1. Forces Chapter 12, Section 1

2. Forces Force – a push or a pull that acts on an object Force – a push or a pull that acts on an object A force can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction. A force can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction. Measurement tool – spring scale Measurement tool – spring scale SI unit – 1 Newton (N) = 1 kgm/s 2 SI unit – 1 Newton (N) = 1 kgm/s 2 Forces can be represented by arrows. Forces can be represented by arrows.

3. Combining Forces Net force – the overall force acting on an object after all the forces are combined Net force – the overall force acting on an object after all the forces are combined Forces can be combined by vector addition. Forces can be combined by vector addition. –Same direction – add the forces –Opposite directions – subtract the forces –Varying directions – Pythagorean Theorem

4. Balanced and Unbalanced Forces Balanced forces (forces that add up to a net force of zero) will cause no change in the object’s motion. (Motion may be sustained.) Balanced forces (forces that add up to a net force of zero) will cause no change in the object’s motion. (Motion may be sustained.) Unbalanced forces (forces that do not add up to a net force of zero) will cause the object to accelerate (in the dir’n of the unbalanced force). Unbalanced forces (forces that do not add up to a net force of zero) will cause the object to accelerate (in the dir’n of the unbalanced force). –Cause a resting object to move. –Accelerate a moving object.  Change the object’s speed  Change the object’s direction

5. Free Body Diagrams Free body diagrams are a visual way of analyzing all of the forces acting on an object. Free body diagrams are a visual way of analyzing all of the forces acting on an object. Force vectors are represented by arrows. Force vectors are represented by arrows. –Length of arrow = size (magnitude) of force –Direction of arrow = direction of force Draw the object first. Then draw an arrow pointing out from the object for each force vector (with the tail touching the object and the head pointing out). Draw the object first. Then draw an arrow pointing out from the object for each force vector (with the tail touching the object and the head pointing out).

6. Free Body Diagram 1 A cup sitting on a table… A cup sitting on a table… Gravitational Force (F grav ) – the downward pull caused by Earth’s gravity Gravitational Force (F grav ) – the downward pull caused by Earth’s gravity Normal Force (F norm ) – the supporting force that balances the gravitational force Normal Force (F norm ) – the supporting force that balances the gravitational force

7. Free Body Diagram 2 A soccer ball being kicked… A soccer ball being kicked… Applied Force (F app ) – the push or pull another object has on the first object Applied Force (F app ) – the push or pull another object has on the first object Frictional Force (F frict ) – the force that opposes the motion of objects that touch as they move past each other Frictional Force (F frict ) – the force that opposes the motion of objects that touch as they move past each other

8. Analyzing the Forces in a Free Body Diagram The vertical and horizontal forces can be inspected separately. The vertical and horizontal forces can be inspected separately. In the previous free body diagram… In the previous free body diagram… The vertical forces are balanced, so there is no vertical acceleration. The vertical forces are balanced, so there is no vertical acceleration. The horizontal forces are unbalanced, so the object will accelerate horizontally. The horizontal forces are unbalanced, so the object will accelerate horizontally.

9. Free Body Diagram 3 A book sliding across a table after being released… A book sliding across a table after being released… Momentum – the product of an object’s mass and its velocity – NOT a force Momentum – the product of an object’s mass and its velocity – NOT a force

10. Free Body Diagram 4 An object in free fall… An object in free fall… Air Resistance (F air ) – the fluid friction acting on an object moving through the air Air Resistance (F air ) – the fluid friction acting on an object moving through the air

11. Free Body Diagram 5 A falling object with forward momentum (A skydiver)… A falling object with forward momentum (A skydiver)…

12. Using Free Body Diagrams Free body diagrams can be used to determine the direction of the net force (F net ) acting on an object. Free body diagrams can be used to determine the direction of the net force (F net ) acting on an object. Vector addition can be used to determine the size (magnitude) of the net force. Vector addition can be used to determine the size (magnitude) of the net force. The object will accelerate in the direction of the net force. The object will accelerate in the direction of the net force. Acceleration – a change in speed or dir’n Acceleration – a change in speed or dir’n

