Forces and Motion 2 2 Force A force is a push or a pull that one object exerts on another object....

Forces and MotionForces and Motion

22Force

• A force is a push or a pull that one object exerts on another object.

• Objects like floors, chairs, and Earth also exert forces on other objects, including people.

Forces and MotionForces and Motion

22Force Has Direction and Size

• Just like velocity and acceleration, a force has both size and direction.

• The direction of a force is the direction of the push or pull.

• Pushing or pulling harder increases the size of the force you exert. The size of a force is measured in newtons (N).

Forces and MotionForces and Motion

22How Forces Combine

• When more than one force acts on an object, the forces combine.

• The combination of all the forces acting on an object is the net force.

Forces and MotionForces and Motion

22Combining Forces in the Same Direction

• When two forces act in the same direction on an object, like a box, the net force is equal to the sum of the two forces.

Forces and MotionForces and Motion

22Combining Forces in Opposite Directions

• If two forces of equal strength act on the box in opposite directions, the forces will cancel, resulting in a net force of zero.

Forces and MotionForces and Motion

22Combining Forces in Opposite Directions

• When two unequal forces act in opposite directions on the box, the net force is the difference of the two forces.

Forces and MotionForces and Motion

22Balanced and Unbalanced Forces

• When the net force on an object is zero, the motion of the object doesn’t change.

• The forces acting on an object are balanced forces if the net force is zero.

• The forces acting on an object are unbalanced forces if the net force is not zero.

Forces and MotionForces and Motion

22Contact and Non-contact Forces

• A force is exerted when one object pushes or pulls on another.

• A force that is exerted only when two objects are touching is a contact force.

• Non-contact forces are forces that can be exerted by one object on another even when the objects aren’t touching.

Forces and MotionForces and Motion

22Gravity

• Gravity is a non-contact force that every object exerts on every other object due to their masses.

• The gravitational force between two objects is an attractive force that tends to pull the two objects closer together.

Forces and MotionForces and Motion

22Gravity

• The size of the gravitational force between two objects depends on the masses of the objects and the distance between them.

• The gravitational force between two objects increases if the mass of one or both of the objects increases.

• If two objects move closer together, the gravitational force between them increases.

Forces and MotionForces and Motion

22Mass and Weight

• Because weight is a force, it is measured in newtons.

• Weight is not the same as mass. Mass is the amount of matter an object contains, and is measured in kilograms.

Forces and MotionForces and Motion

22Friction

• Friction is a contact force that resists the sliding motion of two surfaces that are touching.

• Friction causes a sliding object to slow down and stop.

• Friction also can prevent surfaces from sliding past each other.

Forces and MotionForces and Motion

22Static Friction

• Static friction is the force between two surfaces in contact that keeps them from sliding when a force is applied.

Forces and MotionForces and Motion

22Sliding Friction

• Sliding friction exists between all sliding surfaces that are touching.

• Sliding friction is the force that opposes the motion of twosliding surfaces in contact.

Forces and MotionForces and Motion

22What causes friction?

• When two surfaces are in contact, the surfaces stick to each other where the dips and bumps on one surface touch the dips and bumps on the other surface.

• Friction is caused by the sticking of the two surfaces at these bumps and dips.

Forces and MotionForces and Motion

22The Buoyant Force

• The buoyant force is a force exerted by a fluid on an object that is in the fluid.

• The buoyant force is always upward.• If you are

floating in water, the buoyant force is large enough to balance your weight.

Forces and MotionForces and Motion

22Air Resistance

• Air resistance is a contact force that opposes the motion of objects moving in air.

• Just like friction, air resistance acts in the direction opposite to an object’s motion.

• Air resistance is less for a narrow, pointed object than for a wide, flat object.

22Section CheckSection Check

Question 1

A force is a _______ or a _______.

Force is a push or a pull.

Answer

FL: SC.C.2.3.2

22Section CheckSection Check

Question 2

A force exerted between two objects that are not touching is known as _______.

The answer is a long-range force. Gravity is a long-range force; although we are not touching the Sun, our planet is being acted on by the Sun to keep it in orbit.

