What are Forces?What are Forces?

A force is a push or a pull on an A force is a push or a pull on an object.object.

Unbalanced ForcesUnbalanced Forces

In any situation , the overall force of an In any situation , the overall force of an object after all the forces are added object after all the forces are added together is called the together is called the Net Force.Net Force.

5 N5 N 20 N 20 NAn unbalanced force acting on an object An unbalanced force acting on an object

will change the object’s motion.will change the object’s motion.

Balanced ForcesBalanced Forces

Balanced forces acting on an object will not Balanced forces acting on an object will not change the object’s motion.change the object’s motion.

20 N20 N 20 N 20 N

When you add equal forces exerted in opposite When you add equal forces exerted in opposite directions, the net force is zero.directions, the net force is zero.

Newton’s First Law of MotionNewton’s First Law of Motion

An object at rest will remain at rest. And an An object at rest will remain at rest. And an object that is moving at a constant speed object that is moving at a constant speed will continue moving at constant speed will continue moving at constant speed unless acted upon by an unbalanced unless acted upon by an unbalanced force.force.

This resistance to change is called This resistance to change is called InertiaInertia..

What is the difference between What is the difference between mass and weight?mass and weight?

Mass is the amount of matter in an object.Mass is the amount of matter in an object. This This does not change no matter where you are even does not change no matter where you are even if it is on another planet.if it is on another planet.

Weight is caused by gravity.Weight is caused by gravity. The greater the The greater the gravitational force the greater your weight gravitational force the greater your weight would be.would be.

If you were able to go to the planet, Jupiter, If you were able to go to the planet, Jupiter, your weight would be 27 times greater, because your weight would be 27 times greater, because the mass of Jupiter is 27 times greater. The the mass of Jupiter is 27 times greater. The larger the Mass of an object the greater the larger the Mass of an object the greater the gravitational pull.gravitational pull.

What is weight?What is weight?

When an astronaut moves away from the earth When an astronaut moves away from the earth they “lose” weight. Weight is caused by the they “lose” weight. Weight is caused by the force of attraction between the earth and force of attraction between the earth and objects on its surface. The greater the distance objects on its surface. The greater the distance an object has from the earth, the less the force an object has from the earth, the less the force of attraction which would exist.of attraction which would exist.

What Causes What Causes “Weightlessness”?“Weightlessness”?

Even in the space shuttle, there is a force Even in the space shuttle, there is a force of attraction exerted by the earth on the of attraction exerted by the earth on the shuttle and its contents. shuttle and its contents.

When the space shuttle temporarily “falls” When the space shuttle temporarily “falls” toward the earth, the contents of the toward the earth, the contents of the space shuttle appear to be weightless, space shuttle appear to be weightless, but in fact they are falling with the space but in fact they are falling with the space shuttle. This is called “freefall”.shuttle. This is called “freefall”.

Force , Mass, and Force , Mass, and AccelerationAcceleration

Force = Mass x AccelerationForce = Mass x Acceleration

Newton’s Second Law of Motion

• The net force of an object is equal to the product of its acceleration and its mass

• Force = mass X acceleration• F = ma• Force and acceleration are directly proportional.

The greater the force, the greater the acceleration.

• Mass and acceleration are inversely related. The greater the mass of an object the less the acceleration if the same force is applied.

Newtons

• One Newton equals the force required to accelerate 1 kilogram of mass at 1 meter per second per second.

1N = 1 kg x 1 m/s²

• Sometimes you may want to write the relationship among acceleration, force, and mass in different form.

Acceleration = Force Mass

Example Problems• Force = Mass x Acceleration

–Force = 52 kg x 2m/s²

–104 kg x m/s² = 104 kg•m/s²

–104 N

• What is the force on a 1,000-kg elevator accelerating at 2 m/s²?

• How much force is needed to accelerate a 55-kg cart at 15 m/s²?

Friction and Gravity

What is Friction?• The force that one surface exerts on

another when the two rub against each other is called Friction.

• Friction is a force which opposes motion.

From Glencoe Physical Science

A close up look at two surfaces under a

microscope.

What causes friction?• There are two factors which affect

friction between two surfaces:– Kind of surfaces in contact (rough or

smooth)– Amount of force pressing the surfaces

together.

The rougher the surface and the stronger the force between the surfaces, the greater the amount of friction.

The Three Types of Friction

• Sliding Friction

• Rolling Friction

• Fluid Friction

There is also something called Static Friction

From Glencoe Physical Science

• If you were trying to move a box like pictured below and the box did not move, this would be static friction.

• Static friction is friction between two surfaces which are not moving past each other.

What is Sliding Friction?

• Sliding friction occurs when a force is great enough to overcome the static friction.

Glencoe Physical Science

•The force required to overcome sliding friction is greater than any other type of friction.

Rolling Friction

• Rolling friction is the friction which enables wheels to turn and objects to move. If there were no friction, turning wheels would not enable an object to move.

