Question

1. A spotter assists a weightlifter who is attempting to lift a 1000 N barbell. The spotter exerts an 80 N upward force on the barbell, while the weightlifter exerts a 980 N upward force on the barbell. What is the net vertical force exerted on the barbell?

2. During a long jump, a ground reaction force of 4500 N is acting forward and upward at an angle of 78° relative to the horizontal. The jumper’s mass is 70 kg.

What is the jumper’s weight?

What is the vertical component of the GRF?

What is the horizontal component of the GRF?

3. A punter punts the football. The football leaves the punter’s foot with a vertical velocity of 20 m/s and a horizontal velocity of 15 m/s.

How long is the football in the air?

How far did the football travel?

4. A long jumper leaves the ground at an angle of 25 degrees with respect to the horizontal with a resultant velocity of 30 ft/s.

What is the horizontal velocity of the jump at takeoff in m/s?

What is the vertical velocity of the jump at takeoff in m/s?

How high did the jumper’s center of mass rise above the point of takeoff?

5. A 0.43 kg soccer ball is stationary on the field when Bobby kicks it. Bobby’s foot is in contact with the ball for 0.03 seconds during the kick. The horizontal velocity of the ball as it leaves his foot is 25 m/s. What is the average horizontal force exerted by Bobby’s foot on the ball during the 0.03 seconds of contact?

6. A boxer is punching the heavy bag. The time of impact of the glove with the bag is 0.10 seconds. The mass of the glove and his hand is 3 kg. The velocity of the glove just before impact is 25 m/s. What is the average impact force exerted on the glove?

Answer 1

1. Since the spotter exerts an 80 N upward force on the barbell, and the weightlifter exerts a 980 N upward force on the barbell; net vertical force exerted on the barbell is 80N +980N= 1060N

2. The jumper’s mass is 70 kg; so the jumper’s weight is 70*9.8= 686 N

the vertical component of the GRF is 4500sin78= 401.66N

the horizontal component of the GRF is 4500cos78= 935.6N

3. the  football leaves the punter’s foot with a vertical velocity of 20 m/s . So, 0= 20-9.8*t;

Or, time taken by it to reach the top is t= 2.04s. Again; in this much time it comes back down. So, the football is in the air for 4.08s .

In 4.08s, horizontal distance covered is horizontal velocity times t= 15*4.08= 61.2m

So, the football travel 61.2m far.

4. horizontal velocity of the jump at takeoff in m/s= 30cos25= 27.12ft/s

vertical velocity of the jump at takeoff in m/s= 30sin25= 12.68ft/s

let the jumper’s center of mass rise h ft above the point of takeoff; then it stops momentarily and falls back. So, 0^2- 12.68^2= 2*(-32.2)*h

So, the jumper’s center of mass rise h= 2.5ft above the point of takeoff .

5. From impulse momentum equation, mass of soccer ball*velocity of the ball as it leaves= kicking force*time for which it is exerted

So, 0.43*25= F*0.03;

So, the average horizontal force exerted by Bobby’s foot on the ball during the 0.03 seconds of contact is F= 358.34 N

6. From impulse momentum equation, mass of the glove and his hand*velocity of the glove just before impact = force exerted on the glove*time for which it is exerted

So, 3*25= F*0.10

So, the average impact force exerted on the glove is F= 750 N

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