Chapter 10.3-10.5

8th Grade

• Galileo Galilei– Italian Astronomer– Suggested that once an object is in motion, no

force is needed to keep it moving.

• Force – Needed to change the motion of the object.

http://www.3m.co.uk/intl/uk//3mstreetwiseuk/pupils-force.htm

Newton’s 1st Law of Motion• Restates Galileo’s ideas about force and

motion

• 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.

Newton’s 1st Law of Motion• This means that if an object is not moving, it

will not move until a force acts on it.

• 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.

• Gravity and friction are unbalanced forces that often change an object’s motion.

Inertia• An object moving or not moving it resists

any change to its motion.

• Inertia– The tendency of an object to resist a change in

motion

Inertia• Newton’s 1st Law

– Also called the Law of Inertia

• The greater mass of an object, the greater its inertia, and the greater the force needed to change its motion.

Newton’s 1st Law

• http://www.youtube.com/watch?v=4NQbeZ0EXZQ

Newton’s 2nd Law of Motion• Acceleration

– depends on the object’s MASS and the NET FORCE acting on the object.

– Net force / mass– Units = Newtons (N)

Newton’s 2nd Law of Motion

Newton’s 2nd Law of Motion• How do you increase acceleration?

– Change the FORCE!

• If mass is constant, acceleration and force change in the same way.

• Decrease in mass = increase in acceleration

http://www.youtube.com/watch?v=44dMPz_DPu8

http://www.youtube.com/watch?v=WzvhuQ5RWJE

Newton’s 3rd Law of Motion• Whenever one object exerts a force on a

second object, the second object exerts a force of equal strength in the opposite direction on the first object

Newton’s 3rd Law of Motion• “Action-Reaction” forces

• Ex. When a dog leaps it pushes down on the ground. The reaction force of the ground pushes the dog into the air.

• Do action-reaction pairs

Cancel?

• No! They are acting on

Different objects.

Momentum• “quantity of motion”

• It is the characteristic of a moving object related to the mass and velocity of the object

• Equation: mass x velocity

• Units = kg* m/s

Momentum• The more mass an object has the more

momentum it has, so the harder it is to stop the object.

• Ex. A car moving 20 m/s is harder to stop than a baseball moving at 20 m/s because the car has a greater mass and greater momentum.

Momentum• Amount of momentum objects have is

conserved when they collide.

• It can be transferred from one object to another, but none is lost.

• Law of conservation of momentum– The total amount of momentum of objects that

interact does not change in the absence of outside forces.

– Ex. Friction

http://www.youtube.com/watch?v=Xx9kiF00rts

http://www.youtube.com/watch?v=8bTdMmNZm2M

Determining Acceleration Word Problems

• 1. A speedboat pulls a 55-kg water skier. The force causes the skier to accelerate at 2.0 m/s2. Calculate the net force that causes this acceleration.

Determining Acceleration Word Problems

• 2. What is the net force on a 1,000kg object accelerating at 3m/s2?

Determining Acceleration Word Problems

• 3. What net force is needed to accelerate a 25 kg cart at 14 m/s2?

Determining Acceleration Word Problems• 4. Find the force it would take to accelerate

an 800 kg car at a rate of 5 m/s2?

Determining Acceleration Word Problems

• 5. What is the net force acting on a 0.15 kg hockey puck accelerating at a rate of 12 m.s2?

Momentum Word Problems• Which has more momentum: a 3.0- kg

sledgehammer swung at 1.5 m/s or a 4.0-kg sledgehammer swung at 0.9 m/s?

• Momentum = Mass X Velocity

• Smaller : 3.0 kg X 1.5 m/s = 4.5 kg x m/s

• Larger : 4.0 kg X 0.9 m/s = 3.6 kg x m/s

Momentum Word Problems• A golf ball travels at 16 m/s, while a

baseball moves at 7 m/s. The mass of the golf ball is 0.045 kg and the mass of the baseball is 0.14 kg. Which has a greater momentum?

• Golf Ball: 0.045 kg x 16 m/s = 0.72 kg x m/s

• Baseball: 0.14 kg x 7 m/s = 0.98 kg x m/s

• The baseball has greater momentum

Momentum Word Problems• What is the momentum of a bird with a mass

of 0.018 kg flying at 15 m/s?

• 0.018 kg x 15 m/s = 0.27 kg x m/s

Momentum Word Problems• What is the momentum of a 920 kg car

moving at a speed of 25 m/s?

• 920 kg x 25 m/s = 23,000 kg x m/s

Momentum Word Problems• Which has more momentum? A 250 kg

dolphin swimming at 4 m/s or a 350 kg manatee swimming at 2 m/s?

• Dolphin : 250 x 4 = 1,000 kg x m/s

• Manatee: 350 x 2 = 700 kg x m/s

• The dolphin has more momentum!