Ch. 10

Forces

• force: is a push or pull

• Newton: SI unit used to measure force

• Net force: combination of all forces acting on an object

• Unbalanced forces: forces that produce a nonzero net force, which changes an object’s motion

• Balanced forces: equal forces acting on an object in opposite directions

• The Nature of Force

– What is force?

• In science a force is simple defined as a push or pull – Like velocity and acceleration, a force is described by its

strength and by the direction in which it acts.

» IF you push on a door you exert a force in a different direction than if you pushed to the door.

– Unit for force= Newton (N)

– The direction and strength of a force can be shown with an arrow

– Combining forces

• Often more than one force acts on an object

• The net force decides if an object will move and what direction it will move

• Forces in same direction – Add the strengths of the individual forces

• Ex.

• Forces in opposite directions – Similar to adding a positive and negative number

» Net force always acts in same direction as stronger force

» Ex.

• Unbalanced forces – Occurs when there is net force acting

» Can cause the object to: start moving, stop moving, or change direction

» Ex.

• Balanced forces – Occurs when there is not net force acting

» The object does not move

Example: Brody skates forward with a force of 50N. The friction from the ice is 10N.

Gabby comes up from behind and gives Brody an extra push of 12N. Draw a force

diagram and calculate net force.

• Friction: force that two surfaces exert on each other when they rub against each other

• Static friction: friction that acts on objects that are not moving • Sliding friction: occurs when two solid surfaces slide over each other • Rolling friction: when an object rolls across another surface • Fluid friction: occurs when a solid object moves through a fluid • Fluid: a material that easily flows • Gravity: force that pulls objects towards each other • Weight: force of gravity on a person or object at the surface of a planet • Mass: measure of the amount of matter in an object • Free fall: when the only force acting an object is gravity • Air resistance: a type of fluid friction, occurs when objects fall through

the air • Terminal velocity: greatest velocity a falling object reaches • Projectile: an object that is thrown

• Friction and Gravity – The strength of friction depends on two factors: how

hard the surfaces push together and the types of surfaces involved. • Ex. There is more friction if you rub your hands together

forcefully than if you do it lightly • Ex. Skis slide easily over snow, but would have a difficult

time over sand

– Without friction a moving object might not stop until it hits another object

– Friction acts in the direction opposite to motion • Ex.

• Friction and Gravity – The strength of friction depends on two factors: how

hard the surfaces push together and the types of surfaces involved. • Ex. There is more friction if you rub your hands together

forcefully than if you do it lightly • Ex. Skis slide easily over snow, but would have a difficult

time over sand

– Without friction a moving object might not stop until it hits another object

– Friction acts in the direction opposite to motion • Ex.

Label the pictures with the correct type of friction

– Gravity • The law of universal gravitation states that a force of gravity

acts between all objects in the universe – This means that any two objects without exception will be

attracted to each other

» You don’t notice this because the attraction between objects is small compared to the attraction to Earth

• Two factors affect gravity: mass and distance – The more mass an object has, the greater the gravitational force

» The sun’s large mass makes it exert a strong force on the planets, helping to explain why they orbit the sun

– The further apart to objects are, the smaller the force of gravity

» For example as a spacecraft moves away from Earth the gravitational hold lessons

– Weight and Mass

• When you step on the bathroom scale you are determining the gravitational force Earth is exerting on you – Weight varies with the strength of the gravitational force and

mass does not

– On Earth your weight is 450 Newtons, on the moon your weight is 75 Newtons

» Why?

– Gravity and Motion

• In free fall the force of gravity is an unbalanced force, which causes an object to accelerate – On Earth all objects accelerate at a rate of 9.8m/s2

– In the absence of air all objects accelerated at the same rate regardless of their mass

• Although all objects are supposed to fall at the same rate, this is not always the cause (due to air) – Objects experience air resistance as they fall

» Objects with greater surface area will have more air resistance

• Ex. Dropping a crumbled piece of paper and a flat sheet of paper

• Which landed first? Why?

