1. Forces and Motion Chapter 12

2. What is a FORCE? A force is a push or pull that acts on an object.
A force can cause a resting object to move
A force can accelerate an object by changing its speed or direction.

3. How are forces measured?
Force is measured in Newtons.
A Newton is the force that causes a 1-kilogram mass to accelerate at a rate of 1 m/s2.
1 N = 1kg*m/s2

4. Combining Forces Forces can be combined.
Forces in the same direction can be added.
Forces in opposite direction are subtracted.
The net force is the overall force acting on an object after all the forces are combined.

5. Balanced forces:
When the forces on an object are balanced, the net force is zero
There is no change in the object’s motion.
Balanced forces have a net force of zero.

6. Unbalanced forces:
When an unbalanced force acts on an object, the object accelerates.
Unbalanced forces have a net force
NOT equal to zero.

7. What is meant by friction?
Friction is a force that opposes the motion of objects that touch as they move past each other.
There are four main types of friction:
1. static
2. sliding
3. rolling
4. fluid

8. Static Friction
Static Friction – the friction force that acts on objects that are not moving.
Always acts in the opposite direction to that of the applied force.

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

10. Rolling Friction
Rolling Friction – friction force that acts on rolling objects.
The force of rolling friction is about 100 to 1000 times less than the force of static or sliding friction.

11. Fluid Friction
Fluid Friction – the force that opposes motion through a fluid (liquid or gas).
Fluid resistance increases as the speed of the object moving through the fluid increases.
Air resistance – fluid friction acting on an object moving through the air.

12. Force and motion problem:
A block lies at rest (not moving) on a rough inclined slope; what forces act on the block?
R= Normal Reaction Force

13. Force and motion problem 2:
What forces act on a trailer attached to a tow rope being pulled by a car?

14. Force and motion problem 3:
A ball, attached to the ceiling, is swinging on the end of a string in a simple pendulum motion; modelling the ball as a particle, what forces act on it?

15. Problem 1 Draw a force diagram for a person pushing against a wall with a force of 50 N. The wall and person do not move.
Problem 2 Determine which type of friction is described.
A person is standing on a carpeted floor. What type of friction is present between her feet and the carpet?
A chair is slid out from under a table.. What type of friction is this?
A child is stirring cake batter. What type of friction is this?