Presentation on theme: "Forces and Motion 2 Force"— Presentation transcript:
1Forces and Motion2ForceA 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.
2Force Has Direction and Size Forces and Motion2Force Has Direction and SizeJust 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).
3Forces and Motion2How Forces CombineWhen 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.
4Combining Forces in the Same Direction Forces and Motion2Combining Forces in the Same DirectionWhen 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.
5Combining Forces in Opposite Directions Forces and Motion2Combining Forces in Opposite DirectionsIf two forces of equal strength act on the box in opposite directions, the forces will cancel, resulting in a net force of zero.
6Combining Forces in Opposite Directions Forces and Motion2Combining Forces in Opposite DirectionsWhen two unequal forces act in opposite directions on the box, the net force is the difference of the two forces.
7Balanced and Unbalanced Forces Forces and Motion2Balanced and Unbalanced ForcesWhen 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.
8Contact and Non-contact Forces Forces and Motion2Contact and Non-contact ForcesA 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.
9Forces and Motion2GravityGravity 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.
10Forces and Motion2GravityThe 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.
11Forces and Motion2Mass and WeightBecause 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.
12Forces and Motion2FrictionFriction 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.
13Forces and Motion2Static FrictionStatic friction is the force between two surfaces in contact that keeps them from sliding when a force is applied.
14Forces and Motion2Sliding FrictionSliding friction is the force that opposes the motion of twosliding surfaces in contact.Sliding friction exists between all sliding surfaces that are touching.
15Forces and Motion2What 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.
16Forces and Motion2The Buoyant ForceThe 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.
17Forces and Motion2Air ResistanceAir 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.
18Question 1 Answer 2 A force is a _______ or a _______. Section Check2Question 1A force is a _______ or a _______.AnswerForce is a push or a pull.FL: SC.C.2.3.2
19Section Check2Question 2A force exerted between two objects that are not touching is known as _______.AnswerThe 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.FL: SC.C.2.3.1
20Section Check2Question 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 forcesFL: SC.C.2.3.6
21Section Check2AnswerThe answer is A. When forces are balanced, the net force is zero.FL: SC.C.2.3.6
22The Laws of Motion3The First Law of MotionAccording 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.
23The Laws of Motion3The First Law of MotionThe 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.
24The Laws of Motion3Changing DirectionA moving object changes direction when the net force acting on the object is not in the same direction as theobject’s motion.Then the direction of motion curves toward the direction of the unbalanced net force.
25The Second Law of Motion The Laws of Motion3The Second Law of MotionNewton’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.
26The Laws of Motion3Acceleration and MassAcceleration 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.
27Acceleration and Direction of the Net Force The Laws of Motion3Acceleration and Direction of the Net ForceAccording 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.
28Acceleration and Net Force The Laws of Motion3Acceleration and Net ForceWhen 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.
29The Laws of Motion3The Third Law of Motionobject 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
30The Laws of Motion3Force PairsThe forces two objects exert on each other are called force pairs.The forces in a force pair act in opposite directions and are always equal in size.
31The Laws of Motion3Action and ReactionOne force of the force pair is called the action force, and the other force is the reaction force.For every action force, there is a reaction force that is equal in size, but in the opposite direction.
32The Laws of Motion3Action and ReactionThe 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.
33Section Check3Question 1Newton’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. velocityFL: SC.C.2.3.6
34Section Check3AnswerThe answer is A. Acceleration can be calculated by dividing the net force in newtons by the mass in kilograms.FL: SC.C.2.3.6
35Section Check3Question 2The forces two objects exert on each other are called _______.A. action pairsB. force pairsC. gravityD. reaction pairsFL: SC.C.2.3.3
36Section Check3AnswerThe 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
37Section Check3Question 3When an object slows down, the direction of its acceleration is _______ its direction of motion.A. the same asB. oppositeC. behindD. in front ofFL: SC.C.2.3.5; SC.C.2.3.6
38Section Check3AnswerThe 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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