Basics
Physics is a science that studies matter and energy
Yes ! in physics we study the interaction between matter and energy
The following is a vector example:
- Today, I walked 4 km in 20 minutes, then I drank 1 liter of water !
No.
A vector has direction. None of the given information shows direction
A race car, runs 3 laps in a 1 km track. It starts and finishes in the same point
What is its distance?
What is its displacement?
distance: 3km
displacement: 0 km
A projectile takes 4.2 seconds to reach the maximum height of its trajectory.
How long is the object in the air?
The object will be in the air 8.4 seconds
What is a contact force?
Give 2 examples
Contact forces occur when 2 objects make contact between each other
Some examples:
Applied force, friction, normal force
According the the SI system. What is the unit to measure length?
meter
Vector a is 3N to the north
What is vector -2a?
Vector -2a is 6N to the south
A car moves with a constant speed of 5 miles/hour. It needs to cover 6 miles, how long will it take?
t=6/5 hours
1 hour and 12 minutes
A projectile has an initial velocity in the x direction of 34.8 m/s.
What is the velocity in the x direction after 2 seconds of motion?
In a projectile motion the velocity in x is CONSTANT, so after 2 seconds of motion the velocity in the x direction is 34.8 m/s
An object with a mass of 3kg, is moving with a constant acceleration of 5m/s2
What is the applied force?
F=ma
F=3*5
F=15 N
Convert 45km/h to m/s
12.5 m/s
Draw in the board the following information:
Karina walked 3 km to the north, then 2 km to the west and 1 km 35 degrees south of west
Shown on the board
Observe the graph on the board.
What is the speed in the last 5 seconds?
speed=4/5= 0.8 m/s
A projectile has an initial velocity of 20m/s with an initial angle of 25 degrees.
Is is truth that in the highest point the velocity in the y direction is: 20*sin(25)= 8.5 m/s
False.
In the highest point the velocity in the y-direction is zero!
What is net force magnitude and direction for the following situation:
A 12 kg box that is being pushed in a horizontal flat surface with an applied force of 200N and that experiences 150N of kinetic friction resistance.
Shown on the board
Convert 25000 cm2 to m2
2.5 m2
The initial velocity of a projectile is 30m/s with an initial angle of 15 degrees.
What is the velocity in x direction and what is the velocity in y direction?
In x: 30cos(15)=29.2 m/s
In y: 30sin(15)=7 m/s
An object moving in a straight line starts from rest and reaches a velocity of 50m/s in 4 seconds.
What is the acceleration?
Vf=Vi+at
50=0+a(4)
a=12.5 m/s2
A projectile has an initial velocity of 15m/s and an initial angle of 5 degrees.
What is the velocity in x and in y after 0.05 seconds?
After 0.05 seconds the projectile is still going upwards.
Vix=15*cos(5)=14.94 m/s
Viy=15*sin(5)=1.31 m/s
In x, as the velocity is constant. After 0.05 seconds Vx=14.94 m/s
In y, Vf=Vi+at
Vf=1.31+(-9.81)(0.05)
Vf=0.82m/s
Very clearly state when is the case that
Net force = ma (SECOND LAW)
VS
Net force=0 (FIRST LAW)
Net force=ma when the object is moving with a constant acceleration, so there exist change in velocity
VS
Net force=0 when the object is at rest or the object is moving with constant velocity
The real weight of an object is 2.5 kg.
The measurements you take are:
3.0 kg
2.99 kg
3.10 kg
Can you say that the data is precise?
Yes.
The data is not accurate but it is precise
Maybe the balance was not correctly calibrated
Add:
10 km south + 5 km east
x: 5
y: -10
Resultant: 11.2 km
Angle: 70.5 degrees south of east
I throw upwards a ball with an initial velocity of 8m/s.
What is the reached height?
Vf2=Vi2+2ad
02=82+2(-9.81)d
d=3.3 m
From the top of a 30m building an object is thrown with initial velocity of 18m/s.
What is the horizontal distance reached by the object?
First, get final velocity in y
Vf2=Vi2+2ad
Vf2=00+2(9.81)(30)
Vf2=588.6
Vf=24.26m/s
Second, get the time
Vf=Vi+at
24.26=0+9.81(t)
t=2.47 seconds
Finally, get the horizontal distance
V=d/t
18=d/2.47
d=44.46m
An object is being pushed with a force of 550N. The coefficient of friction between the object and the surface is 0.3. The object has a mass 0f 10kg.
What is its acceleration?
First, find out the normal force. In this case it is the same as the weight. So N=10(9.81)=98.1 N
Friction=Normal * coefficient of friction
friction= 98.1 * 0.3 = 29.43N
The net force in the x direction (where motion occurs) is 550-29.43=520.57N
Then,
Net Force=ma
520.57=10*a
a=52.06m/s2