Notes 10-2 (Part I)

Friction & Gravity

Friction

• A force 2 surfaces exert on each other when they rub against each other

• Acts as an unbalanced force to slow motion down

• The strength of the force of friction depends on the types of surfaces involved and how hard the surfaces push together

• 4 types of friction

1) Static Friction

• Acts on objects that are not moving• You must exert a force greater than the force

of static friction to make the object move

2) Sliding Friction

• Occurs when two solid surfaces slide over each other

3) Rolling Friction

• Occurs when an object rolls across a surface• Rolling friction is less than sliding friction for

similar surfaces

4) Fluid Friction

• Occurs when a solid object moves through a fluid, such as air, water, oil, etc.

• Fluid friction is usually less than sliding friction

Gravity

• Force that pulls towards the center of the earth

• Newton realized that gravity acts everywhere, not just on earth

• Called the Law of Universal Gravitation• Any 2 objects in the universe attract each

other

Gravity Between Objects• The force of gravity between objects increases

with greater mass and decreases with greater distance.

Mass and Weight• Mass is how much matter is in an object• The gravitational force exerted on a person or

object at the surface of a planet is known as weight.

• Weight = Mass x Acceleration due to gravity

Acceleration due to gravity at Earth’s surface = 9.8 m/s2

Free Fall

• If the only force acting on the object is gravity, it is said to be in free fall

• An object in free fall is accelerating because of the force of gravity at a rate of 9.8 m/s/s

• This means that every second an object is free falling, it increases its velocity 9.8 m/s

• Is this affected by mass? If dropped from the same height at the same time, will a heavier object fall faster?

Free Fall

• No! If there are no other forces to consider, then the objects will fall at the same rate, regardless of mass

Air resistance

• A type of fluid friction that acts on objects falling through the air

• An upward force acting on a falling object

Air Resistance

• Falling objects with a greater surface area experience more air resistance.

Projectile Motion

• Occurs when an object is thrown horizantally• Gravity will act on the object in the same way

as it does when an object is dropped vertically

Who can remember Newton’s First Law of Motion?

• An object will remain at rest or moving at a constant velocity unless it is acted upon by an unbalanced force.

Newton’s 2nd Law

• Acceleration depends on the net force acting on the object and on the objects mass

• Acceleration = Net ForceMass

Or Net Force = Mass x Acceleration

Calculating Force• A speedboat pulls a 55-kg water-skier. The skier to accelerates at 2.0

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

• Read and Understand

• What information have you been given?• Mass of the water-skier (m) = 55 kg• Acceleration of the water-skier (a) = 2.0 m/s2

Calculating Force• A speedboat pulls a 55-kg water-skier. The skier accelerates at 2.0 m/s2.

Calculate the net force that causes this acceleration.• Plan and Solve• What quantity are you trying to calculate?• The net force (Fnet) = __• What formula contains the given quantities and the unknown quantity?• a = Fnet/m or Fnet = m x a• Perform the calculation.• Fnet = m x a = 55 kg x 2.0 m/s2

• F = 110 kg • m/s2

• F = 110 N

Calculating Force• A speedboat pulls a 55-kg water-skier. The skier accelerates at 2.0 m/s2.

Calculate the net force that causes this acceleration.

• Look Back and Check

• Does your answer make sense?• A net force of 110 N is required. This does not include the force that

overcomes friction.

Calculating Force• Practice Problem

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

• 3,000 N (1,000 kg x 3 m/s2)

Calculating Force• Practice Problem

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

• 350 N (25 kg x 14 m/s2)