What is important about forces ?
-Forces always arise from an interaction between two objects
- the two forces in an interaction pair
- the forces are of equal size
opposit in direction
the two forces act on different objects
What are the two kinds of interaction pairs?
1. two objects touch eachother
- contrast forces
-as soon as the objects seperate the forces stop
2. called action at adistance
- attracted when near magnets
- forces act all the time
- forces get weaker as the distance increases
What are some examples of interaction pairs?
-push out hot gases
-rocket exerts dowward force o the gases
-gases exert an upward force on the rocket
2. car wheel
What cause the size of the friction?
-depends on external force
- up to a certain limit
limit determined by weight and roughness of the surface and the object
What s frction and what causes it ?
- interaction between two surfaces
- slide against eachother
friction acts in the opposite direction of the force you exert
-caused by roughness of the sliding surfaces
- bumps on two surfaces don't fit- resistance
What is the sum of forces acting on an object?
How can a table exert a force?
- table with a tennis ball is like a school bag on a cushion
- there is a downward force from the ball on the table
-ther is an upward force from the table on the ball
How does freefall work?
- ball dropped
- increaes it speed until it reaches terminal velocity
- falls at a constant speed
What is the difference between displacement and distance?
- distance is the length of the route alon a path
- displacment is the shortest distance from a to b
What is the formula for average speed and acceleration?
Distance(m)= speed(m/s) times time taken (s)
acceleration= change of speed divided by time taken for the change (s)
How do you find speed on a distance time graph?
find the gradient
What is a displacement time graph?
show the path of an object that moves in a straight line
What is special about a velocity time graph?
- can show direction and speed
What is the formula for momentum?
What is important to know about the change of momentum in an interaction pair?
1. the change in momentum is the same
2. the resultant force acting on the objects is the same
3. the time for which the force acts is the same
What is the formula for change in momentum?
What are the safety precautions in cars?
increases time the force acts on the car
-less force on you
-seat belts, air bags
make changes momentum take longer
What are Newtons three laws of motion?
1. If the resultant force acting on an object is zero. the momentum of the object does not change
2. If the ther is a resultant force acting on an object , the momentum of the object will change and the size of the change of momentum is equalto the resultant frce times the time taken for the force to act. the change is in the same direction as the resultant force
3. When two objects interact, each experience ` force. The two forces are equal in size, but opposite in direction.
these laws apply all objects and every situation apart from objects at the subatomic level or moving at the speed od light.
What is the formula for work done, kinectic energy and gravitational potential energy?
Gravitational potentail energy(j)=weight(N) times vertical height
What is the relationship between GPE and kinectic energy?
decrease in GPE=increase in kinectic energy