• The force needed to overcome rolling friction is much less than the force needed to overcome sliding friction.

Fluid Friction

• The friction that occurs when an object moves through a fluid is called fluid friction.

• The force needed to overcome fluid friction is usually less than that needed to overcome sliding friction.

Do we Need Friction?

• Without friction an object would continue to move at a constant speed forever.

• What are two ways to reduce friction?• Ball bearings • Oil or other lubricants.

Gravity

• What is Gravity?–Gravity is the force that pulls

objects toward the center of Earth

• Gravity is a force that acts on us at all times, this force is caused by the Earth and it is what keeps us from floating away.

How Gravity Affects Us

• When the only force acting on a falling object is gravity, the object is said to be in free fall.

– An object in free fall accelerates as it falls because gravity is an unbalanced force pushing down on it.

What is Gravitational Acceleration?

• When objects fall they accelerate toward the ground.

• Using Newton’s 2nd Law we can see this relationship: F = ma

• The force caused by gravity acts upon a certain mass to cause it to accelerate toward the ground at a constant rate.

Acceleration Due to Gravity

• Near the surface of the Earth, the acceleration due to the force of gravity is 9.8 m/s².

• This means that for every second an object is falling , its accelerating at 9.8 m/s².

• Example

–1 Second 9.8 m/s²–2 Seconds 19.6 m/s²–3 Seconds 29.4m/s²

What is a Projectile?

When an object is thrown into the air, two forces and two motions affect the movement of the object.

The object may be thrown horizontally with a certain force, but the vertical force of gravity combines to affect the motion.

Slow projectile - shoot a monkey

Fast projectile - shoot a monkey

Projectile Motion

• An object that is thrown is called a Projectile.

• An object that is simply dropped and one that is thrown horizontally are both in free fall.

• Because the horizontal motion of the thrown object does not interfere with its free fall. Both objects will hit the ground at exactly the same time.

Air Resistance

• Objects falling through air experience a type of Fluid Friction called air resistance.

• Air resistance is an upward force that opposes the falling objects motion.

• Air resistance increases with velocity. So the faster the object falls the greater the air resistance.

• Also the larger the object the greater the air resistance.

Air Resistance

Eventually the air resistance equals the force of gravity and the forces become balanced, therefore the object stops accelerating. This is called terminal velocity.

Calculating Weight

• Weight = Mass X Acceleration due to Gravity

or

Mass X 9.8m/s²

• So a 50- Kilogram person weighs 50 kg x 9.8m/s².

50kg x 9.8m/s² = 490 N

• The Law of Universal Gravitation states that the force of gravity acts between all objects in the universe.

Universal Gravitation

Action-Reaction

What is Newton’s 3rd Law of Motion

• For every action there is an equal but opposite reaction.

• When 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.

Two objects in contact with one another and moving must use the action-reaction law.

Rocket propulsion involves the action-reaction law. The explosion from one end of the rocket causes a force in that direction. This causes an unbalanced force in the opposite direction causing the rocket to accelerate. Video Clip!

What is momentum?

• The momentum of an object is equal to the product of its mass and its velocity. The unit for momentum is kg • m/s

• What is the momentum of a 3-kg sledgehammer swung at 1.5 m/s?

3 kg x 1.5 m/s = 4.5 kg • m/s

Let’s try the practice problems on pg. 67.

Sample Problem

Law of Conservation of Momentum

• The total momentum of any group of objects remains the same unless outside forces act on the objects.

• In physical science the word conservation refers to the conditions before and after an event.

What is the Law of Conservation of Momentum?

• When object collide the momentum of one object is transferred to the other object.

• View this video to see a demonstration of conservation of momentum.

• Notice that the soccer ball has greater momentum since it has a larger mass. Its momentum is transferred to the smaller ball which has a smaller mass and will therefore have a larger velocity. Try this with a soccer ball and a tennis ball!

Orbiting Satellites

• A satellite is an object that travels around another object.

What is centripetal force?

• “Centripetal” means to move toward the center.

• Centripetal acceleration is the acceleration of an object toward the center of a curve or circular path.

• Centripetal force is a force toward the center of a circle which holds the moving object in its circular path.

Newton’s Thoughts on Satellites

What if you threw a projectile from the top of a mountain as fast as you wanted?

– The faster you threw it the farther it would land.– At a certain speed it would match the curve of

the earth, because of this…

Satellites in orbit around Earth continually fall toward Earth, but because Earth is curved they travel around it.

Facts about Satellites

• The speed in which an object must be thrown in order to orbit Earth is 8,100 m/s!

• There are two types of orbits..– Low orbit, in which it takes only 90 minutes to

orbit Earth. Ex. Space shuttle– Higher orbits, where it takes approximately 24

hours to orbit Earth. Approx. 40,000 km from Earths surface.