1. Forces The slope is 9.8. The speed increases by 9.8 m/s each second. Calculating: Calculate the slope of the graph. What does the slope tell you about the object’s motion? - Friction and Gravity

2. Forces 58.8 m/s Predicting: What will the speed of the object be at 6 seconds? - Friction and Gravity

3. Forces - The Nature of Force What is a Force? o A force is a push or pull o Every force has two components:  How strong the force is (in N)  The direction in which the force acts o The strength of a force is measured in Newtons (N)

4. Forces - The Nature of Force Combining Forces The combination of all forces acting on an object is called the net force.

5. Forces - The Nature of Force Balanced Forces If the net force on an object is zero, the forces are said to be balanced. Balanced forces acting on an object do not change the object’s motion.

6. Forces - The Nature of Force Unbalanced Forces If there is a non-zero net force acting on an object, the forces are said to be unbalanced. Unbalanced forces acting on an object will cause the object to accelerate (speed up, slow down, or change direction).

7. Forces - Friction and Gravity Friction Friction is the force that two surfaces exert on each other when they rub against each other. Friction always acts in the direction opposite an object’s motion. The two factors that affect friction are the types of surfaces involved and how hard they are pushed together. Not quite enough friction!

8. Forces - Friction and Gravity Gravity Gravity is a force that pulls all objects toward each other. Two factors affect the gravitational attraction between objects: the mass of the objects and the distance between them.

9. Forces - Friction and Gravity Gravity Strange but True! The force of gravity on Earth causes all objects to fall at the same rate. As long as there is no air resistance, an apple and a feather dropped from the same height at the same time will hit the ground at the same time.

10. Forces - Friction and Gravity Gravity Strange but True! The force of gravity on Earth causes all objects to fall at the same rate. If there were no air resistance, a bullet fired exactly horizontally and a bullet dropped from the same height as the gun will hit the ground at the same time.

11. Forces - 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.” Newton’s First Law In other words : “If an object is not moving, it will not start moving unless a net force acts on it. If an object is moving, it will continue at a constant velocity until a net force acts to change its speed or direction.”

12. Forces - 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.” Newton’s First Law Newton’s First Law tells us what an object will do when it has BALANCED forces acting on it. Newton’s First Law is also called the Law of Inertia. Inertia is the tendency of an object to resist a change in its motion. Inertia is dependent on mass. The greater an object’s mass, the more the object resists a change in its motion. (This is why it is harder to move a heavy object than a light one!)

13. Forces - 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.” Newton’s First Law What are some examples of Newton’s First Law? A hockey puck glides across ice after being hit with the hockey stick A baseball flies after being hit with bat A rock does not begin to roll across the ground on its own

14. Forces - Newton’s Second Law “The acceleration of an object depends on the net force acting on it and its mass.” Newton’s Second Law Newton’s Second Law tells us what an object will do when it has UNBALANCED forces acting on it. A net force acting on an object will cause the object to accelerate (speed up, slow down, or change direction). The greater the net force acting on an object, the larger the acceleration of the object will be. The larger the object’s mass, the greater the force required to accelerate the object.

15. Forces - Newton’s Second Law What are some examples of Newton’s Second Law? It takes more force to speed up a full shopping cart than an empty one When hit with the same amount of force, a lighter baseball will accelerate more than a heavier softball “The acceleration of an object depends on the net force acting on it and its mass.” Newton’s Second Law

16. Forces - Newton’s Second Law “The acceleration of an object depends on the net force acting on it and its mass.” Newton’s Second Law Newton’s Second Law is also written as a math equation: FORCE = MASS X ACCELERATION

17. Forces - Newton’s Second Law “The acceleration of an object depends on the net force acting on it and its mass.” Newton’s Second Law How much force does it take to give each car an acceleration of 0.05 m/s s ? F = ma

18. Forces Calculating Force A speedboat pulls a 55-kg water-skier. The force causes the skier to accelerate at 2.0 m/s 2. Calculate the net force that causes this acceleration. What information have you been given? Mass of the water-skier (m) = 55 kg Acceleration of the water-skier (a) = 2.0 m/s 2 What quantity are you trying to calculate? The net force F = ? What formula will you use? F = m X a Perform the calculation. F = m X a = 55 kg X 2.0 m/s 2 F = 110 kg m/s 2 F = 110 N - Newton’s Second Law

19. Forces Calculating Force Practice Problem What is the net force on a 1,000-kg object accelerating at 3 m/s 2 ? 3,000 N (1,000 kg X 3 m/s 2 ) - Newton’s Second Law

20. Forces Calculating Force Practice Problem What net force is needed to accelerate a 25-kg cart at 14 m/s 2 ? 350 N (25 kg X 14 m/s 2 ) - Newton’s Second Law

21. Forces - Newton’s Third Law “For every action there is an equal and opposite reaction.” Newton’s Third Law In other words : “If one object exerts a force on a second object, the second object always exerts a force of EQUAL strength in the OPPOSITE direction on the first object.”

22. Forces - Newton’s Third Law “For every action there is an equal and opposite reaction.” Newton’s Third Law EXAMPLE: A bird flies by flapping its wings. Action: The wings of a bird push the air downward. Reaction: The air pushes upward on the bird. This is what keeps the bird in the air. For every action, there is an equal (in size) and opposite (in direction) reaction.

23. Forces - Newton’s Third Law “For every action there is an equal and opposite reaction.” Newton’s Third Law EXAMPLE: A car runs into a brick wall, causing the front of the car to be smashed. Action: The car exerts a force on the wall. Reaction: The wall exerts a force on the car. This is what causes the car to be smashed. For every action, there is an equal (in size) and opposite (in direction) reaction.

24. Forces - Newton’s Third Law “For every action there is an equal and opposite reaction.” Newton’s Third Law EXAMPLE: A person on a skateboard pushes off from a wall. Action: The person pushes on the wall. Reaction: The wall pushes on the person, causing the person to roll. For every action, there is an equal (in size) and opposite (in direction) reaction.

25. Forces - Newton’s Third Law “For every action there is an equal and opposite reaction.” Newton’s Third Law EXAMPLE: A balloon flies away when the air is released. Action: The balloon pushes on the air, forcing the air out of the balloon. Reaction: The air pushes on the balloon. This is what causes the balloon to fly away. For every action, there is an equal (in size) and opposite (in direction) reaction.

26. Forces - Newton’s Third Law “For every action there is an equal and opposite reaction.” Newton’s Third Law

27. Forces Click here to see Newton's Laws in action!!