13. Friction Friction – a force that opposes the motion of objects that touch as they move past each other Friction – a force that opposes the motion of objects that touch as they move past each other Four main types of friction: Four main types of friction: –Static –Sliding –Rolling –Fluid

14. Static Friction Static friction – the friction force that acts on objects that are not moving Static friction – the friction force that acts on objects that are not moving Static friction always acts in the direction opposite to that of the applied force. Static friction always acts in the direction opposite to that of the applied force. Examples: Examples: –Trying to push a large piece of furniture –Your shoe remaining at the same spot on the floor as you push off to step forward

15. Sliding Friction Sliding friction – a force that opposes the direction of motion of an object as it slides over a surface Sliding friction – a force that opposes the direction of motion of an object as it slides over a surface Sliding friction is less that static friction. Sliding friction is less that static friction. Less force is needed to keep an object moving than to start it moving. Less force is needed to keep an object moving than to start it moving.

16. Rolling Friction Rolling friction – a friction force that acts on rolling objects, caused by the change in shape at the point of rolling contact Rolling friction – a friction force that acts on rolling objects, caused by the change in shape at the point of rolling contact The force of rolling friction is 100 to 1000 times less than the force of static or sliding friction. The force of rolling friction is 100 to 1000 times less than the force of static or sliding friction. Ball bearings replace sliding friction with rolling friction, causing the friction to be greatly reduced. Ball bearings replace sliding friction with rolling friction, causing the friction to be greatly reduced.

17. Fluid Friction Fluid friction – the force that opposes the motion of an object through a fluid Fluid friction – the force that opposes the motion of an object through a fluid Fluid – liquid or gas (ability to flow) Fluid – liquid or gas (ability to flow) Fluid friction increases as the speed of the object moving through the fluid increases. Fluid friction increases as the speed of the object moving through the fluid increases. Examples: Examples: –A spoon through cake batter –Air resistance – fluid friction acting on an object moving through the air

18. Gravity Gravity – an attractive force (a pull) that acts between any two masses Gravity – an attractive force (a pull) that acts between any two masses Gravity does not require contact (like friction does); it can act over long distances. Gravity does not require contact (like friction does); it can act over long distances. Earth’s gravity acts downward toward the center of the Earth. Earth’s gravity acts downward toward the center of the Earth.

19. Falling Objects Gravity causes objects to accelerate downward. Gravity causes objects to accelerate downward. Air resistance acts in the direction opposite to the motion and reduces acceleration. Air resistance acts in the direction opposite to the motion and reduces acceleration. When these two forces become balanced, the object will stop accelerating. When these two forces become balanced, the object will stop accelerating. Terminal velocity – the constant velocity of a falling object when the force of air resistance equals the force of gravity Terminal velocity – the constant velocity of a falling object when the force of air resistance equals the force of gravity

20. Free Body Diagram 4 (again) An object in free fall… An object in free fall… Air Resistance (F air ) – the fluid friction acting on an object moving through the air Air Resistance (F air ) – the fluid friction acting on an object moving through the air

21. Projectile Motion Projectile motion – the motion of a falling object (projectile) after it is given an initial forward velocity Projectile motion – the motion of a falling object (projectile) after it is given an initial forward velocity Air resistance and gravity are the only forces acting on a projectile. Air resistance and gravity are the only forces acting on a projectile. The combination of an initial forward velocity and the downward vertical force of gravity causes the ball to follow a curved path. The combination of an initial forward velocity and the downward vertical force of gravity causes the ball to follow a curved path.

22. Falling Phenomena Two balls with different masses (but the same shape) that are released at the same time will hit the ground at the same time! Two balls with different masses (but the same shape) that are released at the same time will hit the ground at the same time! A ball that is dropped and a ball (with the same shape) that is released with forward velocity will hit the ground at the same time! A ball that is dropped and a ball (with the same shape) that is released with forward velocity will hit the ground at the same time! Reason: The only two forces acting on the balls are air resistance and gravity, and these forces are identical. Air resistance is the same because the shape is the same. Gravity is always the same (9.8 m/s 2 ). Reason: The only two forces acting on the balls are air resistance and gravity, and these forces are identical. Air resistance is the same because the shape is the same. Gravity is always the same (9.8 m/s 2 ).