1. Physical Science motion and force

2. Objectives: –Explain the effects of unbalanced forces on the motion of objects –Compare and contrast static and kinetic friction –Describe how friction may be either harmful or helpful –Identify ways in which friction can be reduced or increased

3. Force Force- an action exerted on a body in order to change the body’s state of rest or motion; force has magnitude and direction The net force is the combination of all of the forces acting on the object Whenever there is a net force acting on an object, the object accelerates in the direction of the net force. An object will not accelerate if the net force acting on it is zero

4. Balanced forces Balanced forces – when the forces applied to an object produce a net force of zero, the forces are balanced. Balanced forces do not cause a change in motion of a moving object When opposing forces on an object are balanced, the object remains at rest

5. Unbalanced forces When two opposite forces acting on the same object are unequal, the forces are unbalanced. Therefore, a change in motion occurs in the direction of the greater force.

6. Friction Friction- a force that opposes motion between two surfaces that are in contact Static friction- the force that resists the initiation of sliding motion between two surfaces that are in contact and at rest –The friction between surfaces that are stationary

7. Kinetic friction- the force that opposes the movement of two surfaces that are in contact and are sliding over each other –The friction between moving surfaces Two types: Sliding friction Rolling friction (less)

8. Frictional forces and acceleration A. When a car is accelerating, the forces are unbalanced. The force moving the car forward is greater than the opposing force of friction

9. B. When a car is cruising at constant speed, the force moving the car forward is balanced by the force of friction

10. C. A truck on a declining slope will not roll because the force of friction between the brakes and the wheels balance the gravity.

11. Harmful/Helpful friction Reduce friction: –Lubricants –Rolling friction vs. sliding –Smooth surface –Streamlining (aerodynamics)

12. Increase friction: –Make surface rougher –Increase force pushing surfaces together

13. What causes a car to move? A car’s wheels turn, and they push against the road The road pushes back on the car and causes the car to accelerate Friction causes the acceleration (whether speeding up, slowing down, or changing direction)

14. Without friction between the tires and the road, the tires would not be able to push against the road, and the car would not experience a net force. –Ex) hydroplaning

15. Read 331-336 Answer questions 1-5