Unit 3

1. Unit 3 Force and Motion Study Guide

2. Force and Motion • A force is a push or pull. • Only unbalanced forces can cause an object to stop, start, or change directions. • The overall force on an object after all the forces have been added together is the net force. ( A non-zero net force ) • Equal forces acting on an object in opposite directions are balanced forces.

3. Know how to calculate net force in a force diagram 550 N 700 N Net force = 150N Net force = 5 N 25 N 25 N Net force = 0 N

4. Inertia is the tendency of an object to resist its change in motion. • Friction - A force that one surface exerts over another when the two rub against each other.

5. Newtons • Newton- the unit of force. One Newton equals the force required to accelerate one kilogram of mass at one meter per second. • 1 N = 1 kg x 1 m/s (squared)

6. F = m x a • When using the formula • F= m x a Force is measured in Newtons Mass is measured in kilograms Acceleration is measured meters/second2

7. Sample Problem #1 • A 52 kg water-skier is being pulled by a speedboat. The force causes her to accelerate at 2 m/s2. Calculate the net force. We know: Mass = 52 kg Acceleration = 2 m/s2 F = m x a F = 52 kg x 2 m/s2 (multiply numbers and units) F = 104 kg x m/s2 (change kg. m/s2 to N) F = 104 N

8. Sample Problem #2 We know: Force = 104 N Acceleration = 2 m/s2 Need to calculate: Mass M = F/A M = 104 N 2 m/s2 M = 104 kg . m/s2 (Changed Newtons to kg.m/s2) 2 m/s2 M = 104 kg . m/s2(m/s2 cancel out and you’re left with kg) 2 m/s2 M = 52 kg • A water skier is being pulled by a boat with 104 N of force to accelerate a skier at 2 m/s2. What is her mass?

9. Sample Problem # 3 • A 52Kg water skier is being pulled by a boat with 104N of force. What is her acceleration? We know: Mass= 52 kg Force = 104 N Need to calculate: acceleration A = F/M A = 104 N 52 kg M = 104 kg . m/s2 (Changed Newtons to kg.m/s2) 52 kg M = 104 kg . m/s2(kg cancel out and you’re left with m/s2 ) 52 kg M = 2 m/s2

10. Work = Force x Distance • There is no work don’t if there is no movement!! • By making the distance you have to move an object greater, you lessen the force needed to move the object.

11. Having a longer ramp means you have to apply less force to get an object up the ramp. Getting the object into the back of the truck is the job. The amount of work is the same no matter the length of the ramp. What changes is the amount of force needed to do the work.

12. Work = Force x Distance • Work is measured in Joules – J • Force is measured in Newtons – N • Distance is measured in meters – m

13. Work ProblemsSample #1 • How much work is required to pull a sled 5 meters if you use 60 N of force. We know: D = 5 m F = 60 N We need to calculate: W W = FD W = 60 N . 5M W = 300 Nm W = 300 J

14. Work ProblemsSample # 2 • If 300 J of work was done to pull a sled 5 meters, how much force was applied to the sled? We know: W = 300J D = 5 m We need to calculate: F F= W/D F= 300 J 5m F= 300 Nm (Change J to Nm) 5m F= 300 Nm ( m cancel out) 5 m F = 60 N

15. Work ProblemsSample # 3 If 300 J of work was done to pull a sled 5 meters, how much force was applied to the sled? • We know: W = 300J • F = 60 N • We need to calculate: D • D = W/F • D = 300 J • 60 N • D = 300 Nm (Change J to Nm) • 60 N • D = 300 N m ( N cancel out) • 60 N • D = 5 m

16. Force and Motion in plants • Turgor Pressure- is the outward pressure that occurs in a plant cell when the cytoplasm and vacuoles fill up with water and the cell membrane pushes against the cell wall.

17. Geotropism • When a plants roots grow down into the soil, and the shoots grow up toward the surface as a response to gravity.