Physics Midterm

Sometime in the past semester something has happened and you have thought "That's Physics!" Describe the situation and the Physics involved. Did you share this with those around you? How did they react?

A situation was when my parents were looking at the safety rating of my car. My car is a Honda Insight, a small car. The safety rating were good, but I kept telling my mom that in a crash my car will experience the most damage because of Newton's second law which is why the insurance company charges more. My mom reacted by her saying the ratings were good and do you not feel safe in the car.

Your car goes from 0 m/s to 20 m/s in 5 seconds. What is the acceleration? (Hint: a = Δv/t)

(20-0=20) 20/5=4m/s^2

Your car goes from 20 m/s to 32 m/s in 4 seconds. What is the acceleration? (Hint: a = Δv/t)

(32-20=12) 12/4=3m/s^2

You are sitting in a car that is not moving. You toss a ball up in the air and catch it. At the top of its path what is the acceleration of the ball?

0 m/s^2

You are sitting in a car that is not moving. You toss a ball up in the air and catch it. At the top of its path, what is the speed of the ball?

10m/s

3000 kg car A is sitting at a red light. 9000 kg car B going 24 m/s runs into it. What is the speed of the tangled cars after the collision? Show your work.

18 m/s Equation used: (mv)smaller + (mv)larger= (msmaller+mlarger) vf (3000kg)(0)+(9000kg)(24m/s)=(3000kg+9000kg)vf

3000 kg car A is sitting at a red light. 9000 kg car B going 24 m/s runs into it. If the collision lasts 0.3 s, what is car A's acceleration during the collision? (Hint: a = Δv/t) Show your work.

A= 18/0.3= 60m/s^2

3000 kg car A is sitting at a red light. 9000 kg car B going 24 m/s runs into it. If the collision lasts 0.3 s, what is car B's acceleration during the collision? (Hint: a = Δv/t) Show your work.

A=(24-18=6)/0.3= 20m/s^2

3000 kg car A is sitting at a red light. 9000 kg car B going 24 m/s runs into it. Calculate the amount of force was applied to car A during the collision. (Hint: Newton's 2nd Law) Show your work.

F=ma; F=(3000kg) (60m/s^2)= 180000N

3000 kg car A is sitting at a red light. 9000 kg car B going 24 m/s runs into it. Calculate the amount of force was applied to car B during the collision. (Hint: Newton's 2nd Law) Show your work.

F=ma; F=(9000kg)(20)= 180000N

A car slams on the brakes and skids to a stop. What forces act on the car? What net force acts on the car?

Friction and support force the net force while breaking was friction was greater, at rest it is zero

Now you are riding on the interstate at 70 mph. You toss a ball up in the air. What forces are acting on the ball?

Gravity

You are sitting in a car that is not moving. You toss a ball up in the air and catch it. What forces are acting on the ball?

Gravity

When a car is sitting still, what forces act on the car? What is the net force acting on the car?

Gravity and support force, net force is zero (Newton's 1st law supports this)

A car is moving at a constant speed of 70 mph on a straight flat road. What forces act on the car? What net force acts on the car?

Gravity, friction, support force, air resistance, net force is 0 (Newton's first law supports this)

Now you are riding on the interstate at 70 mph. You toss a ball up in the air. At the top of its path what is the acceleration of the ball?

Horizontal= 0m/s^2 Vertical= 10m/s^2

Now you are riding on the interstate at 70 mph. You toss a ball up in the air. At the top of its path, what is the speed of the ball?

Horizontal= 70mph Vertical= 0mph

Describe some part of the event that applies Newton's 3rd Law not yet covered.

If car A exerts a force on car B, then car B exerts a force on car A that is equal and opposite; they will experience the same force

Describe some part of the event that applies Newton's 1st Law not yet covered.

If you're in one of the cars, when the car stops you keep going until you hit the air bag or steering wheel which hits you and stops you

"Motion is relative." What does this mean? Use examples.

It means that it depends on your frame of reference. For example if you're sitting in a classroom, compared to the tables and chairs you're not moving. Compared to the sun, you're moving thousands of mph. Two people can be watching the same event, but have different frame of references.

What if the ball goes out the window? Describe its motion.

It's speed doesn't change, someone on the sidewalk if they get hit it will hit them with the same speed the car was going.

If the cars had been more rigid the time of the collision would have been reduced. What effect would that have had on you?

The effect would have been more force and probably more injury

Describe some part of the event that applies Newton's 2nd Law not yet covered.

The smaller car will accelerate more since it has less mass, and mass and acceleration are inversely proportional.

3000 kg car A is sitting at a red light. 9000 kg car B going 24 m/s runs into it. Did the momentum of the two cars together change? Why or why not?

The total momentum will stay the same because of the law of conservation of momentum

Explain why the accelerations different? (Hint: Newton's 2nd Law)

Their masses are different and according to the second law mass and acceleration are inversely proportional.

Explain the "horse-cart" problem.

There are internal forces between them in the system, but internal forces do not accelerate the system since they act on different objects. The only outside force acting on the system is friction, which acts as the only force. Friction is stronger, so it accelerates forward.

Modern cars have air bags and crumple zones. How do these protect the people in the car during collisions?

These protect people since it adds time before a collusion which lessens force. The more it takes for something to collide, which means less force, which means less injury.

Which car experiences more force? Explain. (Hint: Newton's 3rd Law)

They are equal since according to Newton's second law if A exerts a force on B, the B exerts a force on A that is EQUAL and opposite

3000 kg car A is sitting at a red light. 9000 kg car B going 24 m/s runs into it. Did the momentum of car B change? Why or why not?

Yes Car B changed speeds

3000 kg car A is sitting at a red light. 9000 kg car B going 24 m/s runs into it. Did car A accelerate? Why?

Yes because it went from 0m/s to 18m/s

3000 kg car A is sitting at a red light. 9000 kg car B going 24 m/s runs into it. Did car B accelerate? Why?

Yes because it went from 24m/s to 18m/s

3000 kg car A is sitting at a red light. 9000 kg car B going 24 m/s runs into it. Did the momentum of car A change? Why or why not?

Yes car A changed speeds

Now you are riding on the interstate at 70 mph. You toss a ball up in the air. Is the ball accelerating? Justify your answer.

Yes it changes direction

You are sitting in a car that is not moving. You toss a ball up in the air and catch it. Is the ball accelerating? Justify your answer.

Yes; it changed direction and speed

Now you are riding on the interstate at 70 mph. You toss a ball up in the air. Can you catch it, or will it land behind you? Explain.

You can catch it since you, the ball, and the car are all going 70mph

What is the same about the ball's motion when the car is moving?

You can still catch it and it accelerates since it changes direction

What is different about the ball's motion when the car is moving

You, the car, and ball are all moving at the same speed