Chapter 3 Dynamics Flashcards
1
Q
What is Mass?
A
- Mass is the measure of the amount of matter in an object
- Consequently, this is the property of an object that resists change in motion
- The greater the mass of a body, the smaller the change produced by an applied force
- The SI unit for mass is the kilogram (kg)
2
Q
Weight
A
- is the effect of a gravitational field on a mass
- Since it is a force on an object due to the pull of gravity, it is measured in Newtons (N) and is a vector quantity
- The weight of a body is equal to the product of its mass (m) and the acceleration of free fall (g)
- g is the acceleration due to gravity or the gravitational field strength
- On Earth, this is 9.81 m s−2 (or N kg−1)
3
Q
Mass v Weight
A
- An object’s mass always remains the same, however, its weight will differ depending on the strength of the gravitational field on different planets
4
Q
Force & Acceleration
A
- As stated on the previous page, Newton’s Second Law of Motion tells us that objects will accelerate if there is a resultant force acting upon them
- This acceleration will be in the same direction as this resultant force F= Mg
5
Q
Resultant Force
A
- Since force is a vector, every force on a body has a magnitude and direction
- The resultant force is therefore the vector sum of all the forces acting on the body. The direction is given by either the positive or negative direction as shown in the examples below
- The resultant force could also be at an angle, in which case addition of vectors is used to find the magnitude and direction of the resultant force.
6
Q
Acceleration
A
- Given the mass, Newton’s Second Law means you can find the acceleration of an object
- Since acceleration is also a vector, it can be either positive or negative depending on the direction of the resultant force
- Negative acceleration is deceleration -An object may continue in the same direction however with a resultant force in the opposite direction to its motion, it will slow down and eventually come to a stop
7
Q
Newton’s First Law:
A
A body will remain at rest or move with constant velocity unless acted on by a resultant force
8
Q
Newton’s Second Law:
A
- A resultant force acting on a body will cause a change in momentum in the direction of the force.
- The rate of change in momentum is proportional to the magnitude of the force
- -This can also be written as F = ma
9
Q
Newton’s Third Law:
A
- If body A exerts a force on body B, then body B will exert a force on body A of equal magnitude but in the opposite direction
- Newton’s Third Law force pairs must act on different objects
- Newton’s Third Law force pairs must also be of the same type e.g. gravitational or frictional
10
Q
Linear Momentum
A
- Linear momentum (p) is defined as the product of mass and velocity
- P= mv
- p= momentum (kgms^-1)
- m= mass (kg)
- v=velocity(ms^-1)
11
Q
Momentum is a vector quantity
A
- it has both a magnitude and a direction -This means it can have a negative or positive value
- If an object travelling to the right has positive momentum, an object travelling to the left (in the opposite direction) has a negative momentum
- The SI unit for momentum is kg m s−1
12
Q
Force & Momentum
A
- Force is defined as the rate of change of momentum on a body
- Force is equal to the rate of change in momentum
- The change in momentum is defined as the final momentum minus the initial momentum:
-
pfinal − pinitial
- Force and momentum are vectors so they can be either positive or negative values
-
pfinal − pinitial
- The change in momentum is defined as the final momentum minus the initial momentum:
13
Q
Force is equal to the rate of change in momentum
A
14
Q
Direction of Forces
A
- The force that is equal to the rate of change of momentum is still the resultant force
- A force on an object will be negative if it is directed in the opposite motion to its initial velocity.
- This means that the force is produced by the object it has collided with
15
Q
Maths tip
A
- ‘Rate of change’ describes how one variable changes with respect to another. In maths, how fast something changes with time is represented as dividing by Δt (e.g. acceleration is the rate of change in velocity)
- More specifically, Δt is used for finite and quantifiable changes such as the difference in time between two events