Is Motion Always Positive or Negative in Physics Problems?

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In summary: Positive displacement means it moves up or forward, negative displacement means it moves down or backward.
  • #1
willingtolearn
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Homework Statement
#1
a) A ball is throwing straight upward, Is that motion positive or negative acceleration/velocity/displacement ?
b) A ball is falling down (what go up must come down), Is that a positive or negative acceleration/velocity/displacement ?

#2
a) A car is moving forward then the driver step on brake, Is that motion positive or negative acceleration/velocity ?
b) A car is moving backward then the driver step on brake, Is that motion positive or negative acceleration/velocity/displacement ?
c)A car is moving backward then the driver step on gas, Is that motion positive or negative acceleration/velocity/displacement ?

My answers
#1
a) positive acceleration, negative velocity (the velocity is decreasing), positive displacement
b) negative acceleration, positive velocity (the velocity is increasing), negative displacement
#2
a) negative acceleration (the car is slowing down), negative velocity,
b) same a
c) i got confused
 
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  • #2
careful... positive velocity just means it is moving foward or upward... negative velocity just means it is moving backwards or downwards... it doesn't meant that the speed is decreasing (that gives information about acceleration).

If an object is moving forward and slowing down... that means velocity is positive and acceleration is negative. Also, if an object is moving backwards and speeding up backwards... acceleration is again negative.

Give these another shot.
 
Last edited:
  • #3
#1
a)positive velocity, negative acc
b)negative velocity, negative acc
#2
a)positive velocity, negative acc
b)negative velocity, positive acc
c)negative velocity, negative acc
Is those right ?
 
  • #4
willingtolearn said:
#1
a)positive velocity, negative acc
b)negative velocity, negative acc
#2
a)positive velocity, negative acc
b)negative velocity, positive acc
c)negative velocity, negative acc
Is those right ?

All correct! But you didn't write the displacements.
 
  • #5
oh i forgot, Is the sign of displacements the same with velocity ? positive - up, back
negative back and down
 
  • #6
willingtolearn said:
oh i forgot, Is the sign of displacements the same with velocity ? positive - up, back
negative back and down

Yes, in these cases it is.
 

What is the concept behind a ball being thrown up and down?

The concept behind a ball being thrown up and down is the force of gravity. When a ball is thrown up, it experiences an upward force from the thrower's hand. However, as it travels upwards, the force of gravity pulls it back down towards the ground. This back and forth motion creates the up and down trajectory of the ball.

What factors affect the height and speed of a ball when thrown up and down?

The height and speed of a ball when thrown up and down are affected by the initial force of the throw, the angle at which it is thrown, and the air resistance. The greater the initial force and the steeper the angle, the higher and faster the ball will go. Air resistance, which is affected by the size and shape of the ball, can also impact the height and speed of the ball.

What happens to the energy of the ball as it is thrown up and down?

As a ball is thrown up and down, it will experience a change in its energy. When it is thrown up, it will have a high amount of potential energy due to its height. As it falls back down, this potential energy is converted into kinetic energy, which is the energy of motion. As the ball reaches the bottom of its trajectory, it will have the most kinetic energy before it starts to slow down and the energy is converted back into potential energy.

What is the difference between throwing a ball up and down on Earth versus on the moon?

Throwing a ball up and down on Earth is different from doing so on the moon because of the difference in gravitational pull. On Earth, the force of gravity is stronger, causing the ball to fall back down faster compared to on the moon where the force of gravity is weaker. This means that on the moon, the ball will travel higher and take longer to return to the ground compared to on Earth.

Can a ball thrown up and down ever reach the same height on its way up and down?

Yes, a ball thrown up and down can reach the same height on its way up and down. This is known as the maximum height of the ball and is reached when the ball's upward velocity is zero. At this point, the force of gravity is the only force acting on the ball, causing it to fall back down to the ground. However, factors such as air resistance and friction can affect the ball's maximum height.

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