Question

Two people are pushing a 35 kg box. One person pushes with a force of 45 North and the other person pushes with a force of 60 N South. What is the resulting force on the box?
A cat is running with a horizontal velocity of 2 m/s. It can't stop in time and flies off the edge of a 1.5 m high counter. All of the following statements are true about the cat except
a. its acceleration in the x-direction is -9.8 m/s2
b. its velocity in the x-direction just before it hits the ground is 2 m/s
c. its initial velocity in the y-direction is zero
d. its velocity in the x-direction just before it hits the ground is zero
Which object would have more kinetic energy, a 50 kg person running 10 m/s or a 50 kg person running 15 m/s?
Two weightlifters are each able to lift 120 kg. One is able to lift it in 1.2 seconds and the other is able to lift it in 1.8 seconds. Which person generated more power?
Two moving cars stick together after colliding. What must be true about the cars?

Answer 1

Answer on Question #42111-Physics-Mechanics- Kinematics-Dynamics

Two people are pushing a 35 kg box. One person pushes with a force $F_{1}$ of 45 North and the other person pushes with a force $F_{2}$ of 60 N South. What is the resulting force on the box?

Solution

The resulting force on the box is

$$F = F_{2} - F_{1} = 60 - 45 = 15 \text{ N South}.$$

Answer: 15 N South.

A cat is running with a horizontal velocity of $2\,\mathrm{m/s}$. It can't stop in time and flies off the edge of a $1.5\,\mathrm{m}$ high counter. All of the following statements are true about the cat except

a. its acceleration in the x-direction is $-9.8\,\mathrm{m/s}^2$

b. its velocity in the x-direction just before it hits the ground is $2\,\mathrm{m/s}$

c. its initial velocity in the y-direction is zero

d. its velocity in the x-direction just before it hits the ground is zero

Solution

d. its velocity in the x-direction just before it hits the ground is zero. False: its velocity in the x-direction is always $2\,\mathrm{m/s}$.

Answer: d.

Which object would have more kinetic energy, a 50 kg person running 10 m/s or a 50 kg person running 15 m/s?

Solution

The kinetic energy of first object is

$$K_{1} = \frac{m_{1} v_{1}^{2}}{2} = \frac{50 \cdot 10^{2}}{2} = 2500\,\mathrm{J}.$$

The kinetic energy of first object is

$$K_{2} = \frac{m_{2} v_{2}^{2}}{2} = \frac{50 \cdot 15^{2}}{2} = 5625\,\mathrm{J}.$$ $$K_{2} > K_{1}.$$

Answer: a 50 kg person running 15 m/s has more kinetic energy.

Two weightlifters are each able to lift $120\,\mathrm{kg}$. One is able to lift it in $t_1 = 1.2$ seconds and the other is able to lift it in $t_2 = 1.8$ seconds. Which person generated more power?

Solution

The power is $P = \frac{W}{t}$, where $W$ is the work and $t$ is time to do it. In both cases the work is the same, so

$$P_{1} = \frac{W}{t_{1}} > P_{2} = \frac{W}{t_{2}}.$$

Answer: 1-st person.

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