Force Force: (F) push or pull one object exerts on another.

Presentation on theme: "Force Force: (F) push or pull one object exerts on another."— Presentation transcript:

1

2 Force Force: (F) push or pull one object exerts on another.
Measured in Newtons ( 1 N = 1 kg*m/s2) Forces can cause a change in an object’s motion. More than one force can act on an object at one time. Net Force: the combining of forces when two or more forces act on an object. F = 2 N F = 5 N Net Force = 3 N to the left

3 Forces on an object can be balanced or unbalanced.
Balanced Forces: forces that are equal in size and opposite in direction. Does not cause a change in motion because the net force on the object is 0. F = 4 N F = 4 N Net Force = 0 N

4 Net Force = 6 N to the right
Forces on an object can be balanced or unbalanced. Unbalanced Forces: forces that are not equal in size and opposite in direction. Net force causes the object to move in the direction of the force. F = 2 N F = 3 N F = 7 N Net Force = 6 N to the right

5 Newton’s 1st Law of Motion
Sir Isaac Newton came up with 3 laws in 1687 that describe how forces affect an object’s motion. Newton’s 1st Law: an object will move at a constant velocity until a net force acts on it. An object in motion will stay in motion unless a force acts on it. An object at rest will stay at rest unless a force acts on it. The pins will stay at rest until a force (the bowling ball) acts on them. The bowling ball will stay in motion until a force (the weight of the pins they collide with, and/or friction) acts on them.

6 Newton’s 1st Law of Motion
Newton’s 1st Law is also known as the Law of Inertia Inertia: The tendency of an object to resist a change in motion. The more mass an object has, the more inertia it has. Demo: Place an index card on top of a beaker or cup. Place a penny on top of the index card. Pull the card quickly and the penny will drop straight down into the cup rather than moving with the penny. When you slam on your brakes, your body still flies forward because it wants to stay in motion like it was before you braked. This is why you wear a seatbelt!!

7

8 What kinds of forces change an object’s motion?
Friction: resistance to motion when two objects are in contact with one another. Air resistance: resistance an object feels when travelling through the air. Gravity: the attraction two objects have on one another The force of gravity pulls us towards Earth. Demo before you do this slide: I like to bring in a skateboard and skate around the front of the room. I show them that just standing on the board, I am not in motion. I have to apply a force (kicking off) to change my motion. I don’t push hard so that the board will stop moving on its own. Then I ask them what forces made me stop. That is how we come up with the 3 common forces above!

9 Friction Depends on 3 factors:
The rougher the surface, the greater the friction. Rougher surfaces have greater microwelds (microscopic bumps on surfaces that cause friction) which increases the resistance between the objects. The greater the force pushing the two objects together, the greater the friction. Increases the opposition of the microwelds between the objects. The greater the surface area of the two objects touching, the greater the friction. Increases the contact between the objects.

10 Friction Types of Friction:
Static Friction: force between two surfaces that are NOT moving past each other Sliding Friction: force between two surfaces that are sliding past each other Rolling Friction: force between a rolling object and the surface it rolls on.

11 Air Resistance (drag) “Friction in the air” Determined by: Speed Size
The greater the speed the greater the resistance Size The larger the object the greater the resistance Shape The flatter the object the greater the resistance

12 Law of Universal Gravitation
Any two masses exert an attractive force on each other. Depends on: Mass: the greater the mass, the greater the gravitational attraction Distance: the lower the distance, the greater the gravitational attraction Mass: Explain that this is why you don’t feel a gravitational pull from a pen, because Earth is so much for massive, so your attraction to it is so much greater. Distance: Explain that this is why you don’t feel a gravitational pull from the Sun, because even though it is more massive than Earth, you are much further from it, so your attraction to Earth is much greater.

13 The attraction to Earth that pulls objects in the air downward.
Gravity The attraction to Earth that pulls objects in the air downward. This force causes all falling objects to have an acceleration due to gravity of 9.8 m/s2. Regardless of mass and ignoring air resistance.

