Unit 15 LESSON 1 WHAT ARE FORCES?

Pushing and Pulling A force is a push or pull. Forces can cause an object at rest to move, speed up, slow down, change direction, or stop. Forces can change the shape of an object.

Pushing and Pulling For example, when rowers pull back on oars, the oars push against the water. The water pushes back against the oars, causing the boat to move. The larger the force, the greater the change it can cause to the motion of an object. Smaller forces cause smaller changes. R1 More Force = More change in motion (acceleration)

Pushing and Pulling

Pushing and Pulling Spring scales are used to measure force in units called Newton (N). Weight is the measurement of the pull of gravity on an object. while, Mass is a measurement of the amount of matter something contains The unit of measuring weight is Newton (N) while the unit of measuring mass is g/kg

Pushing and Pulling

Two Common Forces Gravity Gravity is a force of attraction toward the center of Earth. It acts even when objects are not touching.

Two Common Forces Gravity

Two Common Forces Gravity The size of the force increases as the mass of the objects increases. R2 more Mass = more Gravity

Two Common Forces Gravity

Two Common Forces Gravity The size of the force decreases as the distance between the objects increases. R3 More Distance = less gravity

Friction Friction is a force that opposes motion and acts between two objects that are touching. Friction can also exist between air and a moving object. This is called air resistance. Friction can also change the energy of motion into thermal energy. R4 more friction = less motion

Friction R5 smooth surface = less friction rough surface = more friction R6 more mass = more friction

Friction

Balanced or Unbalanced? Balanced forces are forces on an object that are equal in size and opposite in direction. Unbalanced forces are forces that cause a change in motion. A force must overcome the force of friction before an object will move.

Balanced or Unbalanced?

Pull (or Push) Harder!

Pull (or Push) Harder! A large force will cause a large change in the motion of an object. A small force will cause little change. The large force makes the object change its velocity, or accelerate, quickly. Changes in velocity include changes in speed and changes in direction. R1 More Force = More change in motion (acceleration)

I m Not Moving! The larger the force applied to an object, the greater the acceleration.

Pull (or Push) Harder! What should the boy do differently to make the bell ring?

I m Not Moving! When the same force is applied, objects with greater mass accelerate more slowly than objects with less mass. R7 more mass = less acceleration

I m Not Moving! The less an object s mass is, the less force is needed to change its motion. R8 less mass = less force

I m Not Moving!

I m Not Moving! Why does it take the full truck longer to stop than the empty truck?

Let s Go to Mars! An understanding of forces helped to safely send a spacecraft to Mars.

Let s Go to Mars! Balanced forces allowed a spacecraft to move in one direction at a constant velocity through space. Once the spacecraft reached Mars, it accelerated toward the surface because of gravity.

Let s Go to Mars!