Unit 15 – Forces and Motion Lesson 1: Forces

What are Forces? A force is a push or pull which causes a change in motion. It can cause objects to stop moving, start moving, move faster/slower, or change direction.

What are Forces? Forces are measured with spring scales in units called newtons (N). When we use a scale, we are measuring weight: the measure of the force of gravity on an object.

Friction Friction: a force that opposes motion. It acts between two objects that are touching. If it is between air and a moving object, then it is called air resistance.

Friction When two objects have friction between them, we can feel heat coming off of them. This is because the energy of motion is being changed into thermal energy. When I say start, rub your hands together. Do not stop until I say to. What happens?

Unit 15 – Forces and Motion Lesson 1: Forces

Magnetism Magnetism is the force of repulsion (pushing) or attraction (pulling) between poles of magnets. Every magnet has two poles: a south pole and a north pole.

Magnetism The north pole of one magnet is attracted to the south pole of another magnet. The north pole of one magnet repels (pushes away) from the north pole of another magnet.

Magnetic Fields A group of imaginary lines running from the north pole to the south pole on a magnet.

Gravitation Gravity is the force that pulls all objects in the universe toward one another.

Gravitation The strength of gravitation depends on the mass of the objects and the distance between them. More mass = more force Less distance = more force

Gravitation Examples: The mass of 2 ping pong balls is much less than the mass of a ping pong ball and the Earth. There is a much lower gravitational force with the ping pong balls.

Gravitation Examples: Two ping pong balls on the same table will have a stronger force than two ping pong balls across the room because they are closer to one another.

Unit 15 – Forces and Motion Lesson1: Forces

Balanced Forces forces that are equal in size and opposite in direction. Cancel each other out. Object’s do not move or change direction.

Unbalanced Forces When one force is greater than its opposite force. Object’s motion changes – it can stop, speed up, slow down, or change direction.

Forces Always Act in Pairs when one object acts on a second object, the second object also acts on the first object.

Net Force The net force is the combined forces acting on an object at any time.

Net Force If two forces are acting on an object from the same direction, you would add the forces up. If they are acting in opposite directions, you subtract the smaller force from the larger one.

Example You and a friend are helping Ms. Knapp move desks in the classroom. You stand on the same side and both push using a force of 15 N. What is the total force?

Example 2 You and a friend are helping Ms. Knapp move desks in the classroom. You stand on one side, pushing with a force of 25 N, and your friend wants to go the other way. They push with a force of 20 N. What is the total force?

Acceleration any change in an object’s motion. Speed up, slow down, stop, change direction, etc.

Unit 15 – Forces and Motion Lesson 4: Newton’s laws of motion

Newton’s First Law of Motion Objects at rest don’t move unless an outside force makes them move; objects moving don’t stop unless an outside force makes them stop. No acceleration can happen without an unbalanced force.

Newton’s First Law of Motion Inertia: the tendency of an object to resist a change in motion

Newton’s Second Law of Motion Acceleration depends on the amount of force applied to the object and the object’s mass. It can be written as a formula: Force = mass times acceleration F = (m)(a)

Newton’s Second Law of Motion The greater the force, the greater the acceleration. If you gently kick a soccer ball, it will not go as fast or as far as if you kick it with more force. You must use more force to move an object with greater mass. Think about an empty grocery cart compared to a full grocery cart.

Newton’s Third Law of Motion Forces always act in pairs that are equal and opposite to one another. The first force is called the action force and the second force is called the reaction force.

Newton’s Third Law of Motion This explains why a wall does not move when you lean on it. You apply a force onto the wall, but the wall applies the equal and opposite force back on you. This also explains why a ball bounces back up, and why a person rowing a boat moves forward when they push back on the oars.

Motion in Space / Orbit Mass: the amount of matter in an object. Weight: the pull of gravity on an object. Gravity is what holds all things to the Earth, but it also holds the Moon and other objects in an orbit.

Motion in Space / Orbit When an object travels in an orbit, it moves in a circular path around another object. If the object were to be “let go” from its orbit, it would fly off in a straight line.

Motion in Space / Orbit Astronauts on the ISS appear weightless in space due to microgravity. This occurs because Earth’s gravity causes the space station to fall toward Earth at a constant rate. ISS Tour