•

» As a falling object speeds up the air resistance increases

• Eventually a falling object will fall fast enough that the upward force of air resistance become equal to the downward force of gravity acting on the object

• At this point the forces acting on it are balanced

• So, there is not acceleration

• The object continues to fall, but at the same speed

• It has reached its terminal velocity

• The force of air resistance equals the weight of the object

» Throwing a ball horizontally is called a projectile

• Gravity acts the same whether an object is dropped or thrown horizontally

•

• Inertia: tendency of an object to resist a change in motion

• Newton’s First and Second Laws – The first law of motion

• Newton’s first law of motion states that an object at rest will remain at rest, and an object moving at a constant velocity will continue moving at a constant velocity, unless it is acted upon by an unbalanced force. – If an object is not moving, it will not move until a force acts on it

» Clothes on your floor will stay there until you pick them up – If an object is already moving, it will continue to move at a

constant velocity until a force acts to change either its speed or direction

» A tennis ball flies through the air once you hit it with a racket. If your friend doesn’t hit it, the forces of gravity and friction will eventually stop it.

– On Earth gravity and friction are unbalanced forces

• Whether an object is moving or not, it resists any change to its motion. – This resistance is called inertia

– Newton’s first law is also called the law of inertia

– An object with a larger mass will have more inertia

» A full aquarium is more difficult to move than an empty one due to inertia

– The second law of motion

• According to Newton’s second law of motion, acceleration depends on the object’s mass and on the net force acting on the object. – Acceleration = Net force / mass

» a = F/m

– Unit for Force = Newton (N)

» 1 Newton = kg-m/s2

• Think of a Newton a the force required to give a 1kg mass an acceleration of 1 m/s2

A speedboat pulls a 55kg water-skier. The force causes the skier to accelerate

at 2.0m/s2. Calculate the net force that causes this acceleration.

• How can you increase the acceleration of a wagon? – Change the force

» ______________ the force will cause the acceleration to increase.

» This is an inverse/direct relationship

– Change the mass

» ____________ the mass will cause the acceleration to increase

» This is an inverse/direct relationship

– Momentum

• Called “quantity of motion” by Newton

• Can be determined by multiplying the object’s mass and velocity – Momentum = mass x velocity or p = mv

– Unit: kg-m/s

• The more momentum an object has, the harder it is to stop – The ______________ mass, the more momentum

» This is an inverse/direct relationship

– The _____________ force, the more momentum

» This is an inverse/direct relationship

– Conservation of momentum

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

– Collisions with two moving objects (Elastic)

» The objects change velocities, one moves faster and the other moves slower

– Collisions with one moving object (Elastic)

» The moving object stops and the other object moves with the same velocity.

• Momentum is transferred to the other object

– Collisions with connected objects (Inelastic )

» They become one object after the collision

• Together the cars move faster than they did separately

• The mass is doubled and the velocity is halved

A 110 kg running back carries the ball south at 8 m/s. A 130 kg linebacker runs to make the tackle going north at 5 m/s. In their collision, the linebacker wraps his arms around the runner and makes it an inelastic collision. How fast are they moving right

after the collision (before they fall to the ground)?

Teresa and Samir are driving bumper cars. They spot each other across the rink and head straight for each other at full speed. Samir and his car (130 kg total) head east at 3.5 m/s while Teresa and her car (110 kg total) head west at 3.0 m/s. They have an elastic collision when they meet and Samir (and car) bounces backwards at 2.4 m/s. What is Teresa’s speed after the collision?

Satellite: any object that orbits another object in space Centripetal force: any force that causes an object to move in a circular path

• Rockets and Satellites

– How do rockets lift off?

• The ____________ law of motion must be use to overcome the force of gravity

• The gas the rocket expels with a downward action force exerts an equal but opposite reaction force on the rocket.

– What is a satellite?

• Any object that orbits another in space – Natural satellites: objects that naturally orbit in space

– Artificial satellites: device that is launched into orbit » They travel around Earth in an almost circular path

• This is due to centripetal force • It pulls the satellite towards the center of the Earth

» Why don’t they fall? • The faster an object is thrown, the further it travels

before it hits the ground • At a certain speed the speed of the path of the satellite

matches the curve of the Earth • Earth’s surface curves away from the satellite at the

same rate it is falling • This causes the satellite to fall around the Earth

» Satellites don’t need fuel because it continues to move due to inertia • Once thrown at 7,900 m/s it will stay in orbit

» Some satellites are put into orbit at a lower altitude and take less time to orbit the Earth

• These are mapping and observation satellites

» Others are put into orbit at higher altitudes and take longer to orbit the Earth

• Communication

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