The Nature of Force

“An object in motion will remain in motion and an object at rest will remain at rest unless acted on by an unbalanced force.” Newton’s First Law Read Newton’s First Law above and then look at the picture. Write a paragraph predicting the motion of the soccer ball. Will the ball continue to move upward forever? Will it continue to move forward forever? If not, what do you think causes the motion of the ball to change after the player kicks it?

Forces acting on an object can change the object’s motion! Change direction Slow down Speed up

What is a FORCE? A force is a push or pull The strength of a force is measured in Newtons (N) Every force has two components: How strong the force is (in N) The direction in which the force acts

FORCE Forces acting on an object can be drawn using arrows. The length of the arrow represents the strength of the force, and the direction of the arrow shows the direction of the force.

There is usually more than one force acting on an object. NET FORCE There is usually more than one force acting on an object. When two forces act in the same direction, they add together. When forces act in opposite directions, they are combined by subtracting the smaller force from the large force. The direction will be in the direction of the larger force. The combination of all the forces acting on an object is called the net force. Which of these three arrows represents the correct amount of net force of the above two arrows?

NET FORCE If equal forces act on an object in opposite directions, the net force will be zero. If the net force on an object is zero, the forces are said to be balanced. Balanced forces will NOT cause a change in an object’s motion.

REMEMBER NEWTON’S FIRST LAW: NET FORCE If there is a non-zero net force acting on an object, the forces are said to be unbalanced. Unbalanced forces WILL cause a change in an object’s motion (speed up, slow down, change direction). REMEMBER NEWTON’S FIRST LAW: “An object in motion will remain in motion and an object at rest will remain at rest unless acted on by an unbalanced force.”

NET FORCE Unbalanced forces will cause an object to speed up, slow down, or change direction. In other words, an unbalanced force acting on an object will cause the object to ACCELERATE!

NET FORCE Net Force The change in motion of an object is determined by the net force acting on the object. What is the net force for each situation?

NET FORCE

GRAVITY Gravity is a force that pulls all objects toward each other. Gravity acts between any two objects in the universe. Gravity is affected by two factors: The mass of the object (the more mass an object has, the greater its gravitational force) The distance between two objects (the greater the distance between the objects, the weaker the gravitational force between them)

GRAVITY Gravitational Attraction Gravitational attraction depends on two factors: mass and distance. Compare the gravitational force between different planets and their sun.

GRAVITY All objects, regardless of their masses, fall toward Earth with the same acceleration. The force of gravity causes all falling objects to speed up as they fall. The Earth’s gravity causes all objects to be pulled toward the center of the Earth.

The force of gravity is what causes objects to have weight The Earth’s gravity pulls objects toward the Earth. The more mass the object has, the greater the pull of gravity. Therefore, objects with more mass will “weigh” more. Also, the same object will have different “weights” on planets that have different masses. For example, the mass of the moon is less than the mass of the Earth, so the moon’s gravitational force is less. Therefore you would weigh less on the moon than on Earth even though your mass didn’t change.

GRAVITY On which planet would you weigh the most? On which planet would objects fall with the slowest speed?

FRICTION Friction is the force that two surfaces exert on each other when they rub against each other. Friction acts in the direction opposite an object’s motion. The force of friction causes an object to slow down and/or stop. Friction is affected by two factors: the types of surfaces involved how hard the surfaces are pushed together

There are 4 types of Friction:

Find examples of sliding, static, fluid and rolling frictions.

GRAVITY AND FRICTION TERMINAL VELOCITY The force of gravity causes a falling object to speed up (accelerate) as it falls. However, as the object speeds up, the force of friction (air resistance) also becomes greater. When the force of friction acting upward equals the force of gravity pulling down, the object will stop accelerating and will then fall at a constant speed. The speed at which this occurs is called terminal velocity.

Draw a force diagram showing all of the forces that are acting on each of the objects below. the rocket as it moves upward the ball as it is hit by the bat the skydiver as it falls down

Write a paragraph describing the motion of the soccer ball Write a paragraph describing the motion of the soccer ball. Begin with the moment the ball was kicked by the player until the moment the ball stopped rolling on the ground. Include all of the following terms: gravity, friction, unbalanced force, accelerate, at rest, change direction. Draw a force diagram of the ball traveling through the air.

Motion of a Soccer Ball   The ball begins at rest. When the player kicks the ball, he exerts an unbalanced force on the ball that causes the ball to accelerate (speed up). As the ball travels through the air, the unbalanced force of air resistance (fluid friction) acts on the ball which causes it to slow down. At the same time, gravity acts on the ball, causing it to begin to move down toward the ground (change direction). As the ball begins to move down toward the ground, it speeds up. So overall, the combined forces of gravity and friction cause the ball to slow down, change direction, and then speed up. Once the ball hits the ground, it rolls over the grass. The force of rolling friction acts on the ball in the direction opposite the ball’s motion, which causes the ball to slow down. Eventually the ball is at rest again.