Answer

FL: SC.C.2.3.1

22Section CheckSection Check

Question 3You try to push a desk across the floor one direction but a friend is trying to push it the opposite direction. As a result, the desk doesn’t move. This is an example of what kind of forces?

A. balanced forcesB. continuing forcesC. extended forcesD. unbalanced forces

FL: SC.C.2.3.6

22Section CheckSection Check

Answer

The answer is A. When forces are balanced, the net force is zero.

FL: SC.C.2.3.6

The Laws of MotionThe Laws of Motion

33The First Law of Motion

• According to the first law of motion, if the forces acting on an object are balanced, then an object at rest remains at rest and an object in motion keeps moving in a straight line with constant speed.

• When the forces on an object are balanced, the motion of the object doesn’t change.

The Laws of MotionThe Laws of Motion

33The First Law of Motion

• The forces on an object are unbalanced when the net force on the object is not zero.

• According to the first law of motion, for an object to change speed or direction, the net force acting on the object must not be zero.

The Laws of MotionThe Laws of Motion

33Changing Direction

• Then the direction of motion curves toward the direction of the unbalanced net force.

• A moving object changes direction when the net force acting on the object is not in the same direction as theobject’s motion.

The Laws of MotionThe Laws of Motion

33The Second Law of Motion

• Newton’s second law of motion describes how the net force on an object, the mass of the object, and the acceleration of the object are related.

• According to the second law of motion, the acceleration of an object depends on the net force acting on the object and the object’s mass.

The Laws of MotionThe Laws of Motion

33Acceleration and Mass

• Acceleration is larger when the speed of an object changes more quickly.

• If the net force is the same, the acceleration of an object decreases as the mass of an object increases.

The Laws of MotionThe Laws of Motion

33Acceleration and Direction of the Net Force

• According to the second law of motion, when a force acts on an object, its acceleration is in the same direction as the force.

• When an object slows down, the direction of its acceleration is opposite to its direction of motion.

The Laws of MotionThe Laws of Motion

33Acceleration and Net Force

• When you lift your backpack, its speed depends not only on its mass, but also on the force you exert.

• According to the second law of motion, the acceleration of an object increases when the net force on the object increases.

The Laws of MotionThe Laws of Motion

33The Third Law of Motion

object exerts an equal force in the opposite direction on the first object.

• The third law of motion, states that when one object exerts a force on a second object, the second

• The forces in a force pair act in opposite directions and are always equal in size.

The Laws of MotionThe Laws of Motion

33Force Pairs

• The forces two objects exert on each other are called force pairs.

• For every action force, there is a reaction force that is equal in size, but in the opposite direction.

The Laws of MotionThe Laws of Motion

33Action and Reaction

• One force of the force pair is called the action force, and the other force is the reaction force.

The Laws of MotionThe Laws of Motion

33Action and Reaction

• The swimmer’s hands exert a backward action force on the water.

• The water then exerts a forward reaction force on theswimmer. This is the force that propels the swimmer forward.

Section CheckSection Check

33Question 1

Newton’s second law of motion states that _________ of an object is in the same direction as the net force on the object.

A. accelerationB. momentumC. speedD. velocity

FL: SC.C.2.3.6

Section CheckSection Check

33

The answer is A. Acceleration can be calculated by dividing the net force in newtons by the mass in kilograms.

Answer

FL: SC.C.2.3.6

Section CheckSection Check

33Question 2

The forces two objects exert on each other are called _______.

A. action pairsB. force pairsC. gravityD. reaction pairs

FL: SC.C.2.3.3

Section CheckSection Check

33Answer

The answer is B. The forces in a force pair act in opposite directions and are always equal in size.

FL: SC.C.2.3.3

Section CheckSection Check

33Question 3

When an object slows down, the direction of its acceleration is _______ its direction of motion.

A. the same asB. oppositeC. behindD. in front of

FL: SC.C.2.3.5; SC.C.2.3.6

Section CheckSection Check

33Answer

The answer is B. When you stop a ball from rolling, the force of your foot is opposite the direction of motion of the ball.

FL: SC.C.2.3.5; SC.C.2.3.6

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