Tennisball bouncing on wall - momentum

In summary, a tennis ball with a mass of 0.2 kg is thrown horizontally at a brick wall at a speed of 12 m/s and bounces back at 8 m/s. The contact time with the wall is 0.04 s. The magnitude of the average force of the wall on the ball is 100 N. The negative sign indicates that the force is in the opposite direction as the ball's velocity. In the second part of the conversation, the question is asked about the acceleration of an apple placed in orbit at the same distance from Earth as the Moon. The acceleration would be the same as the Moon's, as it is dependent on the gravitational force, not the mass. If Earth had half its present
  • #1
Feodalherren
605
6

Homework Statement



A tennis ball (m  0.2 kg) is thrown at a brick wall. It is
traveling horizontally at 12 m/s just before hitting the
wall and rebounds from the wall at 8 m/s, still traveling
horizontally. The ball is in contact with the wall for 0.04 s.
What is the magnitude of the average force of the wall on
the ball?

Homework Equations


P=mV
FΔt=ΔmV


The Attempt at a Solution



I didn't even know where to begin to I went to the solutions manual which did this:

[-(0.2kg)(8ms)-(0.2kg)(12ms)] / 0.04s = -100N; magnitude would be 100

Where the crap is the negative sign coming from?!
 
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  • #2
Hi Feodalherren, the -ve sign tells u that the 100N force exerted by the wall to the ball is in the opposite direction. Its a consequence of Newton's 3rd law of motion.
 
  • #3
In that case, why are both of the terms negative?
 
  • #4
Only the reverse velocity should be -ve. It seems the method used in ur manual is unclear. This might help:
f=ma=m(v-u)/t
with m=0.2kg, u=12m/s, v=-8m/s & t=0.04s
substitute in the equation:
f=0.2(-8-12)/0.04=-100N
 
  • #5
That actually didn't help at all :(. Can somebody clarify why there are two negative signs? I have test tomorrow and I need to get this down.
 
  • #6
I also had another question,
If an apple were placed in orbit at the same distance from
Earth as the Moon, what acceleration would the apple have?

How can the acceleration be the same if F=ma gives us a = F/m
which says that acceleration IS dependent on mass...?!
 
Last edited:
  • #7
And another, related to the first one:
How would the Sun’s gravitational force on Earth change
if Earth had one-half its present mass? Would Earth’s
acceleration change?

I still don't understand why acceleration doesn't change if f=ma implies it. And I've read all about Galileo's experiment. Why doesn't a=F/m make any sense in this case?
 
  • #8
Feodalherren said:
That actually didn't help at all :(. Can somebody clarify why there are two negative signs? I have test tomorrow and I need to get this down.

if u can't get the obvious, then no one would help u. even angels can't make u understand! open ur eyes!
 
  • #9
You were just doing exactly the same thing as the book. You put two negatives in there without explaining why... Why for an example isn't it -8+12?

ps. There are no angels.
 
  • #10
i didn't imply that angels exist!
That apart, i beleave you know that velocity is a vector (size & direction). In ur case the initial velocity (when the ball is moving toward the wall) is u=12m/s. The final velocity (ball moving away from the wall after being reflected/bounced, this velocity is -ve since it is in the opposite direction), v=-8m/s
u know that f=ma & a=(v-u)/t=(-8-12)/0.04=-500
now f=ma=-500*0.2
hope u get
 
  • #11
Now I did get it. Thank you!
 

1. How does the momentum of a tennis ball affect its bounce on a wall?

The momentum of a tennis ball affects its bounce on a wall in several ways. First, the greater the momentum of the ball, the higher it will bounce off the wall. This is because a higher momentum means the ball has more energy, and this energy is transferred to the wall upon impact, causing the ball to rebound with greater force. Additionally, the direction of the ball's momentum also affects its bounce. If the ball hits the wall at an angle, its momentum will be redirected and the ball will bounce off at a different angle.

2. How does the surface of the wall impact the bounce of a tennis ball?

The surface of the wall can greatly impact the bounce of a tennis ball. A smooth and hard surface, such as a concrete wall, will cause the ball to bounce higher compared to a rough or soft surface, such as a carpeted wall. This is because a smooth and hard surface provides less resistance to the ball, allowing it to maintain more of its momentum and bounce higher. On the other hand, a rough or soft surface will absorb some of the ball's energy, resulting in a lower bounce.

3. What role does gravity play in the bounce of a tennis ball on a wall?

Gravity plays a significant role in the bounce of a tennis ball on a wall. As the ball is thrown or hits the wall, it is pulled down by the force of gravity. The higher the ball bounces, the more it will slow down due to gravity pulling it back to the ground. The strength of gravity also affects the height of the bounce. On Earth, where gravity is stronger compared to other planets, a tennis ball will bounce lower compared to the moon, where gravity is weaker.

4. Can the elasticity of a tennis ball impact its bounce on a wall?

Yes, the elasticity of a tennis ball can greatly impact its bounce on a wall. Elasticity refers to the ability of an object to return to its original shape after being stretched or compressed. A tennis ball with high elasticity will bounce higher on a wall compared to a ball with low elasticity. This is because a ball with high elasticity will be able to quickly regain its shape after hitting the wall, resulting in a stronger and higher bounce.

5. How does the speed of the tennis ball affect its bounce on a wall?

The speed of the tennis ball can affect its bounce on a wall in a few ways. Firstly, the faster the ball is moving, the more momentum it will have, resulting in a higher bounce. Additionally, the speed at which the ball hits the wall can also affect its bounce. If the ball is moving at a slower speed, it may have a lower bounce compared to if it were moving at a faster speed. This is because a slower ball will have less energy to transfer to the wall, resulting in a weaker bounce.

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