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Chapter 7: Q27Q (page 170)

If only an external force can change the momentum of the centre of mass of an object, how can the internal force of the engine accelerate a car?

Short Answer

Expert verified

Newton’s third law helps the internal force of the engine to accelerate a car.

Step by step solution

01

Significance of Newton’s third law

According to Newton’s third law, if a body applies force on an object, the object will also exert a reaction force of identical magnitude on the body.

02

Understanding the acceleration of a car

When an engine applies force on the wheel of the automobile, it starts rotating. The rotating wheel pushes the surface of the road backward. A reaction force exerts on the wheel, pushing it forward due to the third law. This way, the car starts to accelerate using the mechanical energy produced by the chemical energy of the fuel.

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Most popular questions from this chapter

Billiard ball A of mass \({m_{\rm{A}}} = 0.120\;{\rm{kg}}\) moving with speed \({v_{\rm{A}}} = 2.80\;{\rm{m/s}}\) strikes ball B, initially at rest, of mass \({m_{\rm{B}}} = 0.140\;{\rm{kg}}\) As a result of the collision, ball A is deflected off at an angle of 30.0° with a speed \({v'_{\rm{A}}} = 2.10\;{\rm{m/s}}\)

(a) Taking the x axis to be the original direction of motion of ball A, write down the equations expressing the conservation of momentum for the components in the x and y directions separately.

(b) Solve these equations for the speed, \({v'_{\rm{B}}}\), and angle, \({\theta '_{\rm{B}}}\), of ball B after the collision. Do not assume the collision is elastic.

A 980-kg sports car collides into the rear end of a 2300-kg SUV stopped at a red light. The bumpers lock, the brakes are locked, and the two cars skid forward 2.6 m before stopping. The police officer, estimating the coefficient of kinetic friction between tires and road to be 0.80, calculates the speed of the sports car at impact. What was that speed?

Astronomers estimate that a 2.0-km-diameter asteroid collides with the Earth once every million years. The collision could pose a threat to life on Earth. (a) Assume a spherical asteroid has a mass of 3200 kg for each cubic meter of volume and moves toward the Earth at\(15\;{\rm{km/s}}\). How much destructive energy could be released when it embeds itself in the Earth? (b) For comparison, a nuclear bomb could release about\(4.0 \times {10^{16}}\;{\rm{J}}\). How many such bombs would have to explode simultaneously to release the destructive energy of the asteroid collision with the Earth?

Repeat Problem 59 assuming the body bends at the hip joint by about 15°. Estimate, using Fig. 7–27 as a model.

A 0.060-kg tennis ball, moving with a speed of 5.50 m/s has a head-on collision with a 0.090-kg ball initially moving in the same direction at a speed of 3.00 m/s. Assuming a perfectly elastic collision, determine the speed and direction of each ball after the collision?
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