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SOMEONE PLEASE HELP IN PHYSICS!!!?
1) A 0.160kg baseball pitched horizontally at 34.5m/s strikes a bat and is popped straight up to a height of 37.0m .
A) If the contact time between bat and ball is 2.5ms , calculate the magnitude of the average force between the ball and bat during contact. THE ANSWER IS 2800N.
B) Find the direction of the average force on the ball.
PLEASE HELP ME IN THIS.
2) Calculate the impulse experienced when a 64kg person lands on firm ground after jumping from a height of 2.8m . THE ANSWER IS 470KG*M/S
A) Estimate the average force exerted on the person's feet by the ground if the landing is with bent legs. When the legs are bent, assume the body moves about 40 during impact.
PLEASE HELP ME IN THIS PART
1 Answer
- Seamus OLv 78 years ago
1) A) The pitch is horizontal ... but it doesn't say to ignore the affect of gravity ... so assume the ball travels horizontally to the bat so that the momentum vector of the ball to the bat remains horizontal
Now consider the vertical motion of the ball to find the speed the ball came off the bat
s = 37.0 m ... [taking UP as the positive direction]
a = -g = -9.8 m/s²
v(i)(vert) = ?
v(f)(vert) = 0 m/s ... [b/c at max height the ball is at rest for an instant]
v(f)² = v(i)² + 2as
v(i) = √[v(f)² - 2as]
v(i) = √[0 - 2 * (-9.8) * 37.0]
v(i) = 26.9 m/s
Now I = Δp = p(f) - p(i) = m * v(f) - m * v(i)
Draw the vector triangle for the change in momentum at the bat:
Vertical vector for p(f) with magnitude 0.160 * 26.9 = 4.30 Ns
Then from the head of that vector draw the horizontal vector pointing LEFT for -p(i) (b/c we want to add the negative of the initial momentum to the final momentum ... same as subtracting it) with magnitude 0.160 x 34.5 = 5.52 Ns
Δp is the hypotenuse of the momentum vector triangle
so Impulse = Δp = √[4.30² + 5.52²] = 7.00 Ns
Also Impulse = F Δt
Δt = 2.5 ms = 2.5 x 10^(-3) s
so 7.00 = F * 2.5 x 10^(-3)
F = 2800 N
so the magnitude of the average force the bat exerts on the ball is 2800 N
B) Impulse has the same direction as the average force ... so the average force on the ball is in the same direction as Δp
mark the angle between the vertical vector and the hypotenuse as θ
θ = arctan (5.52 / 4.30)
θ = 52°
so the direction of the average force of the bat on the ball is at an angle of 52° from the vertical TOWARDS the pitcher
2) Your answer to the first part has the right magnitude and correct number of sig figs ... and provided you took the positive direction as UP the sign of your answer is also correct
A) Just b4 hitting the ground you would have found the velocity of the jumper to be -7.4 m/s [taking UP as the positive direction]
The jumper comes to rest and in coming to rest his knees bend 40 cm = 0.4 m
so his body travels thru a distance s = 0.4 m
In that distance he slows from -7.4 m/s to 0 m/s ... so the average velocity during the stopping distance = (-7.4 + 0) / 2 = -3.7 m/s
time taken = distance traveled / speed
so Δt = 0.4 / 3.7 = 0.1 s
Impulse = F Δt
so 470 = F * 0.1
F = 4700 N
NOW ... that is the resultant force acting up and they want the force exerted by the ground on the person's feet
so F = force exerted by ground on feet - weight of person
F = F(ground on feet) - mg
F(ground on feet) = F + mg
F(ground on feet) = 4700 + 64 x 9.8
F(ground on feet) = 5327 N = 5000 N (to 1 sig fig)
so the average force exerted by the ground on the person's feet is 5000 N vertically up