted

These speed skaters depend on the low friction level of ice for added speed. These luge competitors also depend on the low friction of ice and snow.

The added friction of these snow tires is highly desirable. The lack of friction on the surface of this Teflon cookware is highly desirable. Whether or not friction is a good thing depends entirely on the situation

Sliding Friction

Sliding Friction - When two surfaces slide over each other Sliding friction can be seen in action in this horse pull. The load that they are pulling is where the sliding friction is occurring.

Rolling Friction

Another is rolling friction, which is probably best demonstrated by ball bearings, roller bearings, and needle bearings.  Rolling friction - when two surfaces roll over each other Rolling friction is easier to overcome than sliding friction.

Fluid Friction

Fluid friction - This occurs when an object moves through a fluid. Example: These swimmers have to fight fluid friction at every arm stroke and every kick of their paws.

Fluid friction is less than sliding friction. Example: Moving parts in machinery are bathed in fluids such as oil and grease because those fluids reduce other types of friction.


Sir Isaac Newton reasoned that there was a force that pulled objects down toward the earth. He called this force gravity. Gravity - The force that pulls objects down toward the earth.

When the only force acting on an object is gravity, the object is in freefall. An object in freefall accelerates as it falls. This is because the force of gravity is an unbalanced force.

If an object is dropped, it accelerates toward the earth at a constant rate. 9.8 m/s2 Acceleration will continue as long as it falls. However, due to air friction, most objects will burn up at some point.

What happens if you throw a ball horizontally and drop another one straight downward? Believe it or not, both balls will reach the ground at the same time! Example: This is called projectile motion.

Around the year 1590 Galileo Galilei wanted to know if different sized objects would fall at different speeds. He went to the top of the Leaning Tower of Pisa with two different sized cannon balls - a small one and a large one.

He dropped both cannon balls at the same time. He learned that both cannon balls hit the ground at the same time. FYI: This may or may not be a true story.

Objects are supposed to fall at the same rate, but this doesn’t always happen. Objects falling toward the earth are slowed down by air resistance. Air resistance is a fluid type of friction. Example: Objects with a greater surface area like leaves, feathers, and parachutes fall more slowly than other objects.

The force of gravity on an object at the surface of a planet is called weight. Example: When you step on a bathroom scale, you are measuring the amount that gravity is pulling you down.

Do not confuse weight with mass! Weight  - The measure of gravity on an object. Mass -  A measure of the amount of matter in that object.
Newton realized that the earth was not the only object that exerted a gravitational force. Example: Gravity is everywhere in the universe! Example: It is the force that keeps the moon in orbit around the earth. Example: It is the force that keeps the planets in orbit around the sun.

What Newton discovered is now called the Law of Universal Gravitation. This law states that the force of gravity acts between all objects in the universe.

Since planets of different sizes have differing amounts of gravitational pull, your weight would change if you left earth’s gravitational pull. Example: You would weigh 1/5 of what you weigh on earth if you went to the moon.

Newton’s Third Law of Motion

When one object exerts a force on another object, there is a force exerted back onto the original object. These forces that are being exerted are equal and opposite. Newton called them “actions” and “reactions.” Newton’s third Law:“If one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object.”

Have you ever been to a hockey game? If so, you know that it can become quite unruly and result in some scuffles. When one skater pushes against another skater, they both go backwards after the push. If one skater is larger, the smaller one will go back further and faster.

Newton’s third law is in action all the time. Example: The ground pushes back on your feet when you walk. Example: The air pushes against a bird’s wings during flight.


Objects can build up momentum. The momentum of an object is a product of its mass and velocity. Example: momentum = mass X velocity

An object’s momentum is what makes it difficult to stop or divert the object. Small objects can have a large amount of momentum. Example: A bullet has a small mass but great speed. It has a lot of momentum.

Large slow moving objects & small fast moving objects can both be difficult to stop or divert. Large objects can have a large amount of momentum. Example: A barge has a huge mass but very little speed. It has a lot of momentum.

The law of conservation of momentum states that the total momentum of the objects that interact does not change. The total momentum of any group of objects remains the same unless outside forces act on the objects. Example: In other words, when objects collide, momentum is not lost, it is transferred.

Satellites and Rockets

What makes a rocket take off? The exploding gases coming out of the back of the rocket create thrust - the upward force. If thrust is greater than gravity, the downward force, the rocket goes up.

What makes a satellite stay in orbit? When a communications satellite is put in orbit, the earth’s gravity wants to pull it down again. Example: The speed of the satellite wants to keep it moving in a straight line, but the earth’s gravity keeps pulling. Because the earth is a sphere, it forces the satellite to form a circular orbit.


Please write a summary and answer the three following questions: 1. What did you know about Forces before we started this PowerPoint? 2. What was something you learned as a result of this PowerPoint? 3. What about Forces is something you would like to investigate more?





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