14 Terminal Velocity Maximum velocity a falling object will reach
Occurs when the force of gravity and air resistance become balanced Net force = 0 therefore no acceleration Force of gravity pulls you down. Force of air resistance opposes the opposite direction. = 55 m/s or mi/hr in Humans

15

16 Newton’s 2nd Law of Motion
How do motion and force relate? Newton’s 2nd Law: A net force acting on an object causes the object to accelerate in the direction of the net force. The greater the mass, the greater the force needed to accelerate it. The greater the net force, the greater the acceleration. Demo before you do this slide: I like to bring in two tennis balls and a medicine ball or other heavier object. I first throw the medicine ball and tennis ball (or push them) both with the same force, so students can see how the same force applied to different masses affects their motion. I also throw (or push) the two tennis balls at the same time with different forces, to show how when mass is constant but the force changes, motion changes. The ball will accelerate in the direction of the net force the mallet applies. The greater the force applied, the faster the ball will move.

17 Newton’s 2nd Law of Motion
Can be summarized as an equation. F = ma F = Force Measured in N m = mass Measured in kg a = acceleration Measured in m/s2 Example #1: You are pushing a friend on a sled. You push with a force of 40N. Your friend and the sled together have a mass of 80 kg. Ignoring friction, what is the acceleration of your friend on the sled? F = 40 N m = 80 kg a = ? a = F m a = 0.5 m/s2 F = ma a = 40 N 80 kg m m

18 Weight Weight: The force of gravity on an object
Because it is a force, you can always calculate weight using F = ma When finding weight, acceleration due to gravity is always used = 9.8 m/s2 Example #2: Find the weight of a suitcase that has a mass of 42 kg. m = 42 kg a = 9.8 m/s2 F = ? F = ma F = ma F = (42)(9.8) F = N

19 Weight vs. Mass Mass = the amount of matter in an object.
Weight = the force of gravity on an object Meaning it can change based on location! The further from Earth you are, the lower your weight On the moon your weight would be 1/6th what it is now. If you could be on another planet your weight would change based on the gravity of that planet On Jupiter your weight would be over twice as much as it is now.

20

21 Newton’s 3rd Law of Motion
Newton’s 3rd Law: every action has an equal and opposite reaction. When one object exerts a force on a second object, the second one exerts a force on the first that is equal in size and opposite in direction. Ex. When you jump on a trampoline, the trampoline exerts the same force on you but pushes you the opposite direction.

22 Law of Conservation of Momentum
Momentum: (p) mass in motion. p = mv p = momentum Measured in kg*m/s m = mass Measured in kg v = velocity Measured in m/s All moving objects have momentum. Momentum is transferred between objects in a collision.

23 Law of Conservation of Momentum
Example #3: What is the momentum of a car with a mass of 1300 kg traveling at a speed of 28 m/s? p = ? m = 1300 kg v = 28 m/s p = (1300 kg)(28 m/s) p = 36,400 kg*m/s p = mv

24 Law of Conservation of Momentum
In a collision, because the forces acting on the two objects are equal and opposite (Newton’s 3rd Law), the transfer of momentum must be the same.  Law of Conservation of Momentum: momentum is never created or destroyed in a collision, it is only transferred.

25 Law of Conservation of Momentum
Example #4: Ball #1 is rolling 11 m/s directly toward a 0.17 kg ball #2 at rest. During the collision, ball #1 stops and ball #2 is launched forward at 9 m/s. What is the mass of ball #1? (think about the Law of Conservation of Momentum) v1 = 11 m/s m2 = 0.17 kg v2 = 9 m/s m1 = ? p = mv p2 = (0.17)(9) m1 = p1 v1 p1 = ? m1 = ? v1 = 11 m/s p2 = ? m2 = 0.17 kg v2 = 9 m/s Only for Honors! Always have them draw a picture!! Due to Law of conservation of momentum… p1 = p2 p2 = 1.53 kg*m/s m1 = 1.53 11 p1 = m1v1 v1 v1 m1 = 0.14 kg m1 = p1 v1