Forces, text 264-272
What is a force? A push, pull or action
What is the unit to measure force? Newtons and pounds
What is weight? The force of gravity that acts on mass
What is the difference between mass and weight? Mass is the amount of matter something contains (# atoms), so it NEVER changes Weight will change if the gravity changes, such as between the Earth and the moon
What is friction? A force that resists motion
In what direction does friction push? In the opposite direction of motion
What are some different types of friction? Static, rolling, sliding, air, viscous
If something isn’t moving what type of friction is occurring? Static friction
Buoyant force (text 82-3, 86) Buoyancy is the upward force that a fluid exerts on a submerged object. It is equal to the weight of fluid that is displaced. If 10 N of water is displaced, the buoyant force is 10 N Use a spring scale or graduated cylinder to measure buoyant force (p 83)
What can you need to measure to know the exact buoyant force What can you need to measure to know the exact buoyant force? (text 82-3, 86) Spring scale or graduated cylinder
Net Force
The overall force acting on an object is called the Net force
What is the net force when all the forces are balanced? Equilibrium
What happens to motion when the forces are balanced? Balanced forces do not change motion If two balanced forces are acting on an object, that object will not change its motion. If it is still, it will stay still. If it is moving, it will continue moving, in the same direction and at the same speed.
What do you do to find the net force in the picture below? You add them if they are in the same direction, as shown
What is the net force? 300N 350 N 300 + 350 = 650 N
What is the net force? 3 N 11 N 3-11 = -8 N
Newton’s First Law
What do balanced forces do to an object’s motion? Balanced forces do not change motion If two balanced forces are acting on an object, that object will not change its motion. If it is still, it will stay still. If it is moving, it will continue moving, in the same direction and at the same speed.
What is Inertia? Inertia is the property of an object that resists changes in motion. Objects with more mass have more inertia and are more resistant to changes in their motion.
What would have the most inertia? Books School bus Frog The school bus because it has the most MASS
Newton’s Second Law
What is the force needed to move a box with a mass of 50 kg at an acceleration of 2 m/s/s? F=m x a F= (50kg) x (2 m/s/s) 100 N
Which object would require more force to move because of its mass? 30 kg present 10 kg cake 2 kg hat 30 kg present!
What two things affect an object’s acceleration? Force causes acceleration, and mass resists acceleration.
If you are pushing a shopping cart with groceries what could you do to make it go faster? Add more force (push it harder)
Speed
How far would Jorge run in 30 minutes at a speed of 5 mi/hr? v T v= D/T D=v x T =5 mi/hr x 0.5 hr =2.5 mi
Using the graph below, what is the speed after 4 seconds? 2.0 1.5 1.0 0.5 1.0 m/s Speed Time
How would you describe the speed in this graph? 2.0 1.5 1.0 0.5 constant Speed Time
What is the speed of the object in this graph? 10 v= d/t =10 m/10 s =1 m/s Position (m) 10 Time (s)
How far did the object move after 5 seconds in this graph? 10 D=s x t =1 m/s x 5 s =5 m Position (m) 5 5 10 Time (s)